Managing Perennial Weeds in Food Plots
W. Carroll Johnson, III
Tifton, GA
wcjohnsoniii@mchsi.com
Worldwide, perennial weeds are among the most troublesome and costly weeds of agriculture. In the book “The World’s Worst Weeds – Distribution and Biology”, eight of the top eighteen weeds in the world are perennials. Closer to home, troublesome weeds in each state are identified by Cooperative Extension Service specialists and many of these are perennial weeds. Common examples of perennial weeds are shown in Figure 1. Perennial weeds earn this dubious distinction by being difficult and costly to control. This is certainly the case in food plots. Research has shown that successful perennial weed management systems are an integration of periodic tillage and systemic herbicides.
Tillage. Perennial weeds are difficult to control due to vegetative structures that allow the weed to survive during dormancy. These vegetative structures include rhizomes, stolons, and tubers (Figure 2). Perennial weeds tend to thrive in reduced-tillage cropping systems where the absence of periodic tillage allows the vegetative structures to become more robust, ensuring their survival to the next season. Successful control of perennial weeds in food plots requires frequent and intense tillage that systematically weakens and depletes the perenniating vegetative structures.
A convenient opportunity for intense tillage will occur before food plot establishment, between plantings of annual forages, or when perennial plantings have petered out. Regular and repeated tillage over a period of several months weakens perennial weeds by cutting the vegetative structures into small pieces, disrupting their ability to re-establish. The results are progressively depleted carbohydrate reserves and desiccated vegetative structures.
The tillage implements of choice are a disk harrow, power-tiller, and moldboard plow. The harrow and power-tiller are the best implements to cut the vegetative structures of perennial weeds into small pieces, while the moldboard plow is the best implement to bury the vegetative structures. Tillage implements should be operated in a manner that achieves the greatest degree of aggressive soil disturbance. Thus, tractor operated implements tend to perform better than ATV operated implements.
Systemic herbicides. Systemic herbicides are readily absorbed by weed foliage and translocated throughout the weed in the vascular system and accumulate in the underground vegetative structures of perennial weeds. The systemic herbicide of choice for perennial weed control is glyphosate (Roundup® and generic brands). A high rate of glyphosate is needed for successful perennial weed control, typically a 2% solution of glyphosate concentrate in water (one gallon glyphosate in 50 gallons water). By all accounts, this is a costly herbicide application, but necessary for effective control of perennial weeds. Glyphosate applied at lower rates may be effective on annual weeds, but ineffective on perennials.
Another factor critical to successful control of perennial weeds using glyphosate is application timing. The glyphosate label has an extensive list of perennial weeds controlled and detailed information on when to apply glyphosate. In general terms, perennial weeds readily translocate photosynthates from leaves downward to the underground vegetative structures in the autumn in preparation for winter dormancy. As these photosynthates move downward, glyphosate also moves downward in the vascular system. Applying glyphosate to perennial weeds at other times of the year results in inferior control since glyphosate is not readily translocated downward in the plant at those times during the growing season.
Another method to apply glyphosate to control tall perennial weeds is using a wiper-apparatus, commonly called a wick-bar. A wick-bar wipes a super-concentrated solution of glyphosate on weeds substantially taller than the low-growing forage. This technique was originally devised to control tall johnsongrass in cotton and was widely used before selective postemergence herbicides were developed. Glyphosate applied with a wick-bar controls most tall weeds in forage plantings. Using a wick-bar to apply glyphosate has been discussed in detail in earlier articles and you can read about wickbars at www.whitetailinstitute.com/info/news/pastnews/.
Postemergence graminicides (Arrest®) provide selective control of perennial grasses. When Arrest® is used to selectively control perennial grasses, the level of control is generally less than what would be reasonably expected with glyphosate. However, the advantage of Arrest® is selectivity – no injury on legume forages. Arrest® should be applied at 2.25 to 3.75 pt./A to perennial grasses from six to 12 inches tall, with applications repeated if regrowth occurs. This regime should be repeated the following season to control survivors or those emerging from seed.
Throughout this discussion on managing perennial weeds, it should be clear there are no simple solutions. An integrated system of fallow tillage and herbicides is the key. Honestly, while densities of perennial weeds can be significantly reduced or suppressed, they will not completely disappear. Perennial weeds are too tough and control measures are too limited to expect complete control – regardless of the setting. I live in southern Georgia and my lawn is centipedegrass. Weed control in centipedegrass is fairly simple, given the diverse array of herbicides available. However, I have a sporadic infestation of a virginia buttonweed – a creeping perennial broadleaf weed. I have tried about every broadleaf herbicide available and I cannot make any noticeable progress in eliminating the weed from my lawn. I now realize that the best I can do is to keep the weed contained to one part of my yard and there will always be a few survivors. In other words, virginia buttonweed is a weed I manage – not fully control. That is the case for many of the perennial weeds found in food plots. You do the best you can and accept a few survivors.