It’s the weed southern farmers already hate, and Midwest farmers are beginning to hate. It can grow an inch or more a day. Pull it out of the ground, and the roots can make a 90-degree beeline back to the soil to re-root. Each weed can deliver nearly a half-million seeds for the next generation of trouble. It costs Georgia cotton farmers alone more than $110 million each year.
This weed has a name, and it’s Palmer amaranth.
The Weed Science Society of America (WSSA) just published its most recent findings on Palmer amaranth, a study in which researchers in Georgia planted the weed in cotton fields between 0 and 12 weeks after the crop.
The study found that as Palmer amaranth and cotton seedlings competed for sunlight, water and nutrients, the weeds established seeds with widely varying success rates. For example, earliest plantings of Palmer Amaranth produced 446,000 seeds per plant. But if weeds were delayed until 9 to 12 weeks into the growing season, the amount of weed seeds drops dramatically, by 89% to 99% respectively.
Cotton yields saw the same large swings. When Palmer amaranth and cotton grew up together, cotton yields took a 67% yield penalty. But by delaying the establishment of weeds until 6 weeks after planting, the study showed yield losses below 30%.
“These results show that early season weed control programs that successfully delay Palmer amaranth establishment can have a large effect on crop yield, weed growth and weed seed production,” the WSSA concludes. “In addition, reducing the soil seedbank is a vital preemptive measure against the next season’s weeds.”
Control of Palmer amaranth should not be less than 100%, according to University of Illinois weed specialist Aaron Hager.
"In other words, the threshold for this invasive and extremely competitive species is zero," the researcher says. "Female Palmer amaranth plants produce tremendous amounts of seed, and in less than five years a few surviving plants can produce enough seed to completely shift the weed spectrum in any particular field."
In this AgWeb article, Hager shares three management options for controlling this noxious weed. Are you dealing with pigweed? Share your management tips to other farmers on the AgWeb discussion boards.