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Velvetleaf: Adaptation & Stress

 

Velvetleaf is a very competitive, well adapted weed in agroecosystems. There are many reasons for this: seed bank longevity, adaptation to physical stress, disease resistance, growth plasticity, competitive ability in many habitats, and resistance to herbicides.



Seed bank longevity.

The seedlings in the picture below are emerging in a refuse dump. This field in adjacent to Lake Erie in Michigan, next to a carp pond. Dead carp are dumped and sit rotting in the sun as velvetleaf exploits the opportunity next to a dead carp fish head:

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Stress adaptation.

The same plant with a broken stem is seen in the pictures below, taken at the same time. On the left you can see the stem has been broken almost completely off. On the right is the branch laying on the ground, but with its leaves doing fine and showing no signs of wilting. Enough vascular strands (very few) are still connected to keep the leaves growing.

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Disease resistance.

Velvetleaf will frequently show signs of infection by verticillium wilt, a disease it seems to survive and thrive with. The plant below is infected with this disease, yet it will still survive and produce seed:

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Plastic growth.

Velvetleaf will adapt its size and productivity to the resources it gets at a particular place.
63t.JPG (10634 bytes) On the left is a plant I cut all the leaves off to reveal the extensive branching and seed capsule formation. This plant was grown with no neighbors; as a consequence it is relatively short and had many leaves and capsules because it had lots of resources.
72t.JPG (12265 bytes) The picture to the left shows my old friend Matt standing next to a pure stand of very tightly packed plants, they tower over his head. Because they lots of resources, but were close together, they grew very tall. They get tall by because when velvetleaf gets shaded, it responds by increasing internode length (the distance between branches) to get taller than its neighbors. This is one of its best competitive traits for success over crops like soybeans.
62t.JPG (9523 bytes) This little velvetleaf plant has only a few leaves and capsules. It is growing in the gravel immediately next to a concrete roadway, not a very favorable or resource-rich habitat. The county road crews would mow along the roadside and cut it down. Despite this adversity it managed to put a few capsules out late in the season and continue the species for another year (or maybe 100). Many feel that managing velvetleaf means never allowing one plant on a farm to set seed. It is difficult to disagree with this strategy to velvetleaf management.


Competitive ability.

Velvetleaf competes in many habitats, agricultural as well as others.
70t.JPG (12614 bytes) Corn grows fast and early in the season, making it much better in competition with velvetleaf (left), but the weed still does well.
81t.JPG (13893 bytes) Notice the large flowering velvetleaf plant in front of the picture window of this house. I guess one adaptation velvetleaf has is the ability to fool urban homeowners that it is an ornamental plant. Hey, whatever it takes.....


Herbicide resistance.

Velvetleaf is susceptible to many herbicides, but it also has some well-expressed herbicide detoxification systems. Below are a few survivors, all which probably set seed later:
1252t.JPG (9757 bytes) This plant (left) was just a little to big to kill with glyphosate, although the leaves on the left are yellow, chlorotic from inhibition of the EPSP synthase in its leaves.
1375t.JPG (11676 bytes) The plant on the left was also just a little to big to kill with 2,4-D, although its stems and leaves are twisted and curved (epinasty) due to differential growth subsequent to blockage of the phloem in its stems.
1506t.JPG (9373 bytes) The leaves of this plant are necrotic and the leaf edges dried and curled up due to a dose of bentazon. a photosynthesis inhibitor. It too will probably survive because the treatment came too late in the plants development to kill it.

©jdekker-1998

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