homepage logo

Preparing for invasion

By Staff | Dec 16, 2016

PRODUCERS AND AGRIBUSINESS professionals take some time to look at and touch a Palmer amaranth weed during a Dec. 8 educational session about the noxious weed that is spreading rapidly through Iowa.

JEFFERSON – A group of agribusiness professionals, as well as several concerned farmers, attended an informational meeting regarding a weed that could prove to be potentially noxious if not soon eradicated in Iowa.

A species of pigweed, Palmer amaranth, has been documented in 48 counties in Iowa in 2016.

Mark Johnson, an Iowa State University Extension field agronomist, said the weed was first discovered in Iowa in 2013 in five southern border counties. That remained the same until this year where the count jumped dramatically throughout the growing season to 48 counties scattered throughout Iowa.

The increase in Palmer amaranth can be credited to the surge in acres that were put in to the USDA’s CRP program in 2016.

“The weed seeds are coming in CRP mixes,” said Johnson.

“The weed seeds are coming in CRP mixes.” —Mark Johnson ISUE agronomist

He added it is difficult to trace back exactly where the seeds are coming from, but advises those who find Palmer amaranth in CRP ground try to determine what seed mixes were used, then report it to a county’s Farm Service Agency office in an attempt to stop additional spread of the noxious weed.

Another option for future CRP acres, Johnson said, is to plant locally-grown seed mixes, as Palmer amaranth is not a native plant to Iowa. Unless those seed fields are infested, there should be no Palmer seeds present.

In fact, one of Iowa’s large native seed producers has no evidence of Palmer amaranth, which has convinced Johnson the weed is being transported into the state.

Palmer amaranth is a native plant for the south, but has been slowly making its way to the north through a variety of ways, including mechanical transportation through farming equipment, dairy feed consisting of cotton seed and gin-trash.

Because the weed is not indigenous to the state, Johnson said Iowans have a lot to learn about it.

What he does know is that the Palmer amaranth seed can rest in the soil up to three to five years before germination and that the growth habit of the weed is extreme.

“Its growth is faster, with a denser canopy,” said Johnson. “It is a competitive weed with documented yield losses in soybeans of up to 80 percent.”

The bright side

But Johnson said there is some good news.

Because the weed hasn’t seemed to have shown up in any crop acres yet, farmers still have a chance to eradicate it.

Eradicating such a noxious weed in CRP ground could be tricky and he suggests getting the permission from your county’s FSA office as to how to handle it, but says to get out there and get those weeds killed or pulled and taken off of the land as carefully as possible.

In order to help eradicate the weed, Johnson gave the example of a Crawford County producer who, along with a crew, took to his CRP ground and dug up each and every Palmer amaranth weed.

Johnson said that resulted in 100 percent eradication.


Identifying Palmer amaranth is key to helping getting it controlled. However, that is a difficult task.

As a seedling there are a few differences that can possibly be spotted between Palmer amaranth and waterhemp; it is not until later in the growing season where identification gets a little easier.

Palmer amaranth, Johnson said, has a denser canopy, but that isn’t always the case, so he suggests not always using that as a way of identifying the weed.

The bracts of the plant, later in the growing season, are the key to identifying Palmer amaranth, Johnson said.

A bract, he described, is a modified seed leaf and is much longer than the seed. The bract of the Palmer amaranth gets hard and sharp and when grabbed, it can be felt. Johnson said that’s how producers can identify the weed as Palmer amaranth versus waterhemp and other pigweeds.

This, however, is only on the female plants, not the male plants.

“In June, they all feel the same, but by July through October, if you grab them, they will hurt,” said Johnson.

Palmer amaranth, he said, will also put up height faster than waterhemp, being known to grow 3 to 4 inches a day in some studies with one particular study showing the weed to grow at a rapid rate of 5 inches a day.

“That’s why this weed is so competitive,” said Johnson.

Although it is good to know if your fields have Palmer amaranth, Johnson recommends trying to eradicate all pigweeds, as they are all yield-robbing pests, especially waterhemp.

Finding the proper herbicide program is important for weed control, but especially for controlling any potential Palmer amaranth growth.

Johnson said in order to minimize the impact of Palmer amaranth for the next growing season it will be imperative that producers prevent any new introductions of the weed.

“Purchase locally-grown seed, unless it is mixed from the south, it should be clean,” said Johnson. “Be wary of seed and bedding coming from some of those infested areas.”

If Palmer amaranth becomes present in crop acres, Johnson recommends putting a pre-plant residual herbicide down. Then when spraying early-post-applied herbicides to add another residual in that mix to hopefully keep control of those weeds germinating past July.

“We want layered residuals because these weeds germinate the entire season and we should be doing this for the waterhemp issues already,” said Johnson. “Waterhemp is our biggest problem and will continue to be, so be sure to go after those patches.”

Please Enter Your Facebook App ID. Required for FB Comments. Click here for FB Comments Settings page