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Stubborn, ‘unbelievably competitive’ Palmer pigweed could do damage to Kentucky crops

By Tim Thornberry
KyForward correspondent

Keeping weeds out of cropland has long been an issue for farmers and one usually handled with the use of chemicals. But more producers are coming across a herbicide-resistant weed that can do real damage if not handled properly.

The Palmer pigweed, or Palmer amaranth, has been making its way up from the southeast and has been present in Kentucky for the past four or five years, according to University of Kentucky College of Agriculture, Food and Environment Extension Weeds Specialist Jim Martin.


Palmer pigweed stems can get as big as a ‘baseball bat,’ UK expert Jim Martin said. (Photo from Wikimedia Commons)

“It’s mostly in the western part of the state but we are seeing evidence of it in Central Kentucky,” he said. “Wherever soybeans are grown, that’s where it raising its ugly head.”

Ugly could be an understatement considering the plant is difficult to get rid of once it has been established. And it can establish itself in other row crops as well as soybeans.

“Palmer pigweed has gotten here by a number of ways. We think it came in through the floodwaters and backwaters when we had the spring floods of 2010 and 2011,” said Martin. “When the rivers drained, it left the pigweed seed in those flood plains.”

There is also evidence it has come by way of transported cottonseed or cottonseed hulls which are often used as cattle and dairy cow feed. Palmer pigweed has been found in many southern cotton fields where the cotton seed originates. The pigweed seeds can also been transported through combines or equipment that have been used to harvest crops where the weed was present.

Martin said that in the case of cottonseed that is being used as livestock feed, manure from those cows, and used as a fertilizer, can contain pigweed seeds and thus cause the plant to spread in that way.

Regardless of how it is moved, Martin said it doesn’t take long for it to become a major problem. There are other pigweed species, but Palmer seems to present the biggest threat to farmers in this region.

“It’s unbelievably competitive, more so than other pigweeds we know of and the stems can get quite big; as big as a baseball bat or as big around as your wrist,” he said. “And Palmer can produce hundreds of thousands of seeds. With just a few plants, in the matter of a year or two, you’ve got a pretty good stand throughout a field. Once you get a few plants that have gone to seed, you can be into a pretty significant population in a somewhat short period of time.”

If conditions are right, the weed can grow 2 or more inches a day, and the plants can grow so large that yields can be reduced in significant numbers even to the point of ruining a crop if the stand is large enough.

Martin said indeed the problem for some growers has been so bad they had to disk up their fields with nothing left to harvest.

“That doesn’t happen a lot but there are cases where it does happen,” he said.

Martin also said there is strong evidence that Palmer pigweed is moving father north, a fact Aaron Hager, a weed specialist at the University of Illinois can attest to.

He said one thing that seems certain for the plant, which is native to the southwest, it’s not a matter of if it gets to that part of the country, it’s a matter of when.


Once Palmer pigweed is established, it doesn’t take long for it to become a major problem. (Photo from Purdue.edu)

“We’ve seen these populations now in parts of the state where it’s never been seen before,” he said. “We’re going to continue to see that and we’ve tried to further emphasize the fact that prevention of Palmer is a much more preferable technique than trying to eradicate it once it’s here.”

Those preventive efforts include trying to keep the seeds of the plant out of the area to begin with and, in the case of areas where the plant is already present, trying to prevent established plants from making any additional seeds.

“Unfortunately because of all the different vectors in which these seeds can move, I think the prevention of seed introduction is going to be very, very difficult,” said Hager.

He added that now it’s important to get people to focus on doing all that can be done to prevent populations that are present and those that will be found from creating any additional seeds that would be added into the soil seed bank.

“If we go toe-to-toe with the plant and we think we can just spray our way out of a Palmer amaranth problem, we’re going to lose and we’re going to lose big because we can’t do it,” said Hager. “Based on the biology of this thing and how rapidly it grows and how quickly it can infest a field, if we try to battle the plant we’re not going to be successful.”

He noted that the key to winning the battle against something like Palmer amaranth is exploiting the weakness and the life cycle of the species.

“What we do know, based on the work that has been done in many different areas across the U.S. in many good research laboratories, the seed of Palmer amaranth doesn’t stay viable in the soil profile indefinitely,” said Hager. “The viability of these very, very small seeded pigweed species may only be five to seven years, for example. So our message is, if we can keep a lot of seeds from being either introduced or can we do anything to keep those female plants from adding seed to the seed bank, that is how I think we are going to win this battle in the long term.”

There are research efforts being made to figure out just how to do that but Hager said producers need to take this seriously and keep an eye out for it.

“In reality, this is a weed species that has literally put people out of business because they have not taken it seriously in the early stages,” he said. “Based on the experience that we’ve looked at from other states where these populations have really become established and problematic, there is no easy way to deal with Palmer.”

Some producers have hired hoeing crews to literally go through fields and chop out the weeds and some success has been found when applying an herbicide before planting as a preventative measure.

The USDA recommends a series of steps that can possibly help including early detection before the seed stage when herbicides can be more effective; high–residue cover crops in conservation tillage systems which offers effective weed suppression; rotation of crops and herbicides; and pre- and post-emergence herbicides.

Tim Thornberry is a freelance writer and photographer who has covered Kentucky agricultural and rural issues for various publications since 1995.

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