An agronomy sales rep visiting a farm near Cochranville in southeastern Lancaster County recently spotted a suspicious-looking pigweed.
Thousands of the plants were growing in a soybean field.
The prolific plant turned out to be Palmer pigweed, a variety still rare in Pennsylvania, according to Bill Curran, a Penn State weed science professor.
Redroot and smooth pigweed have been common in the state for years, but Palmer pigweed, also known as Palmer amaranth, has so far mostly been restricted to Southern states.
The Cochranville infestation appeared in a planting of soybeans that had been double-cropped after wheat, Curran said. The pigweed appears to have come from contaminated manure because it was thick in some places and thin in others.
The manure came from a dairy farm that has used cotton seed in its feed, and cotton seed has been linked to the spread of Palmer pigweed in Michigan and Ohio, Curran said.
“It becomes pretty complicated” the more one looks for the source of a pigweed outbreak, he said. The Amish dairy farmer bought his hay from a broker, who got it from someone else.
Fortunately, the dairy farmer transported his manure only to the crop farmer who had the pigweed, so dealing with the plant will be straightforward.
The Cochranville outbreak is “relatively contained,” Curran said. “Now that we know where it is, we can kind of monitor it.”
Still, it makes Curran wonder whether more pigweed could be out there, unreported, somewhere else in Pennsylvania.
A field agronomist saw an unusual pigweed while driving by a Cumberland County soybean field earlier this month. He pulled over and destroyed the few plants in that field.
Curran said he received only pictures of those weeds, not live plants, but they look like they were Palmer pigweed.
“This is the time of year to recognize this kind of thing,” Curran said.
Pigweed varieties look almost identical as seedlings but become easier to distinguish when they get more mature in the fall.
“Managing it is going to be a whole different beast” at this time of year, Curran said. Farmers should be wary of driving a combine through a field with known pigweed to avoid spreading seeds.
Although Palmer pigweed has the usual traits that make weeds so obnoxious — preference for disturbed soil, swift emergence, competitiveness — it causes additional headaches because it is glyphosate-resistant.
Many pigweeds have been atrazine-resistant for a quarter-century, but glyphosate resistance is unique to Palmer at this point.
Like many weeds, Palmer pigweed produces a monstrous number of seeds that can spread easily because they are so tiny. Those seeds can also become dormant, holding off germination during a lean year until a more favorable time.
Palmer pigweed is unusual among pigweeds in that it can survive in many environments. The weed is originally from the Americas, but it probably came from points south such as the U.S. Southwest, Mexico or even South America, Curran said.
“I think we know that it can be successful here,” he said, but the question is whether it will be as destructive in the Northeast as it has been in states like Georgia.
A pigweed identification guide from Aaron Hager, a University of Illinois Extension weed specialist, indicates that Palmer pigweed can grow faster than other pigweeds, at 3 inches a day, and can cause catastrophic yield losses, nearing 80 percent in soybeans and topping 90 percent in corn.
Plants like Palmer pigweed outcompete many field crops, especially soybeans, because humans have bred the crops for food quality rather than aggressiveness, Curran said.
That is not to say that farmers are not trying to turn pigweed into an edible product.
Grain amaranth, a variety developed from wild pigweed, is growing in popularity as a human food source elsewhere in the world because it is a high-protein grain, though it is not well-known in the United States yet, he said.
Pennsylvania farmers are familiar with redroot and smooth pigweed, Curran said, but Palmer and spiny amaranth are moving in from the Southeast. Tumble pigweed has “been around,” too, Curran said.
Pigweeds have very plastic genetics, meaning they can adapt to stressors quickly, he said.
A mature Palmer pigweed can be distinguished, especially from the similar redroot pigweed, by the long, cylindrical seed head on top of the plant.
This flowering part of the plant can grow 8 to 20 inches long, much larger than other varieties’ flowers.
Palmer is also hairless, whereas redroot is covered in fine hairs, and Palmer has petioles at least twice as long as the leaves they connect to the stem. Other pigweed petioles are about as long as the leaves.
If a pigweed turns up in a field, “don’t freak out, because there’s lots of pigweed around,” Curran said.
A farmer should consider how his management practices may have allowed pigweed to survive. If the weeds tower over Roundup Ready soybeans, even after a glyphosate application, that should set off an alarm, Curran said.
Farmers should report abnormal-looking pigweed to Curran, their county Extension offices or their crop consultants.
“The main thing is to bring somebody on board to see if you really have a problem,” Curran said.