A perfect pest — Palmer Amaranth, a new threat to Pennsylvania agriculture

CM-MR-3-Palmerby Steve Wagner

Woody Sigmin of Memphis, TN is very vocal about Palmer Amaranth, a species of pigweed recently introduced to Pennsylvania agriculture.

“I’m from the Mississippi delta, and believe me, you do not want this plant on your farm!” he said while riding in a canopied wagon with three dozen attendees during the 2014 Farming for Success seminar at Penn State’s research station in Lancaster County, Pennsylvania.

Pigweed is not grown in these trial fields, but samples have been brought in. “It’s unbelievable. It is horrible, it is terrible,” Sigmin said. “We have to rotate completely different crops just trying to clean out that one weed.”

In the southern parts of Mississippi and Arkansas, they have clocked three inches of growth per day, added Penn State weed specialist Dwight Lingenfelter. It is less ambitious in the Keystone State, largely due to density and frequency of humidity, far different from the Deep South. Perhaps it will grow only an inch a day in Pennsylvania.

Lingenfelter broke down the pigweed varieties: redroot/smooth, tumble, spiny, powell, palmer and waterhemp. “Redroot,” he said, “is commonly found and has lots of hairs on the stem of the plant. Tumble has long narrow leaves and more of a bright green appearance as it gets older. Its seedlings are dark green but become a brighter green with maturity.” Picking up the smallest plant in the smallest container, Lingenfelter identified it as waterhemp, earmarked by long leaves and a smooth stem. “The last one we see primarily in pasture situations. If you look closely at it, you will see sharp spines within the nodes of the plant. Obviously, this is spiny amaranth.” Whereas many farmers view the pigweed threat as something akin to children’s movie Transformer-type terrors, a handout speaks in more conservative terms, saying they pose “unique management challenges.” But there’s more to the problem.

Containing new infestations and preventing its spread is a critical first step to managing this threat, the literature tells us. “The risk from this new weed comes from its competitive growth habit, prolific seed production (greater than 100,000 seeds per plant) along with its potential resistance to Roundup and other glyphosates and the Group 2 herbicides (ALS-inhibitors).” Lingenfelter referenced the Palmer as being roughly six feet tall, but said it could rise to eight feet and possibly higher. “It almost has a poinsettia appearance to it,” he notes, “based on its leaf architecture.”

Weed watchers to the south and west of Lancaster and Pennsylvania suggest this plant can hybridize with other pigweed species, primarily waterhemp. Scientists in Iowa and Illinois additionally posit that it is difficult to distinguish the two different species in the wake of such hybridization. The University of Illinois is working on a DNA test to determine whether the species is waterhemp or palmer. Others say it doesn’t matter because they will cross-pollinate anyway. If that wasn’t enough, pigweed adapts very well to drought and resists attempts to pull it out of the ground. So far, it seems to have racked up all the obnoxious traits of the perfect weed pest. It tolerates both tillage and no-till and over the past five years has spread into Delmarva. In Pennsylvania, it has been found in the southeast counties this year, and last year in non-crop areas, field edges and in soybeans.

Pollen from male plants can travel on the wind to susceptible female plants, “and if the male is herbicide-resistant, a portion of the offspring will also be resistant,” according to a Palmer Amaranth handout. Seed predation is a whole topic unto itself, but it is clear that is a major factor. Because this weed is an aggressor, it must be aggressively managed. If you find it in your crops, report it at once to a Penn State Extension office. This sets the battle wheels in motion and prevents you from having to cope and/or guess at what measures might or might not work in tackling the problem. “If there’s any way to plant corn,” Lingenfelter throws out, “because we are finding out that corn is going to be our best bet. Also Liberty Link soybeans can be somewhat of a benefit.

“Start clean!” he said, which means tillage or an effective burndown. “I realize that tillage can be a dirty word but it can be a savior with a weed like this.” PRE (Pre-emergent) herbicides are essential. “Unfortunately, we have not done any studies on this weed in the field yet,” Lingenfelter said. “We haven’t found a population big enough yet to do this. Don’t forget POST emergence [herbicides]. Timeliness is important. Once they get to three or four inches tall, that is a bit too late. One to two inches is where you want to nail them.”

One ray of hope is that some of agriculture’s southern colleagues are finding that pigweed does not tolerate shade very well. The key to managing this is to get up and above it.

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