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Controlling Palmer amaranth

By Staff | Jan 30, 2017

PALMER AMARANTH is now known to be in 48 Iowa counties. It’s an aggressive, fast-growing weed that will produce an estimated 300,000 seeds per plant, and develops resistance to herbicides rapidly.

OKOBOJI – Palmer amaranth is one of the newest nuisance weeds on the radar.

But according to Bob Hartzler, Iowa State University professor and Extension weed specialist, early action can at least slow down its spread.

Palmer amaranth is a weed that is native to the southwestern United States, and has been moving northward to the Corn Belt over the past 10 years. In Iowa, it was first reported in Harrison County in August 2013. By the end of 2016, 48 counties in Iowa were known to have been infested.

Hartzler said many of the weed identifications were associated with the planting of native seed mixes used for conservation plantings, which were contaminated with the Palmer amaranth seed.

He said at least six counties were identified where it was introduced by traditional farming practices such as importing of animal feed or bedding, or movement of contaminated equipment.

“Eradication is only economically feasible when a weed is found within two or three years of introduction.” —Bob Hartzler ISUE weed specialist

The weed, part of the pigweed species (along with waterhemp, to which it is nearly identical) has become more prevalent because it quickly evolves resistance to herbicides and grows rapidly with high levels of competitiveness.


In Iowa, ISU weed specialists have found populations resistant to five different herbicide groups. He said resistance profiles of the populations found in Iowa are not yet known.

Hartzler said one more positive way to identify the weed, since is looks very similar to waterhemp, is that the length of the plant-to-leaf stem is longer than the leaf blade itself.

“Palmer amaranth also has a denser canopy than waterhemp and has kind of a poinsettia appearance,” said Hartzler. “Often it will grow five feet tall and five feet wide.”

He said the plants, early on, will develop “bracts” – a flower of sorts that is sharp and extends far past the flower itself. He said waterhemp also has the bracts, but are not as long.

The female plant is spiky in appearance, and is the plant that spreads the seeds. He said male and female plants don’t need to be near each other in order to produce seeds because it’s a wind-driven pollen which does not rely on insects.

Yield losses

Palmer amaranth can grow two to three inches per day later in the season and can create up to an 80 percent yield loss in soybeans. By comparison, waterhemp typically reduces soybean yield by 55 percent and the railroad pigweed reduces yield by 38 percent.

Biomass yield losses to Palmer amaranth are even more significant.

Hartzler said producers must step up their approaches to Palmer amaranth, by comparison to waterhemp, which does not significantly reduce yields when emerging late in the growing season. Palmer amaranth differs in that it will reduce yields if not managed well. He said mowing will not control it.

“Eradication is only economically feasible when a weed is found within two or three years of introduction,” said Hartzler. “That’s why it’s so important to carefully scout fields in 2017 to identify Palmer amaranth when it first moves into an area.”

Pigweed management

Because the weed became so prevalent in 2016, Hartzler said it will quickly become a problem across Iowa. He said since it and waterhemp are both pigweed species, they respond similarly to management programs. Since waterhemp is prevalent, management programs are developed to control pigweeds.

“This will reduce its ability to quickly overwhelm the state, but the acceptance of marginal waterhemp control proves an opportunity for Palmer amaranth to replace waterhemp as our number one weed problem,” he said.

Hartzler added full rates of effective pre-emergence herbicides are key to targeting Palmer amaranth, since it has a prolonged emerging pattern. Cutting rates, he said, allows late emerging plants to continue growth.

He said there are several effective post-emergence herbicides in corn and soybeans, but resistance is reducing the number of options, especially in soybeans. Once the plants reach two to three inches in height, effectiveness of the herbicide begins to diminish. According to Hartzler, its rapid growth creates a smaller window of opportunity for effective application of post-emergence herbicides, and reinforces the need for full rates of pre-emergence herbicides.

The inclusion of pre-emergence herbicide in early post-emergence applications is a good method of extending control until the crop canopy develops – especially in 30-inch rows, Hartzler said.

He added walking fields early and pulling out the weeds wouldn’t take very long because of the lower population at that time of year.

He encouraged producers to be observant because Palmer amaranth can be easily overlooked.

Hartzler said cultivation could be a viable option for problem areas

Keeping equipment clean will also be important to avoid spreading the seed from field to field.

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