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• North Carolina State Weed Scientist Wes Everman says a good first step for farmers in dealing with Palmer pigweed is to be able to accurately identify it at the seedling stage and to differentiate it from smooth and redroot pigweed species.
PIGWEED NUMBERS after grain and cotton harvest can cause big problems for next year’s crop.
Second, pull up a suspect pigweed at ground level. Break the petiole at the stem and fold it back on itself at the base of the leaf. If it’s Palmer amaranth, the petiole will be the length of the leaf or longer. Petiole length of Palmer pigweed is significantly longer than other pigweed species.
Finally, on suspect pigweed look carefully at the top of the leaf for a notch. Then, look even closer at the notch to determine whether there is a hair in the notch. Redroot and smooth pigweed will not have the hair in the notch on top of the leaf.
Once it is determined the weeds are Palmer amaranth, then the fun begins — how to get rid of it. Pulling Palmer pigweed isn’t easy. “I’ve seen the root system of a mature Palmer pigweed run across three or four rows of cotton or soybeans,” he says.
It’s real easy to pull up a Palmer pigweed plant and snap the plant off at root level and think you have solved the problem — at least for that weed. However, many times the weed just sprouts new branches and keeps on growing.
Palmer pigweed puts on seed until first frost and can be a problem most of the year in the Southeast.
Everman says growers should be diligent all the time in looking for pigweed, especially those that may have developed resistance to glyphosate or any number of ALS-based herbicides used to control them.
If a grower is using any of the PPO herbicides for postemergence control, they must be very timely in application. Otherwise, when Palmer pigweed gets more than four inches or so tall, control becomes ‘iffy’ at best and can be significantly influenced — in a bad way — by weather.
Most growers know how to control weeds and once pigweed, or most weed species get to a mature size, growers can easily recognize them and manage them.
Pigweed are a little different because of this blend of resilience and their prolific reproductive capabilities.
“I’ve sprayed plots with a backpack sprayer and with the spray going out in front of me, I’ve left a footprint in the soil. I can come back some time later and see where pigweed have germinated around the edge of footprint,” the North Carolina State researcher says.
“As growers head into some down time after harvesting their summer crops, it is an easy time to get complacent on weed control, but that can be a big mistake with Palmer pigweed,” Everman says.