Stuck in the Weeds: The Challenge of Palmer Amaranth and Waterhemp

As cousins in the pigweed family, Palmer amaranth and waterhemp are aggressive, invasive weeds and a threat to corn yields throughout the Midwest. There are populations that are glyphosate-resistant, making them more challenging to contain.

Native to the southwestern part of the United States and Mexico, Palmer amaranth is the more destructive of the two weeds, which was recently documented in 28 states; it has a reported yield loss in corn of up to 91 percent.i Waterhemp is thought to have originated in the Midwestern part of the United States. It has since spread to 40 states, but resides primarily in the Great Lakes and Great Plains. Reported yield loss in corn due to waterhemp is of up to 15 percent.ii

Managing these two resistant weeds continues to become more difficult. Understanding how to identify and control these problem weeds is more important than ever.

Identifying Palmer amaranth and waterhemp

Waterhemp and Palmer amaranth have similar physical characteristics and are difficult to distinguish from each other. They both have the capacity to grow upwards of six feet in height. Waterhemp stands tall, with a glossy appearance and a stem that can be green or red. The waterhemp leaf and stem are both hairless, and the leaf shape tends to be long, slender and either egg- or lance-shaped with a shorter petiole.

Palmer amaranth is a summer annual. It may have a single hair at the leaf tip, with the rest of its leaf and stem also being hairless. Its leaf shape is much wider and ovate than waterhemp, and its petiole is longer than its leaf. The leaves often have a white “V” patterned into them. Finally, Palmer amaranth plants have a taller female seedhead, which can reach to over two feet in length.

Waterhemp and Palmer amaranth seed production

Both plants are dioecious, meaning they have separate male and female plants. They produce hundreds of thousands of seeds that are less than 1/32 inch in diameter. This is the first of many challenges with both waterhemp and Palmer amaranth. These seeds are tiny and easily escapable. They transfer through wind, weather, manure and animals, as well as in machinery. Millions of seeds mean that these weeds have an optimal environment to develop into the current problem that they are.

Resistance in Palmer amaranth and waterhemp make management a challenge

The challenge doesn’t end there. Both plants are biologically adaptable and can withstand climate limitations. They bloom from the early spring to well past the first frost. This allows them to be genetically diverse and develop resistance to single site-of-action herbicides.

“This means that Palmer amaranth and waterhemp are also already resistant to new active ingredients that are discovered within these known site of action groups," says Frank Rittemann, selective corn herbicides product manager at Bayer. "This causes problems for resistance management.”

Managing these weeds will require a holistic approach, including crop rotation, tillage, cover crops and monitoring ditches and borders. Using pre- and post-emergence or multiple mode-of-action herbicides are other ways to control resistant weeds. Maintaining extreme vigilance helps capture them at the earliest stages of development.

Zero tolerance of resistant weeds is key

Both Palmer amaranth and waterhemp can grow up to three inches per day. That means capturing them at their first emergence is key to keeping these weeds from going to seed. Once these prolific, resistant weeds get started with seed production, they are difficult to slow down. A zero-tolerance policy and scouting 10 to 12 days post-planting protect fields from these resistant weeds.

If resistant weeds are present in a field, Rittemann recommends a two-pass herbicide program. This includes pre- and post-emergence applications of herbicides with multiple effective sites of action.

Utilizing products from Bayer in a program approach, Corvus^® herbicide (Groups 2 and 27), tankmixed with Harness^® Xtra herbicide (Groups 5 and 15), followed by DiFlexx^® DUO herbicide (Groups 4 and 27), tankmixed with a Roundup^® brand agricultural herbicide (Group 9), helps provide zero tolerance control against resistant weeds.

Corvus offers burndown to help take out early weeds, residual control to help prevent new weeds and reactivation with as little as a half inch of rain. Harness Xtra provides broad spectrum control of more than 40 grasses and broadleaf weeds.

DiFlexx DUO offers highly effective post-emergence protection on glyphosate-resistant and other tough-to-control weeds, including Palmer amaranth and waterhemp. With two effective sites of action, DiFlexx DUO provides built-in resistance management, while also offering exceptional crop safety for all types of corn and soil types.

“Some other products are not effective on resistant weeds. In instances where some other products may fail, DiFlexx DUO continues to provide control,” says Rittemann. “Controlling these weeds really serves two main purposes. First, in season, these weeds compete with the crop for the nutrients in the ground. So, you want to control them to raise a good crop. Secondly, you also want to prepare your soil for the next year.”

Local agronomists or University extension representatives can confirm the presence of resistant Palmer amaranth or waterhemp in fields.

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