Tolerance of Herbaceous Perennials to Preemergence Herbicides
Russell Smith, Ted Whitwell, and William
Department of Horticulture, Clemson University
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Herbaceous perennial production is increasing both in total plants produced
and in number of new species grown. In the past several years this trend
has steadily increased as more consumers demand long-lived color in the
landscape. Weed control caused a major production problem because there
are few labeled herbicides for herbaceous perennials particularly newly
potted plants and newly introduced species.
A particularly toublesome seed in South Carolina is Hairy Bittercress
). Bittercress is a major weed problem in the
production of greenhouse and nursery crops, particularly in the fall through
spring seasons. One bittercress plant can forcefully expel a seed to a distance
of 42 inches, and a single plant can produce up to 5000 offspring carpeting
a circle 6 feet in diameter within 5 weeks. While well suited for winter
growth in South Carolina, bittercress especially enjoys the winter protection
afforded by Remay, a fabric commonly used for winter protection of many
herbaceous perennials. Remay traps solar heat during the day and helps hold
heat in during the night. When Remay is removed in the spring, bittercress
has completly covered the pot.
To address this weed problem, we initiated a research project at Stacy's
Greenhouse in York, SC. There were two objectives to the study. The first
was to determine the phytotoxicity of granular preemergence herbicides to
herbaceous perennials. The second was to determine effectiveness of granular
preemergence herbicides on hairy bittercress. The primary goal was to find
an herbicide(s), which could be applied before plants are covered with Remay,
that effectively controls bittercress through the spring with no or minimum
injury on several species of perennials.
Materials and Methods
This study was conducted on outside production beds at Stacy's Greenhouse,
York SC. Twelve species of herbaceous perennials were evaluated for tolerance
to four preemergence herbicides and bittercress control was also determined.
Species in the study were: Achillea millefolium
'Red Beauty,' Coreopsis
'Moonbeam,' Dianthus gratianapolitanus
Dianthus x allwoodii
'Doris,' Gaillardia grandiflora
, Phlox subulata
Blue,' Salvia x superba
'Blue Queen,' Sedum
'Rosea,' Verbena canadensis
plugs or crowns were potted up in the fall and summer in 1 gal pots with
a 85% pine bark and 15% sand media.
Four herbicides were applied at two rates each along with an untreated
control. The herbicide treatments were Southern Weedgrass Control (pendimethalin)
2 & 4 lb ai/A, Snapshot TG (isoxaben + trifluralin) 2 & 4 lb ai/A,
Stakeout (dithiopyrl) 1 & 2 lb ai/A, and Rout (oxyfluorfen + oryzalin)
1.5 & 3 lb ai/A.
Plants were evaluated for herbicide injury (phytotoxicity) every week
for the first six weeks on a scale from 0% to 100% where no visual injury
= 0% and plant death is 100% injury. After the first sex weeks, evaluation
of the plants went to alternate weeks. During this time plants were dormant
and very little information could be obtained. In early spring (March 10,
121 Days After Treatment), evaluation returned to every week. Plants were
examined for phytotoxicity, delay in green-up, and stunted growth.
Bittercress was seeded in two species, Achillea
on November 9. Bittercress seeds was mixed with sand so that one tablespoon
of sand contained about 20 viable bittercress seeds and one tablespoon of
sand/seed mix was applied to each pot of Achillea
. Percent weed control were determined at each visit by ranking effectiveness
of control on a scale of 0% to 100% compared to the untreated check. To
determine the length of weed control exhibited by each herbicide, and additional
two species were seeded with bittercress. On March 17 (129 DAT), bittercress
seeds were added to Iberis
Results and Discussion
On the first evaluation (20 DAT-Nov. 29), Hosta treated with Rout began
to show injury symptoms. This injury was expected, since the product's label
urged precaution when used on Hosta. Hosta and similar plants will channel
the granules toward the leaf base, resulting in basal burn of the foliage.
This basal injury caused a wilting of the foliage. All of the Hosta went
dormant by the December 15 evaluation (36 DAT). Hosta began to emerge uniformly
in the spring (129 DAT-March 17). All new growth was healthy and free of
Rout injury was also observed in Plox subulata
on the December 15 evaluation (36 DAT). These symptoms appeared as necrotic
foliage in the center portion of the plant. This necrotic area grew progressively
larger at each of the later evaluations. On January 3 (55 DAT), cold damage
was observed on all of the Phlox. The cold damage and herbicide injury symptoms
were similar, however, the plants treated with Rout had more injury. The
spring growth of Phlox masked the herbicide damage and by March 24 (136
DAT) the injury was barely observable.
In early spring (129 DAT-March 17), Salvia
Queen' began to show herbicide injury from Snapshot TG and Stakeout. This
injury appeared as plant stunting, and floral bud delay. Rout injury was
also evident on this species by March 24 (136 DAT). Injury was distinctly
greater for all three herbicides at the higher rates. Salvia began flowering
on April 21 (164 DAT) in the control and low rate of Rout, but plants treated
with the high rate of Rout, both rates of Snapshot TG, and both rates of
Stakeout were considerably shorter and showed few to no flowers. Southern
Weedgrass Control was the only herbicide that did not injure Salvia.
Stakeout injured Sedum 'Blue Spruce' in the early spring (129 DAT-March
17, evaluation) by stunting and reddening the plant foliage. The damage
was quite severe, and grew progressively worse at later evaluations. Sedum
was not injured by the other herbicides.
Rout, Snapshot TG, and Stakeout controlled bittercress. However, bittercress
was not effectively controlled with Southern Weedgrass Control herbicide
at the rates used.
Bittercress control is presented in Table
. The control illustrated from the evaluation on February 24 (108
DAT) reflects the bittercress seed planted into the pots on November 9,
and the data on April 21 (164 DAT), represents the control from the March
17 (129 DAT) bittercress sowing. Southern Weedgrass Control gave only about
40-60% control. This is inadequate, since with only 50% control, many seeds
will still be produced and germinate under Remay. Rout, Snapshot TG, and
Stakeout provided excellent bittercress control.
Southern Weedgrass Control herbicide did not injure any of the twelve
species, however, it was relatively ineffective in controlling bittercress.
While Southern Weedgrass Control was not effective on bittercress it could
have great potential in future work with the control of other weeds in herbaceous
perennials. Rout, Stakeout, and Snapshot TG controlled bittercress, but
injured one or more perennials. From this study, no single herbicide was
identified that controlled bittercress without phytotoxicity on the desirable
plant. Therefore, prescription herbicide selection may be needed to effectively
control bittercress without injury to the crop. Different herbicides would
be used for different perennials, for example, one might use Stakeout on
Phlox and Hosta, but not on Sedum.
We are grateful to Mr. Louis Stacy and the staff at Stacy's Greenhouse
in York, S.C. for donating plant materials and all their assistance with
We are especially grateful to Louis Stacy for not getting too mad when
we told him we were deliberately seeding bittercress in his nursery.
Last Updated 2/1/97