By Dusty Sonnenberg, CCA, Ohio Field Leader

From a distance, Palmer amaranth, common waterhemp, and redroot pigweed can easily be mistaken for each other, but proper identification is a key to effective management. Each weed species is a growing problem in Ohio.

Palmer amaranth and waterhemp are relatively easy to separate from redroot pigweed when taking a closer look, because both the Palmer amaranth and waterhemp have smooth stems with no hair on them. Redroot pigweed has fine hairs on the stem.

“It is not critical to separately identify the Palmer amaranth from common waterhemp because the management strategy

Jeff Stachler, OSU Extension

is going to be similar for both,” said Jeff Stachler, Ohio State University Extension educator in Auglaize County. “Palmer amaranth and waterhemp are both dioecious plants, meaning they produce separate male plants and separate female plants. That causes a problem from the standpoint of diversity as every flower on a female plant could hypothetically have a unique male plant that pollinated each of the flowers. This leads to genetic diversity with all kinds of variations, such as plant height and color and also the level of herbicide resistance due to the genetic capacity the plant has.”

The herbicide resistance is real.

“Herbicide resistant waterhemp happens because of the genetic diversity, and because farmers do the same thing over and over again,” Stachler said.

In Missouri and Illinois, waterhemp has developed resistance to six different groups (modes of action) of herbicides. That resistance includes the Group 2 herbicides, which is ALS resistance (such as Pursuit), also the Group 4 herbicides which are the growth regulators (such as 2,4-D), Group 5 which is Atrazine, Group 9 which is glyphosate (Round-Up), Group 14 PPO Inhibitors (such as Flexstar and Cobra), Group 27 which is HPPD Inhibitors, (like Calisto, Laudis, and Impact).

“Group 15 herbicides could be use pre-emergence. These would include products like Dual, Harness, Zidua and Outlook. In Illinois, however, the waterhemp is resistant to Group 15 herbicides,” Stachler said. “In soybeans the only two post emergence options those farmers have left are dicamba and Liberty.”

Fortunately, Ohio has not seen the same levels of resistance, yet.

“Resistance is building to in Ohio, but it is not at the levels found in Missouri and Illinois yet,” Stachler said. “Farmers will not achieve control of common waterhemp with a total post-emerge single application program.”

Waterhemp is an annual weed with enormous genetic diversity. It begins emerging in early May and continues emerging until late July. (Palmer Amaranth will continue to germinate into September.) Waterhemp is a prolific seed producer.

“Most plants will produce at least 100,000 seeds per plant. Some plants can produce over 250,000 to 500,000 seeds per plant. It is possible that a single plant could produce over 1,000,000 seeds,” Stachler said. “Waterhemp is a weed that grows quickly, however the seeds do not have great longevity. Approximately 95% of the seeds are gone in 4 years.”

Waterhemp is a weed that needs to be controlled. If left untreated, it will compete with soybeans all season long, and can reduce yields by 44%.

“Effective control will take a combination of a pre-emerge application followed by one or two post-emerge applications,” Stachler said. “Ideally the second post-emerge application would also contain a residual.

“Pre-emerge options would include the Group 14 herbicides, which are the PPO inhibitors such as Flexstar and Cobra. The Group 5, PSII inhibitors such as Metribuzin, as well as the Group 15, mitosis inhibitors such as Metolachlor and acetochlor like Dual Magnum, Harness and Warrant are all available. The Group 3, Mitotic inhibitors such as Treflan, are also options.”

Post-emerge options include glyphosate (Round-up), with the Herbicide Planning and Technology Tips with Mark Loux on Ohio Field Leaderunderstanding that most waterhemp populations have resistance already developed at some level. The PPO inhibitors in Group 15 like Cobra and Flexstar can be used, however 25% to 50% of the waterhemp populations are showing resistance at some level to these products. Glufosinate, which is Liberty, is a good product with no known resistance, along with dicamba which also has no known resistance and is a good product are both post-emerge options, but neither of these are perfect. Another option is 2,4-D post-emerge, however this is not perfect, and resistance has been found in Missouri, Illinois, and Nebraska, Stachler said.

“There are a few limitations to some of the post-emerge products. Dual II Magnum and Zidua need to be applied before the soybean reaches the third trifoliate according to the product label. Dual Magnum has a 90-day pre harvest interval, and Outlook needs to be applied prior to the fifth trifoliate stage in the soybeans. Warrant can be applied up to R2,” Stachler said. “Farmers need to keep in mind that each of these products has a maximum total amount that can be applied per acre in a season, so if the product is used in multiple applications, that needs to be factored in.”

Palmer Amaranth has been found in 36 counties in Ohio. Those counties include: Williams and Fulton, Paulding, Putnam, Hancock, Sandusky, Seneca, Lorain, Wyandot,

Palmer Amaranth is an invasive pigweed. Get tips for controlling it from Ohio Field Leader.
Palmar amaranth

Hardin, Richmond, Wayne, Mahoning, Columbiana, Mercer, Shelby, Champaign, Delaware, Knox, Licking, Tuscarawas, Clark, Madison, Fairfield, Preble, Green, Fayette, Pickaway, Ross, Warren, Clinton, Highland, Brown, Adams, Scioto, and Vinton. Only nine of those counties are known to have established populations. Soybeans can have a 79% yield loss when Palmer amaranth emerges with the crop and is left untreated.

“Palmer Amaranth management is going to be similar to what was described for waterhemp management,” Stachler said.

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