Hairy Bittercress Seed Production, Dispersal, and Control in Propagation Beds
Gary Bachman and Ted Whitwell
Department of Horticuluture, Clemson University
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(image: Hairy Bittercress seed pods) Bittercress (Cardamine hirsuta
) is a major southeastern weed
that germinates all year during propagation of landscape plants. Bittercresss
produces a great wuantitiy of seeds that are forcefully expelled. Plastic
containers are commonly reused during propagation with media residue from
previous crops clonging to the sides of the containers. The residue in used
containers could hold some of these seeds to germinate during the next crop
cycle. The objectives of this study were to (1) investigate seed number
and dispersal from hairy bittercress and (2) determine the effectiveness
of certain preemergence herbicides for bittercress control.
Materials and Methods
Thirty bittercress seedlings from a population occurring in a greenhouse
at Clemson, SC were transplanted into 4" square plastic containers.
After 10 days. eight plnats were selected for uniformity of growth. Data
were collected from these eight plants concerning silique development, seed
production, and seed dispersal (distance and pattern).
The second phase was conducted at Carolina Nurseries in Moncks Corner,
SC.Two inch square containers with media residue were collected. These containers
had a heavy hairy bittercress population during the previous propagation
cycle. Half of the containers were rinsed (clean) with a pressure nozzle
attached to a water hose and the other half the residue were not rinsed
(dirty). This resulted in two container treatments. Propagation media, 50%
peat moss and 50% perlite, was sampled at the outdoor storage area from
two spots, the top and bottom of the pile, for two media treatments. The
containers were placed into 18" sqaure trays holding 64 containers.
There were 16 replicates of each container/media combination per tray. The
trays were placed in the center of 4' x 4' square blocks on the gravel floor.
(image: Hairy Bittercress seed dispersal) Three uniform plants were selected from the bittercress that emerged
from the unwashed containers. Each plant was placed in the center of a five
tray (7.5') by five tray (7.5') block and allowed to seed. Each tray contained
thirty-six 3" square new uninfested plastic containers with Ilex
'Helleri' cuttings. Bittercress seedlings were counted on 21
Jan 94. The resulting counts were placed in a grid to determine the dispersal
) by distance from the mother plant. All
data was analyzed using analysis of variance and regression analysis. Twelve
preemergence herbicides at one rate was evaluated for control of bittercress.
Liners of Ligustrum lucidum
were potted into one gallon pots and
herbicides were applied using either a shaker can or a CO2
sprayer. Bittercress seed (~50) were seeded in the pots after herbicide
application and 30 days later. Evaluation of control were made at 30 days
and 60 days after application.
Results and Discussion
After five weeks, the average number of siliques per plant was 68, however
the nember of siliques per plant ranged from 27 to 182. The average number
of seeds per silique was 29. The total number of seeds ranged from 675 to
4980 per plant.
Hairy bittercress seeds are dispersed by a spring-like action of the
locule (sides) of the silique rolling back on both sides perpendicular to
each other. The seed are thrown to each side of the silique on the same
plane as the rolling locules. The average seed dispersal distance was 19.7".
Germination of bittercress seeds was 90% after 13 days. Germination began
after five days with no new germination after 13 days. Greatest germination
occurred between 6 aqnd 8 days after sowing. Presumably the other 10% of
the seeds were either not viable or more likely have a dormancy mechanism
for later germination.
Emergence of bittercress from unwashed containers int he study at Carolina
Nurseries correlated with previous results. All seedlings were within 1
cm of the edge of the container indicating the seed was in the media residue
clinging to the container. There were several seedlings that germinated
on the outside of the containers where media residue remained, but these
seedling did not survive. There was a difference between the number of weed seedlings
emerging from the dirty compared to clean (rinsed) containers. Location
of media collection revealed no difference in number of weed seedlings from
the media from different locations indicating that the new media was not
a significant source for hairy bittercress seed.
Seed dispersal was first observed 16 Dec with seedlings emergence beginning
4 Jan. Silique development, seed dispersal and seedling emergence continued
until the end of the study 21 Jan. There were 3799 and 5128 seedlings produced
from the two mother plants, respectively. Seedlings were counted up to 42"
away from the mother plant with the mean dispersal distance being 24".
Seedling density at the mean dispersal distance was 7.8 seedlings/in2
Overall, seedling density was 1.2 seedlings/in2
across the total
dispersal area of 5542 in 2
All preemergence herbicides (Table 1
except Southern Weedgrass Control and Kerb provided greater than 95% control
at 30 and 60 days after application.
Hairy bittercress has the potential to quickly become a problem in a
nursery. There were six times more bittercress seedlings in the unwashed
containeres compared to the washed containers. The problem is amplified
by not being able to use preemergence herbicides in propagation.
Significance to the industry
: Hairy bittercress is a prolific
seed producer and disperses them efficiently in the propagation areas of
nurseries. In trying to control the hairy bittercress, the cycle of seed
production must be interrupted. Once this is accomplished a program of hairy
bittercress prevention and sanitation should be followed.
Last Updated 7/16/98