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KEYWORDS: spiny aster/herbicide sprays/herbicide pene-
tration/ecology/anatomy.

METRIC UNITS — ENGLISH EQUIVALENTS

Metric
Unit
Centimeter
Gram
Hectare

Kilogram per hectare
Kilometer

Liter

Meter

Microgram per gram
Milliliter

Millimeter

Degrees Celsius >< 9/5 + 32

Mention of a trademark or a proprietary product does not constb-Qx ,-

English
Equivalent

0.4 inch

0.035 ounce (weight)
2.47 acres

0.89 pounds per acre
0.62 statute mile
0.26 gallon

3.28 feet

One part per million
0.034 ounce (volume)
0.04 inch

Degrees Fahrenheit

tute a guarantee or warranty of the product by the U.S. Depar

ment of Agriculture or the Texas Agricultural Experiment Station.
and does not imply its approval to the exclusion of other products

that also may be suitable.

Contents

METRIC UNITS — ENGLISH EQUIVALENTS . . . . . . . . . . . . .. inside front cover

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3

Botanical Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3

Distribution and Status as a Weed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

METHODS AND MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Study Area . . . . . . . . . . . . . . . . . . . . . . . . . .1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Autecological Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Leaf and Stem Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Herbicide Penetration and Translocation . . . . . . . . . . . . . . . . . . . . . . . . .  5

Field Applications of Herbicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7

Autecological Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Leaf and Stem Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Herbicide Penetration and Translocation . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Response to Herbicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11

Fall and Spring Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11

Applications to Regrowth Following Shredding . . . . . . . . . . . . . . . . .. 12

Natural Revegetation Following Control . . . . . . . . . . . . . . . . . . . . . . .. 14

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15

LITERATURE CITED  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Summary

Spiny aster occurs as an incidental species on fertile bottomland range
sites throughout most of Texas. However, on portions of the Coastal Prairie,
spiny aster eliminates forage production on substantial areas of native range-
land. After initial invasion of rangelands, seed production does not appear to
be important in development of spiny aster stands. Reproduction is primarily
through vegetative propagation by rhizomes which allows spiny aster to toler-
ate top removal. Since spiny aster cannot be controlled with conventional
mechanical treatments used on range and pastures, several herbicides and
herbicide combinations were evaluated as broadcast foliar sprays applied to
both shredded and undisturbed stands.

Applied in the fall, broadcast sprays of 2,4-D, dicamba, or picloram, alone
or in combination at rates of 2.24 kilograms per hectare or less, did not effec-
tively control spiny aster. Applied in spring when leaves were present on spiny
aster stems, only picloram at 2.24 kilograms per hectare satisfactorily reduced
spiny aster stem densities. Glyphosate, atrazine, paraquat, 2,4-D, or dicamba
applied in spring at 2.24 kilograms per hectare or less did not control spiny
aster. Shredding during the winter prior to application of herbicidal sprays in
the spring substantially improved the initial response of spiny aster to all her-
bicides. However, control with sprays in combination with shredding was
short-lived except where an established perennial grass was present to com-
pete with the weed. »

The apparent low susceptibility of spiny aster to foliar sprays is attributed
to the low degree of penetration of topgrowth by herbicides. Herbicide trans-
location is also slow in spiny aster. Neither picloram nor 2,4-D could be de-
tected in rhizomes up to 5 days after foliar application. Herbicide uptake by
leaves and stems may be limited by small stomatal size and number, compared
to other plants, but cuticle thickness is apparently not a factor.

Herbicide penetration of rapidly growing stems 2.5 months after shred-
ding was slightly greater than that of live undisturbed stems. Stomatal densities
of regrowth stem tips were about 10 percent greater than those of undisturbed
stem tips, but stomatal aperture size did not differ. Increased initial control of
spiny aster regrowth with herbicide sprays, compared to undisturbed growth,
probably results from increased herbicide penetration and improved spray
deposition and interception. Dead plant material accounts for about 50 percen-
of undisturbed spiny aster canopy; shredding removes the standing dead
stems without substantially reducing the number of live stems in regrowth.
Thus, spray effectiveness is not reduced by deposition on dead plant material.

5$0m€ Factors Affecting

the Response of Spiny Aster
to Herbicide Sprays

H. S. Mayeux, Jr., C. J. Scifres, and R. E. Meyer*

Botanical Description

Spiny aster (Aster spinosus Benth.), also called
wolfweed or Mexican devil-weed, is a perennial
plant that spreads by rhizomes and, consequently,
occurs primarily in clumps or continuous stands
rather than as individuals. Stems of spiny aster are
slender, erect, and bright green (Figure 1). Stem
height ranges from 0.5 to 2 meters, but usually aver-
ages about 1 meter. Dense stands develop in which
stem densities exceed 100 stems per square meter.
Short thorns, variable in size and number, occur on
mature stems. The stems are apparently responsible
for photosynthetic activity, since leaves are present
for only a brief period in the spring (Figure 2). The
oblong leaves are as long as 8 centimeters near the
base of stems, but smaller, tapered leaves occur in
the upper canopy.

Little is known about sexual reproduction of
spiny aster. The flower heads are small, measuring
about 1 centimeter across, and contain white ray and
yellow disk florets. The flower heads occur through-
out the summer and fall months. The seed (2500 per
gram) is an achene equipped with a persistent pap-
pus (tuft of hairs).

Distribution and Status as a Weed

Spiny aster occurs throughout the American
Southwest and is common on roadsides and irri-
gated areas of northern Mexico. Correll and Johnston
(5) describe the species as ”locally very abundant” in
the south and west half of Texas.

Spiny aster occasionally invades upland sites
with clay soils of high water-holding capacity. More
commonly, spiny aster occurs in small stands along
waterways, lake shores, and other wet sites, even if
periodically flooded. On portions of the Texas
Coastal Prairie, however, spiny aster occurs in dense,
continuous stands over extensive areas, exclud-

*Respectively, research agronomist, Science and Education Ad-
\ministration —Agricultural Research (SEA — AR), U. S. Depart-
ment of Agriculture, Temple, Texas; professor, The Texas Ag-
ricultural Experiment Station (Department of Range Science);
and plant physiologist, SEA —- AR, U. S. Department of Agricul-
ture, College Station, Texas.

ing almost all other vegetation (Figure 3). Infested
areas are primarily native rangeland. Sites domi-
nated by spiny aster often are fertile bottomlands
which receive additional water as runoff from sur-
rounding upland sites. Thus, the impact of spiny
aster as a weed is significant, even though the geo-
graphical area affected is relatively limited in size.

Spiny aster is a tenacious weed on sites where it
is well adapted. Shredding several times a year for
many years in succession or tillage associated with
production of grain sorghum (Sorghum vulgare
Pers.) has not eliminated spiny aster‘.

Although spiny aster is not considered an im-
portant forage plant, cattle occasionally graze succu-
lent new growth. The authors have often observed
browsing of spiny aster stem tips by white-tailed
deer (Odocoileus virginianus Bod.).

Specific objectives of this research included in-
vestigations of aspects of spiny aster ecology and
anatomy that are pertinent to its control with herbi-
cidal sprays and the evaluation of selected herbi-
cides and mixtures, alone and in combination with a
mechanical pretreatment, for control of spiny aster.

METHODS AND MATERIALS

Study Area

Field studies were conducted near Bloomington
in Calhoun County, Texas. All experiments were po-
sitioned within a level swale encompassing about 90
hectares. The swale receives runoff from surround-
ing areas of slightly higher elevation, but standing
water does not persist during years of normal rain-
fall.

Vegetation in the swale consists of an almost
pure stand of undisturbed spiny aster. Peripheral
areas of the depression support sea oxeye daisy
[Borrichia frutescens (L.) DC.] and Carolina
wolfberry (Lycium carolinianum Walt.) in association
with spiny aster. Although not common, a few her-
baceous plants grow where spiny aster stem den-
sities are low. These include an annual groundsel

lPhillips, Tommy. 1975. Personal communication. Ranch man-
ager, Greenlake Ranch, Victoria, TX 77015.

(Senecio imparipinnatus Klatt.), Missouri ironweed
(Vernonia missurica Raf.), and barnyardgrass
[Echinochloa crusgalli (L.) Beauv.]. A portion of the
swale was plowed and seeded to kleingrass
(Panicum coloratum L.) about 5 years prior to initia-
tion of this research. A good stand resulted, but the
grass is now heavily infested with spiny aster.

Soils of the swale are clays of the Victoria series
(Typic Pellustert). Clay content is about 60 percent in
the upper horizons. Soil reaction is basic, and the
soil is slightly saline, with 2,180 parts per million of
salts near the surface, increasing to 2,700 parts per
million at 45 centimeters. Less than 3 percent or-
ganic matter occurs in the A horizon. No B horizon is
apparent. The soil is calcareous throughout and very
dark in color. Soils of this series are typically fertile.

Annual rainfall of the area averages 93.5 cen-
timeters (Figure 4). The precipitation pattern is
characterized by slight rainfall peaks in spring and
fall. The mean maximum temperature for July is 33.3°
Celsius, and the mean minimum temperature in
January is 8.3° Celsius (2).

Figure 2. Spiny aster stems photographed near Bloomington,
Texas, in April, while leaves are present.

Autecological Observations

Although growth and phenology of spiny aster
were not studied in detail, time and duration of leaf
presence and stem growth were observed periodi-
cally during 1975 and 1976. Du ring June of each year,
numbers of new stems, ovenivintered stems, and I
standing dead stems were counted in 0.25-square-
meter quadrats on both undisturbed and previously
shredded stands.

The relative importance of sexual and vegetative
reproduction was also evaluated. Flower heads on
main stems and numbers of achenesper head were
counted on plants in the field and in a greenhouse at
College Station. Mature, filled achenes from both
locations were tested for germinability at 20° Celsius
for 16 hoursyand 30° Celsius for 8 hours with light. In
March, 100 rhizome sections 10 to 20 centimeters
long with at least two nodes were transplanted to the
greenhouse. These were planted in 20-centimeter
diameter pots with Victoria clay soil. Numbers of
new stems and the time required for resprouting
were recorded. Regrowth plants were maintained in
a slathouse to provide leaves and stems for sub-
sequent experiments.

Leaf and Stem Anatomy

Potted plants grown outdoors were the source
of organs and tissues for study. Impressions of
upper and lower leaf surfaces were obtained by the
plastic stripping method (24). Impressions were also
made of main stem and fine terminal stem surfaces.
Numbers of stomata were counted within ten 0.05-
square-millimeter areas on at least five leaves
stems with a light microscope. A filar micrometer» "

Figure 1. A leafless spiny aster shoot with rhizome and newly eyepiece mounted on a light microscope was used
initiated stem. to measure stomatal apertures.

4

 

Figure 3. A dense stand of spiny aster covering about 100 hectares
near Bloomington, Texas; the stake in the foreground is 1.5 meters
tall.

ln general, the procedures of Johanson (11) and
Sass (22) were followed in the preparation of perma-
nent slide mounts of leaf and stem transections. Or-
gans were fixed in a Craf solution and dehydrated in
an ethanol-tert-butyl alcohol series. Tissues were
embedded in Paraplast and 12- or 14-micrometer
sections were cut with a rotary microtome. Tissue
sections were stained for 30 minutes in safranin-O
and then for 5 minutes in fast green FCF. Transec-
tions were photographed with a 10- by 12.7-
centimeter camera using transmitted light. To obtain
micrographs of upper and lower leaf surfaces, sam-

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ples were dehydrated, coated with gold, and photo-
graphed at 10 kilovolts in a Cambridge S-4 Stereo-
scan electron microscope.

Herbicide Penetration and Translocation

Leaves and stems of spiny aster were exposed to
aqueous solutions of picloram (4-amino-3,5,6-
trichloropicolinic acid) and 2,4-D [(2,4-
dichlorophenoxy)acetic acid] to evaluate rate and
extent of herbicide uptake (3). Leaves of sunflower
(Helianthus annuus L.) were included as a reference,
since sunflower is easily controlled with these her-
bicides. The objective of these experiments was to
investigate potential morphological barriers to her-
bicide uptake by spiny aster.

Undamaged leaves of uniform size and maturity
were removed from the same location on stems, be-
tween 4 and 10 centimeters from the growing tip.
Spiny aster stems were collected from plants in the
field during June. Stem sections, 20 centimeters
long, were cut from the large, main portion of
current-year shoots. Similar stem samples were col-
lected from an undisturbed stand and from regrowth
following shredding to a 4-centimeter stubble height
90 days previously. All stems were kept moist and
transported over ice (but not frozen) to the labora-
tory.

Cut ends of spiny aster stem sections were
dipped in paraffin wax and coated with stopcock
grease to prevent herbicide movement through the
open end. Six leaves of sunflower or spiny aster, two
spiny aster main stem sections, or five spiny aster
stern tips were suspended around the inside edges
of 20-centimeter diameter culture dishes. Leaves or
stems were adjusted so that the acropetal halves

were immersed in herbicide solutions. The solutions
(10"3 molar) were prepared with analytical grade pic-
loram or 2,4-D acid in distilled water with pH ad-

 

Figure 4. Average monthly temperature

and precipitation at Victoria, about 22

kilometers from the spiny aster research
_ area near Bloomington, Texas (1).

justed to 11 with potassium hydroxide. The her-
bicide solutions were then adjusted to pH 6.8 with
hydrochloric acid. The solutions were slowly swirled
and care was taken to maintain the immersed organs
freely suspended in the solutions. After 2, 4, or 6
hours, the organs were washed in a stream of
acidified acetone. Leaves and stem sections were cut
at the solution line, and the immersed portions were
blotted dry, weighed, and stored frozen.

Comparisons included herbicide uptake by
leaves of the two species, and uptake by leaves,
main stems, undisturbed stem tips, and stem tips
from regrowth of spiny aster only. Each treatment
was triplicated in each of two experiments. Hierar-
chical analyses of variance were conducted with
herbicide as main effects, plant part as subplot ef-
fects, and time as the sub-subplot effects (25).
Means were compared with Duncan's new multiple
range test at the 95-percent level of confidence.

Rate of foliar herbicide uptake and extent of
translocation to rhizomes were investigated at the
Bloomington study site on March 12, 1976, when
spiny aster was in full leaf, and again on June 24,
1976, when stems were leafless. Herbicide treat-
ments consisted of either 1.12 kilograms per hectare
of the potassium salt of picloram or 2.24 kilograms
per hectare of diethylamine salt of 2,4-D applied in
water with 0.5 percent (volume basis) commercial
surfactant (containing sodium dodecyl, benzene sul-
fonate, trimethyl nonylether, and ethylene glycols).
Treatments were applied in early morning to 16-
square-meter plots with a back-pack sprayer.

In March, herbicide uptake was determined by
removing four separate, random samples of 10
leaves each. Each sample was rinsed in 0.1 normal
ammonium hydroxide (NH4OH), blotted dry, and
placed in plastic bags. Leaf rinses were also kept for
analysis. Sampling was conducted immediately after
herbicide application, after 8 hours, and then daily
for 5 days. Terminal stems were collected in lune on
the same time schedule and rinsed as described for
leaves. Rhizome samples, about 10 centimeters long
and with large aerial stems, were collected on the
same schedule. All samples were stored frozen until
extraction and analysis.

Established techniques were followed for the
extraction and preparation of samples containing
picloram (4) and 2,4-D (10, 12) and for chromato-
graphic determinations (12, 16). Stems and rhizome
samples were cleaned of soil and ground in a Wiley
mill. Picloram and 2,4-D were extracted by blending
tissue samples with 30 or 60 milliliters acidified
acetone (4 milliliters hydrochloric acid per liter
acetone) for 2 minutes. The resulting macerate was
filtered under vacuum, tissue and filter paper were
blended a second time, and the filtrates were com-
bined. After the acetone was evaporated to near
dryness and 10 milliliters water were added, the
aqueous solution was made basic with potassium
hydroxide. Interfering compounds were partially
removed by shaking in a separatory funnel with

6

three, 30-milliliter diethylether washes. After acidifi-
cation with hydrochloric acid, herbicides were ex-

tracted by shaking three times with BO-millilitefi
aliquots of ether. Extracts were combined and the .

ether evaporated.

Both herbicides were esterified with 8 milliliters
boron trifloride-methanol solution (10 to 12 percent).
Samples containing 2,4-D were briefly heated during
methylation, but picloram samples were heated on
an oscillating hotplate to near dryness (26). After
cooling, herbicides were dissolved in 10 milliliters of
distilled hexane.

Herbicide concentrations were determined with
a Barber Coleman Model 5360 gas chromatograph
with a radium-226 electron-capture detector and a
column packed with 1.5 percent SE 30 Chromasorb
‘W’. Prepurified nitrogen gas was used as carrier at a
flow rate of 70 milliliters per minute. Injector, col-
umn, and detector temperatures for picloram were
260, 200, and 230° Celsius, respectively. Injector and
detector temperatures were unchanged for 2,4-D
determinations, but column temperature was ad-
justed to 185° Celsius.

Two microliters of the herbicide methyl ester-
hexane solution were injected onto the column.
Herbicide concentrations were determined by com-
paring peak heights to those of fortified samples.
Recovery percentages of picloram and 2,4-D ex-
tracted from leaves and rinses were 96 and 86 per-
cent, respectively. Herbicide recovery from
rhizomes was about 70 percent.

Foliar herbicide uptake was expressed as the
ratio of herbicide concentration in leaf tissue di-
vided by the total herbicide concentration in tissue
and rinse. Hierarchical analysis of herbicide uptake
data included herbicide treatments or mechanical
pretreatment as main plot effect and time periods as
subplot effects. Mean differences were compared
with Duncan's new multiple range test at the 95-
percent level of confidence.

Field Applications of Herbicides

Herbicides were applied to spiny aster with a
tractor-mounted, small-plot boom sprayer, which
delivered 200 liters per hectare of water containing
0.5 percent commercial surfactant. Air temperature,
relative humidity, and wind speed were measured
during herbicide application. Replicated soil sam-
ples were collected at three depths for gravimetric
moisture determinations.-

Experiments were designed as randomized
complete blocks with three or four replications of
treatments applied to 0.02-hectare plots. Numbers
of live spiny aster stems were counted in eight,
0.25-square-meter quadrats systematically located
down the center of each plot at 7 to 9 months, 1 year,

2 years, and 3 years after treatment. §

Herbicides were applied to leafless spiny aster
in the fall (October 5, 1974) and to leafed spiny aster
in the spring (March 20, 1975, and March 16, 1976).

v
~ t

 

Fall treatments included the dimethylamine salt of
dicamba (3,6-dichloro-o-anisic acid), butoxyethanol
ester of 2,4-D, and the potassium salt of picloram
applied alone at 0.5, 1.12, and 2.24 kilograms per
hectare. Also, dicamba or 2,4-D was combined with
picloram at 1.12 and 2.24 kilograms per hectare total
herbicide. Treatments applied in the fall, and at-
razine [2-chloro-4-(ethylamino)-6-(isopropylamino)-
s-triazine], glyphosate [N-(phosphonomethyhgly-
cine], and paraquat (1,1’-dimethyl-4,4’-bipyridinium
ion) were applied in the spring. Glyphosate, atra-
zine, and paraquat were applied at 1.12 and 2.24
kilograms per hectare.

A portion of the spiny aster stand was shredded
to a height of 4 centimeters in March 1975. Leafless
regrowth had reached 40 centimeters on June 2,
1975, when herbicides were applied. Dicamba,
2,4-D, glyphosate, and atrazine were applied indi-
vidually at 1.12 and 2.24 kilograms per hectare. Pic-
loram was applied at the lower rate only.

On June 17, 1976, herbicide treatments were
applied to spiny aster regrowth following shredding
during the previous winter within the portion of the
swale seeded to kleingrass. Broadcast sprays were
repeated on the same day in an adjacent area cov-
ered with dense, undisturbed spiny aster.

RESULTS AND DISCUSSION

Autecological Observations

Leaves appeared on spiny aster in the field dur-
ing late January in both 1975 and 1976 and were
present until mid-April. Overwintered stems bore
widely scattered groups of leaves that resembled
rosettes and occurred only in the upper portions of
the canopy. Leaves on developing stems occurred
singly, alternately, and were uniformly distributed
from stem base to tip.

Shoots from rhizomes appeared continuously
during January and February (Figure 5), elongated
rapidly, and reached heights of about 50 centimeters
by June. Branching occurred during this period, and
a single shoot had as many as 20 stem tips by mid-
summer. Progressive narrowing of diameter as up-
ward growth and branching progressed transformed
the stems from thick and fleshy to a fine, flexible
appearance.

Current year's growth accounted for over 75
percent of the living portion of the spiny aster
canopy in lune (Table 1). Live stem density was al-
most equaled by density of standing dead stems.
Although ages of older stems could not be deter-
mined, it was doubtful that many survived beyond
the second growing season. Top removal by shred-
ding during winter did not substantially alter num-
bers of new stems. In lune, stem density on a shred-
ded portion of the study area averaged 92 per square

 eter.

Stem growth first appeared 2 weeks after trans-
planting spiny aster rhizomes into pots during

Figure 5. Spiny aster shoot about 8 centimeters tall, which de-
veloped from a rhizome, photographed in early March at the study
site near Bloomington, Texas.

March and 86 percent of the rhizomes ultimately de-
veloped aerial stems. Most pots contained two to
four individual shoots which grew to a height of 80
centimeters by August when growth ceased. The
appearance of stems of potted plants did not out-
wardly differ-from that of plants in the field. Leaves
of potted spiny aster appeared soon after growth
began and lasted through May.

A few flower heads formed on spiny aster in the
field site and in pots during the summer. Flower
head numbers reached a maximum in October, and
blooming ceased in late November. Flower heads
ranged from 5 and 25 per stem and averaged 17 in
the field. Counts of achenes in undamaged flower
heads collected in the field averaged 63 and ranged
from 44 to 84. Potted spiny aster plants averaged 60
achenes per head, rangin from 40 to 84. Approxi-
mately one-third of all lower heads inspected,

Table 1. Average density of new, overwintered, and dead stems of
spiny aster in June near Bloomington, Texas

Stem numbers/mz

Live stem age
>1 yr Total
113 35 148 139

<1 yr Dead stems

whether on potted or field-grown plants, contained
a small, unidentified insect larva that fed on the de-
veloping seeds.

Few spiny aster seeds germinated at alternating
temperatures of 20° and 30° Celsius with light. Aver-
age germination of well-filled achenes collected in
the field was 1.4 percent, whereas 5.7 percent of the
achenes produced by potted plants germinated. Ex-
periments were initiated shortly after achenes were
collected so that aging and storage did not affect
viability. A dormancy mechanism may be involved,
however, and temperature or light conditions other
than those employed may be required for high ger-
mination of spiny aster seed.

Leaf and Stem Anatomy

Stomatal concentrations on upper leaf surfaces
are about twice those on lower leaf surfaces of spiny
aster (Table 2). Most plants have highest stomatal
densities on lower surfaces (8), and spiny aster
leaves have few stomata relative to other species.
For example, stomatal densities of honey mesquite
(Prosopis glandu/osa Torr. var. glandulosa) are about
twice those of spiny aster (17).

The epidermis of terminal stems contains more
and larger stomata than do leaves (Table 2). Also,
mechanical pretreatment increased stomatal density
in the upper spiny aster canopy. Stomatal densities
averaged 344 per square millimeter on regrowth
stems 12 weeks after top removal. Densities on ter-
minal stems collected from undisturbed top-growth
were lower, averaging 309 per square millimeter.

Stomatal densities on main stems located in the
middle portion of the spiny aster canopy were lower
than on stem tips. Densities of stomata increased
from the main stem outward to the shoot tip. Al-
though stomatal apertures of main stems were not
measured, they appeared to be larger than terminal
stem stomata.

A few uniseriate papillae (hairs) occurred on the
upper surface of spiny aster leaves, but none were
noted on the lower epidermis (Figure 6). Epidermal
cells of both leaf surfaces are irregular in shape and
size, especially those of the lower surface.

According to the classification of Martens (13),
the cuticular structure of the upper surface is
strongly ridged, while that of the lower leaf surface
is an intergrade between ridged and smooth. Sub-

Table 2. Average stomatal densities and aperture lengths of upper
and lower leaf surfaces and stems of spiny aster

Density Aperture length

Location (stomata/mmz) (pm)
Upper leaf surface l 109 15
Lower leaf surface 53 17
Undisturbed stem tip 309 39
Regrowth stem tip 344 38
Main stem 78

sidiary cells of the lower surface are more noticeably
ridged than ordinary epidermal cells.

Spiny aster does not have heavy leaflcuticle€

(Figure 7). Cuticle development does not appear to
differ on upper and lower leaf surfaces, and cuticle
thickness is uniform across epidermal layers of both
surfaces.

Leaf mesophyll of spiny aster is relatively dense,
with little intercellular space (Figure 7). The
mesophyll consists of a single layer of palisade
parenchyma cells and a well-developed spongy
parenchyma. A resin canal (oil duct) is associated
with each vascular bundle. Similar structures have
been reported in western ironweed and other
weedy members of the Composite family (15).

Both larger main stems and fine, terminal stems
of spiny aster are herbaceous in cross section. Ter-
minal stems are somewhat costate (ridged) (Figure
8), but main stems are rounded (Figure 9). Fiber
bundles and resin canals are located at the ridges of
terminal stems, separated by dense cortex. Resin ca-
nals in main stems are not associated with fiber bun-
dles, however. Large, thin-walled cortical paren-
chyma cells, possibly epithelial oil cells, are grouped
adjacent to the xylem in the terminal stem and be-
tween the fiber bundles in the main stem.

Pith constitutes a large portion of spiny aster
stem volume, especially in the main stem. Xylem oc-
curs as a continuous ring adjacent to the pith in both
spiny aster stem types, while phloem occurs only
adjacent to fiber bundles. Stomates occur on the
epidermis of both terminal and main stems.

Herbicide Penetration and Translocation

Leaves of spiny aster absorbed little herbicide
relative to that in annual sunflower (Table 3). Con-
centrations of 2,4-D and picloram in leaves of annual
sunflower were substantially higher than concen-
trations reported in live oak (3) and honey mesquite
(14) leaves exposed to 2,4,5-T using the same ex-
perimental method. After 2 hours, for instance,
sunflower leaves contained about 24 micrograms
2,4-D per gram of leaf tissue, while less than 2 mi-
crograms per gram was detected in spiny aster
leaves. Picloram was not detected in spiny aster
leaves until after 4 hours (Table 3). Therefore, foliar
absorption may be the limiting factor in response of
spiny aster to broadcast applications of 2,4-D or pic-
loram.

Leaves of both spiny aster and sunflower gener-
ally had absorbed more of the potassium salt of
2,4-D than picloram after 4 and 6 hours but not after
2 hours in the laboratory. Increased uptake of 2,4-D
with time was pronounced in leaves of annual
sunflower. However, significant differences in the
concentrations of the two herbicides in spiny aster

leaves occurred only after the longest exposure§

time, 6 hours.

The various organs of spiny aster clearly differed
in regard to rate of herbicide penetration (Table 4).

»

Initially, leaves and main stems absorbed either her-
bicide rather slowly under laboratory conditions.
a Picloram content of leaves after 2 hours was below
he limit of detection (0.01 microgram picloram per
gram leaf tissue). Terminal stems, whether from un-
disturbed growth or previously shredded spiny as-
ter, apparently absorbed at least three times more
herbicide than did leaves and main stems during the
initial 2-hour period. Picloram accumulation in
leaves equaled or exceeded that in terminal and
main stems at 4 and 6 hours, however.

The accumulation of high amounts of either
herbicide in terminal stems early in the experiments
without continued accumulation of comparatively
high concentrations compared to other organs
seems questionable. Epidermal surfaces of terminal
and main stems do not visibly differ. These results
suggest mass flow of the herbicide solutions into the
terminal stems upon immersion rather than actual
passage of herbicides through morphological bar-
riers. A possible route of herbicide entry may be
through the meristematic tips of the fine stems,
which abort when growth ceases. A small,
irregularly-shaped callus forms, which may readily
admit solutions. However, stems of regrowth spiny
aster appeared to be actively growing when col-
lected in lune. Terminal tips were soft, pale green,
and rounded; those of undisturbed shoots were in-
flexible, dark green, and callused. lf herbicide so-
lutions easily entered through aborted tips, terminal
stems from spiny aster shoots without mechanical
pretreatment should have absorbed more herbicide
than those from regrowth. However, the reverse oc-

curred, although differences were not always signif-
icant. The higher stomatal concentrations observed
on regrowth stem tips may be responsible for the
differential absorption. A relationship between
stomatal abundance and herbicide uptake has been
noted in foliage of several woody species (6).

No portion of the spiny aster canopy is readily
penetrated by 2,4-D or picloram, relative to foliage
of annual sunflower. Apparently, spiny aster leaf
presence is not an important consideration in timing
of broadcast sprays, since leaf uptake of herbicide is
only slightly greater than stem uptake. However, leaf
presence would increase the total absorptive area of
the spiny aster canopy and thus improve spray inter-
ception.

ln the first field experiment, penetration of
spiny aster leaves by 2,4-D and picloram proceeded
at the same rate until 96 hours after treatment (Table
5). At 96 and 120 hours after treatment, absorption
ratios for 2,4-D were significantly higher than those
for picloram.

Absorption ratios for spiny aster leaves are
comparable to some previously reported for other
species. For instance, blackberry (Rubus procerus
P.]. Muell.) absorbed only 38 to 59 percent of
applied 2,4,5-T (21). Blackberry is not controlled by a
single application of 2,4,5-T (21). On the other hand,
honey mesquite leaves absorbed about 40 percent of
available 2,4,5-T after 1 day and 90 percent after 5
days (23). Que Hee and Sutherland (20) found that 65
to 70 percent of the dimethylamine salt of 2,4-D
penetrated sunflower leaves within 13 hours of

Table 3. Herbicide concentrations (micrograms per gram fresh weight) in detached leaves of sunflower and spiny aster after exposure to
1073 molar aqueous solutions of 2,4-D or picloram for 2, 4, or 6 hours“

Time elapsed (hr)

Species 2 4 6
2,4-D Picloram 2,4-D Picloram 2,4-D Picloram
Sunflower 24.4 d 21.5 d 54.3 g 28.3 e 77.2 h 33.1 f
Spiny aster 1.7 a NDb 10.3 b 9.8 b I 16.2 c 11.7 b

“Means followed by the same letter do not differ significantly at the 95-percent confidence level using Duncan's multiple range test.

bND denotes no detectable herbicide.

Table 4. Herbicide concentrations (micrograms per gram fresh weight) in detached leaves, terminal stems of undisturbed growth, terminal
stems of regrowth following shredding, and main stems of spiny aster exposed to 10_3 molar aqueous solutions of 2,4-D or picloram

for 2, 4, or 6 hours“

Time elapsed (hr)

Organ 2 4 6
2,4-D Picloram 2,4-D Picloram 2,4-D Picloram
Leaves 1.7 b NDb 10.3 g 9.8 fg 16.2 i 11.7 gh
Undisturbed
stem tips 6.9 de 4.9 c 11.5 gh 7.2 de 12.8 h 9.6 fg
Regrowth
stem tips 9.7 fg 7.0 de 13.4 h 8.5 ef 15.8 i 10.9 g
ain stems 2.0 b 1.9 b 6.6 cde 5.3 cd ’ 11.4 gh 9.8 fg

“\

“Means followed by the same letter do not differ significantly at the 95-percent confidence level using Duncan's multiple range test.

bND denotes no detectable herbicide.

spraying; penetration of long-chain amine formula-
tions reached 90 percent in only 4 hours. Leaf ab-
sorption of the dimethylamine salt of 2,4-D by spiny
aster in the field was only 2O percent (absorption
ratio >< 100 = percent) after 1 day and 49 percent
after 5 days (Table 5).

Herbicide concentrations within leaves of spiny
aster sprayed with 2.24 kilograms per hectare of
2,4-D reached a maximum of about 15 micrograms

per gram fresh weight at 24 and 48 hours after appli-
cation (Table 5). Thereafter, 2,4-D concentrations

were reduced by about half. Picloram concen- 
trations in leaves of spiny aster treated with 1.12 ‘p

kilograms per hectare were also greatest at 24 hours
and declined rapidly at subsequent samplings. The
rapid decrease in picloram concentration in spiny
aster leaves may be attributed to greater mobility
and more rapid export, compared to 2,4-D. Picloram

is more mobile than 2,4,5-T in certain woody species
(7) but less mobile than 2,4-D in skeleton weed
(Chondrilla juncea L.) (9). The comparative extent of
degradation of the two herbicides in spiny aster
leaves may also be a factor.

ln general, 2,4-D behaved similarly when
applied to leafless spiny aster in June (Table 6) and to
leafy spiny aster in March (Table 5). Concentrations
of 2,4-D in terminal stems of regrowth 74 days after
shredding rose rapidly to almost 19 micrograms per
gram at 24 hours after application and fell gradually
to about 5 micrograms per gram after 120 hours.
Maximum 2,4-D concentration in terminal stems of
undisturbed spiny aster was less than 14 micrograms
per gram. Picloram concentrations in terminal stems
were highly variable.

There were usually no significant differences
between herbicide concentrations in shredded and
nonshredded terminal stems. However, herbicide
concentrations averaged across time tended to be
greater in regrowth stems than in undisturbed plants
(Table 6). Elapsed time was significant for 2,4-D
penetration but not for picloram.

No 2,4-D or picloram was detected in rhizomes
of spiny aster in either field experiment. Apparently,
downward translocation of these herbicides by spiny
aster is slight. Least detectable limits, determined
with fortified samples of rhizomes, were 0.025 mi-
crogram per gram for 2,4-D and 0.01 microgram per
gram for picloram.

Table 5. Herbicide absorption ratios and concentrations (micro-
grams per gram fresh weight) in spiny aster leaves at various inter-
vals after broadcast sprays of 2.24 kilograms per hectare 2,4-D or
1.12 kilograms per hectare picloram in March“

Herbicide
Time elapsed Absorption ratiob concentration
(hr) 2,4-D Picloram 2,4-D Picloram
1 0.01 a 0.01 a 0.5 a 0.5 a
__ 8 0.07 a 0.07 a 6.9 c-e 6.4 c-e
  24 0.20 b 0.22 b 15.1 f 16.9 f
i 4a 0.22 b 0.19 b 14.8 f 9.0 e
72 0.26 b 0.21 b 6.4 c-e 6.2 b-e
96 0.40 cd 0.23 b 7.4 de 3.0 a-c
120 0.49 d 0.31 bc 5.1 b-d 2.3 ab

aMeans beneath a major heading (absorption ratio or herbicide
concentration) followed by the same letter do not differ signifi‘
cantly at the 95-percent level using Duncan's multiple range test. - ”

bAbsorption ratio = micrograms herbicide in leaves/(micrograms
herbicide in leaves + micrograms herbicide in leaf rinses).

Figure 6. Spiny aster upper leaf surface, X620 (upper). Note ridged
surfaces. Lower leaf surface, X620 (lower), with relatively smooth
cuticle. Symbols are H, hair; S, stomate.

10

 nmgggi‘,  

1%
‘ill;
fiwfiQw

Response to Herbicides

Conditions of drought or excessive moisture did
not occur when herbicides were applied. Soil water
content ranged from 26 to 32 percent in the upper 45
centimeters of the soil profile when sprays were
apphed.

Fall and Spring Applications

When herbicides were applied on October 4,
1974, spiny aster stems were leafless, stem elonga-
tion had ceased, and most stems were in full bloom.

ew stems were completely killed, regardless of

< f-nerbicide or rate applied, in May of the following

year (Table 7). Lower portions of main stems were
green, and new stems were growing from old stem

Figure 7. Transection of midrib area of
spinyaster leaf, X170. Symbols are BS,
bundle sheath; Cu, cuticle; E, epidermis;
Mi, midrib; Ph, phloem; PP, palisade
parenchyma; RC, resin canal; S, stomate;
SP, spongy parenchyma; V, vein; X, xylem.

Figure 8. Transection of terminal portion
of spiny aster stem, 2 centimeters from tip,
X90. Symbols are Ca, cambi um; Co, cortex;
Cu, cuticle; E, epidermis; FB, fiber bundle;
P, pith; Ph, phloem; RC, resin canal; S,
stomate; X, xylem. -

bases and rhizomes. Only 2,4-D plus picloram
applied at 2.24 kilograms per hectare significantly
reduced live spiny aster stem numbers. Three years
after fall applications, spiny aster stem densities in
treated plots were essentially equal to those of un-
treated plots (Table 7).

Spiny aster leaves were fully expanded and
stems were elongating rapidly when herbicides were
applied in March. Control from spring applications
was substantially higher than that obtained in the
fall. Seven months after treatment, 2,4-D or dicamba
significantly reduced stem densities only at the
highest rate, 2.24 kilograms per hectare (Table 8). In
combination, 2,4-D and dicamba tended to be more
effective, reducing stems by 50 percent at 2.24 kilo-

11

Applied alone to undisturbed, leafy stems, at-
razine proved to be ineffective (Table 9). Glyphosate
applied at 1.12 kilograms per hectare was no more _
effective than 2,4-D or dicamba. Applied at 2.24  "
kilograms per hectare, however, glyphosate re- l‘

grams per hectare, but the increases in control were
not significantly greater than where the herbicides
were applied alone.

Picloram was the most effective herbicide

applied in the spring to leafy spiny aster. Stem den-
sities were reduced by 72, 86, and 95 percent fol-
lowing picloram applications of 0.56, 1.12, and 2.24
kilograms per hectare, respectively, 7 months after
applications. Picloram alone at 1.12 or 2.24 kilograms
per hectare and in combination with 2,4-D at 2.24
kilograms per hectare total herbicide were the only
treatments which maintained significant stem den-
sity reductions 1 year after application (Table 8).
After 3 years, plots treated with picloram alone at the
highest rate still supported 63 percent fewer spiny
aster stems than untreated plots. The combination
of 2,4-D and picloram at 2.24 kilograms per hectare
total herbicide also significantly reduced stem den-
sity at the final evaluation.

Table 6. Herbicide concentrations (micrograms per gram fresh
weight) in terminal stems from shred regrowth or undisturbed
spiny aster at various time intervals after broadcast sprays of 2.24
kilograms per hectare 2,4-D or 1 .12 kilograms per hectare picloram
in lune“

Time 2,4-o Picloram
elapsed
(hr) Shredded Undisturbed Shredded Nonshredded
1 0.1 a 0.2 a 0.5 a 0.1 a
12.3 d 13.6 d 10.0 c 11.5 cd
24 18.7 e 13.2 d 9.2 bc 9.1 bc
48 12.0 d 9.7 cd 7.6 bc 7.4 bc"
72 8.7 bcd 7.7 bcd 14.2 d 11.1‘ cd
96 7.7 bcd 7.6 bcd 14.5 d 6.0 b
120 5.2 b 6.8 bc 14.8 d 9.3 bc
Avg 9.2 8.4 10.1 7.8

“Means beneath a major heading (2,4-D or picloram) followed by
the same letter do not differ significantly at the 95-percent level
using Duncan's multiple range test.

duced spiny aster stem density by 62 percent, about
twice the reduction obtained by applications of
2,4-D or dicamba at the same rate, 9 months after
application. Glyphosate was not as effective as pic-
loram, however.

Applications to Regrowth Following Shredding

Shredding alone reduced live stem numbers by
24 percent on the previously undisturbed spiny aster

Table 7. Percent reduction of spiny aster stem density 7 and 36
months after foliar applications of selected herbicides and com-
binations of herbicides in October 1974

Months after
Treatment treatment“
Rateb 7 A 36
Herbicide(s) (kg/ha)
None — 0 a 0 a
2,4-D 0.56 7 a 0 a
2,4-D 1.12 6 a 0 a
2,4-D 2.24 17 ab 6 a
Dicamba 0.56 3 a 0 a
Dicamba _ 1.12 3 a 0 a
Dicamba 2.24 1 a 0 a
Picloram 0.56 8 a 0 a
Picloram 1.12 8 a 0 a
Picloram 2.24 28 ab 11 a
2,4-D + Dicamba 1.12 10 a 0 a
2,4-D + Dicamba 2.24 10 a 1 a
2,4-D + Picloram 1.12 11 a 0 a
2,4-D + Picloram 2.24 35 b 10 a

aMeans followed by the same letter do not differ significantly at
the 95-percent confidence level using Duncan's multiple range
test.

bRates shown are total herbicide applied; herbicides were com-
bined in equal ratios.

Figure 9. Transection of main stem from
midcanopy of spiny aster, X90. Symbols are
Ca, cambium; Co, cortex; Cu, cuticle; E,

phloem; RC, resin canal; S, stomate; X,
xylem.

epidermis; FB, fiber bundle; P, pith; Ph, w»

stand (Table 9) and by 58 percent on the seeded area
(Table 10). As a pretreatment, shredding at least

"doubled the effectiveness of most herbicide applica-

tions in both experiments. While differences be-
tween responses to spraying only and to shredding
plus spraying were significant for each herbicide and
rate, responses to rates of the same herbicide
applied with or without pretreatment were not sig-
nificantly different. The major factor, then, was pre-
treatment versus no pretreatment rather than rate of
herbicide, within the narrow range of rates applied.

Glyphosate applied at 1.12 kilograms per hec-
tare in March without pretreatment reduced live
stern density by 24 percent, which was the same re-
sponse obtained by shredding alone (Table 9). Twice
that rate of glyphosate reduced live stems of undis-
tu rbed spiny aster by 62 percent, which was no more
effective than the lower rate applied to regrowth.
The higher rate, 2.24 kilograms per hectare, in com-
bination with shredding reduced live stem density
by 93 percent at 9 months after application. In the
1976 experiment, spiny aster responded similarly
when glyphosate was applied without the shredding
pretreatment (Table 10). However, after 1 year, re-
sponses to glyphosate applied to regrowth at 1 or 2
kilograms per hectare did not differ.

Of the herbicides compared, atrazine re-
sponded most positively from the mechanical pre-
treatment. Spiny aster control was increased from
less than 10 percent reduction in stem density fol-
lowing applications of atrazine to leafy plants in
March to 54 and 68 percent, respectively, when 1.12
and 2.24 kilograms per hectare were applied to re-
growth (Table 9). Either the pretreatment resulted in
increased foliar uptake of the herbicide, or rapid
stem regrowth following shredding contributed to
greater root uptake of atrazine. Applied to regrowth,
atrazine was as effective as 2,4-D or dicamba but was
not as effective as picloram.

In combination with shredding, picloram
applied at 1.12 kilograms per hectare in 1975 com-

Table 8. Percent reduction of spiny aster stem density 7, 12, and 36
months after foliar applications of selected herbicides and com-
binations of herbicides in March 1975 and 1976

Treatment Months after treatment“
Rateb 7 12 3e
Herbicide(s) (kg/ha)
None 0 a 0 a 0 a
2,4-D 0.56 10 ab 7 a 5 a
2,4-D 1.12 22 ab 20 ab 5 a
2,4-D 2.24 31 b 25 ab 9 a
Dicamba 0.56 6 a 10 a 4 a
Dicamba -- 1.12 24 ab 24 ab 3 a
Dicamba ‘ 2.24 34 b 25 ab 6 a
Picloram 0.56 72 cd 24 ab 14 a
Picloram 1.12 86 d 39 b 21 ab
Picloram 2.24 95 d 77 c 63 c
2,4-D + Dicamba 1.12 21 ab 28 ab 7 a
2,4-D + Dicamba 2.24 50 bc 31 ab 10 a
2,4-D + Picloram 1.12 70 cd 19 ab 12 a
2,4-D + Picloram 2.24 91 d 33 b 30 b

“Means within a column followed by the same letter do not differ
significantly at the 95-percent confidence level using Duncan's
multiple range test.

“Rates shown are total herbicide applied; herbicides were com-
bined in equal ratios.

Table 9. Percent reduction of spiny aster stem density 9 months
after foliar applications of selected herbicides to undisturbed
stands in March 1975 or to shred regrowth in lune 1975

Treatment Stem density reduction“
Rate Spray alone Shred + spray
Herbicide (kg/ha)
None 0 a 24 ab
2,4-D 1.12 22 ab 58 Cd
2,4-D 2.24 31 bc 78 def
Dicamba 1.12 19 ab 55 cd
Dicamba 2.24 ~ 34 bc 68 de
Atrazine 1.12 7 ab 54 cd
Atrazine 2.24 4 ab 68 de
Glyphosate 1.12 24 ab 63 d
Glyphosate 2.24 62 d 93 ef
Picloram 1.12 86 def 100 f

“Means followed by the same letter do not differ significantly at
the 95-percent level using Duncan's multiple range test.

Table 10. Percent reduction of spiny aster stem density 1 and 2 years after foliar applications of selected herbicides in June 1976 to un-
disturbed stands or to shred regrowth in an area previously seeded to kleingrass

Treatment Years after treatment“
Rate 1 2

Herbicide (kg/ha) Spray alone Shred + spray Spray alone Shred + spray

None 0 a 58 cd 0 a 43 c

2,4-D 1.12 36 bc 80 e 12 a 69 ef

2,4-D 2.24 3S b 77 e 7 a 71 ef

Glyphosate i 1.12 33 b e6 de 5 a as def

Glyphosate 2.24 54 cd 67 de 19 ab 37 bc

Picloram 0.56 28 b 64 de 1 a 67 ef

Picloram 1.12 27 b 76 e 0 a 79 f
“Paraquat 1.12 31 b 63 de 9 a 46 cd

/.'Paraquat 2.24 42 bc 63 de 20 ab 57 cde

“Means within a time after treatment subheading (1 year or 2 years) followed by the same letter do not differ significantly at the 95-percent

confidence level using Duncan's multiple range test.

13

7' " a;

pletely eliminated spiny aster by the first evaluation
(Table 9). Picloram was not as effective in 1976 (Table
10), whether applied with or without pretreatment
on the area seeded to kleingrass. Subsequent evalu-
ations of plots shredded and sprayed in the dense,
grassless spiny aster stand indicated rapid reestab-
lishment of the weed. Three years after treatment,
stem density reductions averaged 38 percent in plots
shredded and sprayed with 1.12 kilograms per hec-
tare of picloram and 39 percent in plots shredded
and sprayed with 2.24 kilograms per hectare of
glyphosate (data not shown). Live stem densities in
plots receiving other treatments in 1975 did not dif-
fer from those in untreated plots 3 years after her-
bicide application.

Spiny aster control remained high in plots
shredded and sprayed where kleingrass was estab-
lished, however. After 2 years, all herbicides applied
to regrowth maintained significantly greater stem
reductions than shredding alone except for the
higher rate of glyphosate and plots treated with
paraquat (Table 10). Picloram or 2,4-D, applied to
regrowth in 1976, removed 70 to 80 percent of the
spiny aster stems after 2 years. Applied to plots
without mechanical pretreatment or grass seeding,
no herbicidal treatment significantly reduced the
density of live spiny aster stems for 2 years.

Natural Revegetation Following Control

Revegetation following effective control of
spiny aster had not occurred by the end of the sec-
ond growing season after spraying (Figure 10), ex-
cept where kleingrass was already established. The
herbaceous plants that invaded treated plots during

14

the third and subsequent years were barnyardgrass,
bushy bluestem [Andropogon glomeratus (Walt.)
B.S.P.], common bermudagrass [Cynodon dactylo

(L.) Pers.], and diamond-leaf frogfruit [Phylaw ‘
strigulosa (Mart. & Gal.) Moldenke]. Substantial for-

age production occurred on only a single nonshred-

ded plot. lt had been treated with 2.24 kilograms per
hectare picloram in March 1975. The equivalent of
4215 kilograms per hectar foven-dry standing for-

age occurred on the plot 4 months after treatment.

Several conditions may have been responsible
for the general lack of natural revegetation of de-
nuded plots. The complete dominance of a site for
prolonged periods by spiny aster may eliminate the
seed sources required for establishment of forage
plants or other weeds. Dead stems from the dense
spiny aster growth may provide a physical or chemi-
cal hindrance to seed germination of other species.
Depth of the weed stem litter on bare plots was
about 2 centimeters, and decomposition was slow,
requiring about 2 years.

Herbicide residues may be suspected of sup-
pressing seedling establishment, but plots treated
with the same herbicides and rates in several other
studies revegetated naturally within a few months.
Mutz et al. (18) reported excellent forage responses
after treatment of spiny aster with pelleted picloram
if the weed infestation had not eliminated the re-
sidual forage seed source. However, in areas where
competition from dense stands of spiny aster had
eliminated the forage plants, control did not give an 4
equally favorable forage response. Artificial revege-
tation by seeding with adapted grasses will probably

Figure 10. Spiny aster plot
sprayed with 1.12 kilo-
grams per hectare piclo-

ram in lune 1975, 74 days

after shredding, in Cal-
houn County, Texas
(photographed in summer

1976). Note the heavy ac- s
cumulation of dead stems 
on the soil surface and the
absence‘ of grasses and
forbs.

be required on such sites where spiny aster is suc-
cessfully controlled.

CONCLUSIONS

Clonal increase by rhizome growth is apparently
the primary method of spiny aster reproduction.
Seed viability appears to be extremely low under
standard laboratory seed testing conditions. How-
ever, spiny aster seed may have rather unique ger-
mination requirements. /

Shredding does not effectively controlgspiny as-
ter. Vigorous resprouting by rhizomes quickly re-
places the canopy after either mechanical or ineffec-
tive chemical top removal. Control of spiny aster was
not obtained with fall applications of several her-
bicides and mixtures. Susceptibility to herbicides is
probably lowered by physiological inactivity during
the fall. Applications of 2,4-D, dicamba, atrazine, or
glyphosate at 1.12 or 2.24 kilograms per hectare dur-
ing the brief spring period when leaves are present
are more successful than in the fall, but control
levels are not satisfactory. Picloram applied during

the spring gives temporary control of spiny aster, _

but 2.24 kilograms per hectare are required.

Shredding spiny aster during early spring prior
to spraying greatly increases the effectiveness of
herbicides. Picloram or glyphosate applied at 1.12 or
2.24 kilograms per hectare, respectively, to regrowth
effectively controlled spiny aster for a single year,
but revegetation by other species rarely occurred.
Density of live stems was reduced 70 to 80 percent
during the second year after treatment where com-
binations of shredding and spraying were applied to
a mixture of spiny aster and kleingrass. Apparently,
the competitive influence of an established, peren-
nial grass contributes to the success of the weed
control treatments. However, even with mechanical
pretreatment, most broadcast sprays did not effec-
tively control spiny aster.

The effectiveness of hormone-like herbicides is
limited by poor herbicide uptake by spiny aster top-
growth. Herbicide translocation to rhizomes, re-
quired for control of spiny aster, could not be dem-
onstrated in the field. Although epicuticular waxes
or other barriers may be important, the relatively
low accumulations of 2,4-D and picloram by foliar
organs in the laboratory are not adequately ex-
plained by cuticle thickness. Chemical characteris-
tics of the epidermal covering may be more respon-
sible for high resistance to herbicide penetration
than thickness (19).

Comparatively low stomatal densities on leaves
may limit herbicide uptake from aqueous solutions.
However, high densities of stomata with relatively
large apertures do not substantially enhance her-
bicide uptake by fine terminal stems. As a mechani-
.al pretreatment, shredding slightly decreases resis-
tance to herbicide penetration of the terminal stems.
Furthermore, shredding removes older living stems
and standing dead shoots without reducing den-

sities of current-year stems. This contributes to im-
proved herbicide effectiveness by providing a
greater proportion of aerial plant surface which is
more easily penetrated. Since about half of the un-
disturbed spiny aster canopy consists of dead stems,
herbicide interception by living tissues is substan-
tially reduced in undisturbed stands.

The stimulation of stem initiation and rapid
elongation which results from top removal may
promote root uptake of herbicides. In view of the
latter and the inherent resistance of spiny aster top-
growth to herbicide penetration, further efforts to
develop methods for control of spiny aster should
stress the evaluation of soil-applied herbicides (18).
Greater spiny aster responses to picloram may be
attributed to the relatively high soil activity of this
herbicide (9), even though the clay and organic mat-
ter content of the soils which generally support
dense stands of spiny aster have high herbicide-
binding potential.

LITERATURE CITED

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2. Anonymous. 1973. Texas almanac and state industrial guide.
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3. Baur, J. R., R. W. Bovey, R. D. Baker, and Imogene Riley.
1971. Absorption and penetration of picloram and 2,4,5-T into
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6. Davis, F. S., R. W. Bovey, and M. G. Merkle. 1968. The role of
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7. Davis, F. S., M. G.’ Merkle, and R. W. Bovey. 1968. Effect of
moisture stress on the absorption and transport of herbicides
in woody plants. Bot. Gaz. 129:183-189.

8. Esau, Katherine. 1960. Anatomy of seed plants. John Wiley
and Sons, New York, London, and Sydney. 376 pp.

9. Greenham, C. G. 1973. Investigations on skeleton weed
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13:243-253.

10. Hagin, R. D., and D. L. Linscott. 1965. Determination of
4-(2,4-dichlorophenoxy)-butyric acid (2,4-DB) and 2,4-
dichlorophenoxyacetic acid (2,4-D) in forage plants. J. Agr.
Food Chem. 13:123-125.

11. Johanson, D. A. 1940. Plant microtechnique. McGraw-Hill
Book Co., New York and London. 523 pp.

12. Ketchersid, M. L., O. H. Fletchall, P. W. Santelmann, and
M. G. Merkle. 1970. Residues in sorghum treated with the
isocotyl ester of 2,4-D. Pestic. Monit. J. 4:111-113.

13. Martens, P. 1934. Cellule 43:289. In J. T. Martin and B. E.
Juniper. 1970. The cuticle of plants.‘ R. & R. Clark, Edinburgh.
p. 100.

14. McCall, H. G., C. J. Scifres, and M. G. Merkle. 1974. Influ-
ence of foam adjuvants in activity of selected herbicides.
Weed Sci. 22:384-388.

15

15. McCarty, M. K., and C. J. Scifres. 1967. Western ironweed:
Research on anatomy, physiology, life history, and control.
Nebraska Agr. Exp. Sta. Res. Bull. 231. 36 pp.

16. Merkle, M. (1., R. W. Bovey, and R. Hall. 1966. The determi-
nation of picloram residues in soil using gas chromatography.
Weeds 14:161-164.

17. Meyer, R. E., H. L. Morton, R. H. Haas, E. D. Robinson, and
T. E. Riley. 1971. Morphology and anatomy of honey mes-
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18. Mutz, J. L., C. J. Scifres, W. C. Mohr, and D. L. Drawe. 1979.
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19. Norris, R. F. 1974. Penetration of 2,4-D in relation to cuticle
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20. Que Hee, S. S., and R. G. Sutherland. 1973. Penetration of
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21. Richardson, R. G. 1975. Foliar penetration and translocation
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22. Sass, J. E. 1958. Botanical microtechnique. lowa State College
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23. Scifres, C. J., J. R. Baur, and R. W. Bovey. 1973. Absorption of
2,4,5-T in various carriers to honey mesquite. Weed Sci.
21:94-96.

24. Sinclair, C. B., and D. B. Dunn. 1961. Surface printing of plant
leaves for phyllogenetic studies. Stain Technol. 35:299-304.

25. Steel, R. C. D., and J. H. Torrie. 1960. Principles and proce-
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15:168-170.

ACKNOWLEDGMENTS

This study is part of a dissertation submitted by
the senior author to the Department of Range Sci-
ence, Texas A&M University. The authors extend
special thanks to P. H. Welder and Tommy Phillips
of the Greenlake Ranch for their active interest and
cooperation. Many former students and employees
of the university kindly assisted with field work and
lab analyses. Jim Mutz and Andy Crane were espe-
cially helpful. Shirlee Meola, USDA, SEA-AR
Entomology Laboratory, College Station, provided
the scanning electron micrographs. Morris Merkle
made available the gas-liquid chromatograph, and
Mary Ketchersid contributed her expertise in her-
bicide extraction and analysis. The manuscript was
typed by Brenda Lytle, USDA, SEA-AR, Temple, TX.

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