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Field Fiends: Pigweeds


Everything you need to know about waterhemp and Palmer amaranth

As much as we enjoy visiting as many fields as possible, there are certain visitors that call for maximum security. Pigweeds such as waterhemp and Palmer amaranth are sneaky species that have a tendency to make an appearance in fields across the Corn Belt. Learn about these pesky pigweeds and develop your management plan for this season.

Identifying Waterhemp and Palmer Amaranth

In the image below, you can see the loose, open canopy of waterhemp on the left, along with Palmer amaranth’s densely filled canopy on the right. As it grows, Palmer amaranth also begins flowering, making it easier to distinguish from waterhemp.

Late season waterhemp in soybean

In the photo on the left, you can see how waterhemp’s expanded appearance stands out in a soybean field. Both waterhemp and Palmer amaranth can grow much taller than the average soybean plant, making it quite noticeable as you scout fields.

Contact your local agronomist for help identifying and confirming waterhemp or Palmer amaranth in your fields.

These pigweeds are usually most prominent later in the season, around August, depending on your location. Late-season appearances of Palmer amaranth and waterhemp are usually due to a delay between herbicide application and pigweed invasion, or an herbicide application that didn’t quite cut it.


The phrase “growing like a weed” might as well say “growing like a pigweed,” because these species spread like wildfire through fields, and develop herbicide resistance just as fast. Waterhemp specifically is resistant to five different herbicide groups: HG 2, 5, 9, 14 and 27. As commonly used herbicides, Iowa State University Extension expects that pigweed resistance to dicamba, 2,4-D or glufosinate may come next.

Although pigweeds are resistant to many herbicides, as late-emerging weeds, timing your herbicide application later in the season is more effective in reducing viable seed production. Waterhemp and Palmer amaranth can produce more than 500,000 seeds per plant, and those seeds can remain viable in the soil for 4-5 years.

With that high of a reproduction rate, missing even a few plants would increase the population of the pigweeds for multiple years to come, so controlling the population and preventing spread is imperative.

The first step in combating pigweeds is scouting your fields, especially in late-season. Because herbicide resistance is so common with waterhemp and Palmer amaranth, scouting and removing the plants is often the best solution. It’s also important to note that at this point—noticing fully grown pigweeds in your fields—the effectiveness of late-season herbicides is highly variable and questionable.

Scouting fields should continue from the end of July well into September, depending on your location and planting/harvest timing. Include roadsides, waterways, fencelines and the edges of your fields when scouting. If you see waterhemp or Palmer amaranth, take care of business and pull the entire weed out to prevent regrowth from partial cutting, followed by seed production and spread.

Staying on top of pigweeds and being prepared before the season begins can help you manage these field fiends.