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Beware of Super Weed

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Palmer amaranth in a newly seeded Conservation Reserve Program field. (Photos courtesy of Bob Hartzler)

Palmer amaranth can reach seven feet tall. These weeds were pulled from a corn field and are displayed here by Meaghan Anderson, an Iowa State University extension field agronomist.

 

Fast spreading weed becoming a serious threat

By Ted Pennekamp

 

Area farmers might have another insidious weed that is relatively new to the region to think about for this upcoming planting season. Palmer amaranth has been found in Clayton County, Iowa and in Grant and Iowa counties in Wisconsin and has a proclivity for crowding out crops. 

In 2013, Palmer amaranth made its initial appearance in five Iowa counties and has since spread to nearly half of the counties in Iowa. The destructive weed is native to the Southwestern United States, but has been spreading to the east and north.

“Palmer amaranth is a close relative of waterhemp, Iowa’s number one weed problem,” said Bob Hartzler, a professor of weed science at Iowa State University. “The biggest difference between the two species is that Palmer is much more competitive, thus it poses a bigger threat to crop yields.  At high densities it can cause complete crop failures, at lower densities the sturdy plants can interfere with harvest equipment.”

Farmers should not take Palmer amaranth lightly, said Hartzler. The weed can grow to seven feet tall and has proven to be devastating. In the Southwestern United States, farmers called it the “super weed” because it grows fast and is resistant to many herbicides.

“One problem is that it is very similar in appearance to waterhemp, thus farmers need to pay closer attention to their weed escapes than normal,” said Hartzler. “It is very easy to overlook Palmer amaranth thinking it is just the normal waterhemp.”

Hartzler said the most distinguishing trait is the large, sharp bracts on seedheads of female Palmer amaranth plants. The bracts give the seedhead a ‘spiky’ appearance and are painful when you grab them. The best way to stop the spread is to be alert when in fields looking for plants that don’t look like the average pigweed (Palmer amaranth and waterhemp are members of the pigweed family, there are many weedy pigweeds). 

“People involved in invasive species management promote early detection and rapid response and this is what is needed,” Hartzler said. “If a farmer can identify the invader in the first year or two after introduction they have a good chance of being able to eradicate the weed. If they don’t observe it, or ignore it, in the first couple years, it will establish a persistent seedbank and become a permanent component of the weed community.”

Palmer amaranth produces several hundred thousand seeds per plant. The primary dispersal mechanism of these seeds are human activities. 

“Farms that import feed or bedding from states south of us are at risk because these supplies are easily contaminated,” said Hartzler. “Palmer amaranth has also been brought in on equipment that was used in the south where Palmer amaranth is present. Once in a field, the seeds are easily moved by farming equipment to all the fields a farmer manages. Combines are especially effective at transporting the seed.”

This year, Iowa, and other states to a lesser extent, had Palmer amaranth introduced via the planting of native seed mixes that were contaminated with Palmer amaranth, noted Hartzler. Because of a large increase in demand for these seed mixes, local seed producers had to purchase certain species from out of state, and it is believed these imported seeds were the source of Palmer amaranth. Also, the spread of Palmer amaranth has been tied to the increase in planting conservation habitats such as prairies to help pollinators such as bees and Monarch butterflies because some planters ended up buying seeds from out of state.

Palmer amaranth is more easily controlled if caught early. “It is an annual weed, so the best time to control it is early in the growing season while seeds are germinating or when the seedlings are still small,” Hartzler said. “Unfortunately, at that time of year it can be difficult to distinguish Palmer from waterhemp. The best time to identify it is after it has flowered, and that typically happens in late July or August. In conservation plantings it can still be identified by looking for seedheads on standing plants, but if your area has had much snow most of the plants probably would have been knocked down.”

Given Palmer amaranth’s ability to spread quickly, it is poised to surpass other weeds as a primary concern for farmers. One way to combat Palmer amaranth is through herbicides. Another way is through tillage.

“To sum it up, one could say Palmer amaranth is a ‘better’ weed than what most farmers are facing now,” said Hartzler. “It combines the prolific seed production of waterhemp with the high competitiveness of giant ragweed. Farmers can save themselves money over the long run by increasing the intensity of field scouting so they find Palmer amaranth soon after it enters their fields.  Once it is found, they need to adjust their management practices to prevent any escapes that would build a permanent seedbank.”

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