Catch Palmer amaranth before seed production for true control
Palmer amaranth is a seed production bomb. Called pigweed, hogweed, devilweed or a dozen more colorful expletives, Palmer plays a cruel numbers game fueled by phenomenal seed production. Even in a field situation, competing for resources alongside crops, herbicide-resistant Palmer can easily churn out several hundred thousand seeds.
Resistant pigweed makes up for tiny seed size with incredible volume and germination window.
Palmer is an annual weed, typically emerging in early spring and dying in late fall. During its seven- to 10-month life span, Palmer can produce 500,000 to 1 million seeds. Smaller than a pinhead, the sheer number of seeds allows for a wide germination window. In some Southern states, it germinates every month except January and February.
Seed production depends on when Palmer emerges during the season. “It varies, but in the fall after emergence, you’re dealing with viable seed 28 days later,” says Jason Norsworthy, University of Arkansas weed specialist.
The tiny seed spreads by water, wildlife and wind, but travels best on the free ride offered by humans, adds Stanley Culpepper, University of Georgia Extension weed specialist.
Palmer seed piggybacks on cotton lint, crop residue, farm machinery and crop seed, particularly cotton and soybeans. At harvest, combines spread billions of seeds across farmland. It’s a devil’s bargain: Crops come in, Palmer seed scatters and farmers prepare for another fight the following spring.
“When glyphosate-resistant pigweed blew up, we went from not having pigweed on the roadsides to an abundance,” Norsworthy says. In the fall of 2013, Norsworthy’s research team sampled 550 roadside sites from the Missouri Bootheel to the Arkansas-Louisiana line. They found 393 randomly chosen spots infested with Palmer—and 95% of those locations were resistant to glyphosate.
Ten years ago, 15 states had Palmer presence. As of 2015, 28 states have confirmed glyphosate-resistant Palmer. Although the majority of seed quickly loses viability, numbers are deceiving.
“We think 99% of Palmer seed is nonviable within four to five years, but when a plant is pumping out nearly
1 million seeds, that 1% fraction goes a long way,” Norsworthy explains. “There will still be resistant seeds in the soil bank. One or two escapes mean the soil seed bank is replenished and the process begins again.”
In essence, Palmer math equates to persistence. “Even though the seed is tiny and arguably vulnerable to weather and other factors, that weakness is overcome with volume,” Culpepper says. “Waterhemp is a big challenger, but nothing matches Palmer seed numbers.”
A little goes a long way in dealing with Palmer seed, Norsworthy emphasizes. In 2007, he placed 20,000 seeds, a tiny amount for Palmer, on four Roundup Ready cotton fields (two to three acres in size). Three years later, he’d completely lost the cotton crop, which relied solely on glyphosate because it was taken over by pigweed corner-to-corner.
Even old-school hand chopping is a pointless weapon against Palmer seed. Chopping can end seed production, but it doesn’t address the seed bank, Norsworthy explains.
“I’ve seen chopping where pigweed is left behind to shatter or is pulled up prior to irrigation or rain and it actually re-roots,” he says. “At least get the weeds out of the field, but even then, the real concern is to catch pigweed before seed production. Long-term success in the Palmer fight is about driving down the soil seed bank.”
Culpepper says hand pulling and removing pigweed from fields greatly reduces the seed bank with visual and mathematical benefits. “We’ve experienced reduced populations from hand weeding along with a good weed management program. Hand weeding is one of the keys to our success.”