Invasive Spotlight: Palmer Amaranth

Amaranthus palmeri, otherwise known as Palmer Amaranth or Palmer Pigweed is a broadleaf weed. It is an annual plant native to the arid southwestern United States and northwestern Mexico. Once cultivated by Native Americans, it was prized for its ability to produce large amounts of vegetation and vast amounts of seeds. This prolific nature is the reason why today Palmer Amaranth is identified as a noxious weed in Ohio.

“Palmer” is an extremely aggressive plant. Growing 2 to 3 inches per day, it can out compete any field crop. Plants can produce as many as 100,000 –500,000 seeds. With no surrounding crop, just one plant can produce 1 to 1.8 million seeds. In addition, it has dioecious reproduction, meaning there are male and female plants. Through this process, one plant is able to rapidly pass its herbicide resistant traits onto the next generation. In-field tests revealed that Palmer was able withstand up to nearly 200 oz. per acre levels of glyphosate applications as well as other ALS inhibiting herbicides.

Identification

It is easy to misidentify Palmer amaranth because it looks similar to three other common amaranth species: redroot pigweed (Amaranthus retroflexus), smooth pigweed (Amaranthus hybridus), and common waterhemp (Amaranthus rudis). The resemblance is especially strong during the seedling stages of growth

Palmer has several unique identifying features that can help it stand out from other Pigweed species.

Leaf Shape

The leaf shapes of amaranths can vary quite a bit within a single species; however, there are general shapes that distinguish the species

Palmer amaranth leaves are wider and ovate to diamond-shaped.

Common waterhemp leaves are generally long, linear, and lanceolate.

Redroot and smooth pigweed leaves are similar to Palmer leaves and have a round to ovate shape — redroot and smooth pigweed leaves, however, have hairs while Palmer and common waterhemp leaves do not.

Plants that have been sprayed and survived multiple herbicide applications (especially PPO-inhibitors) can exhibit variable leaf shapes that may not correctly represent the species.

Petiole Length

Seed Heads

It is easier to distinguish Palmer from Waterhemp once the plants have started flowering. Palmer seed heads tend to be thicker (up to 1” in diameter) than those of Waterhemp. Terminal branches of Palmer are long, sometimes exceeding three feet in length.

Female seed heads have stiff, sharp bracts making them prickly when touched.

Leaf Marking

Leaves sometimes have white or purple chevrons

Palmer In Ohio

Palmer has been identified in several Ohio counties. Commonly found in dairy feed additives such as cottonseed hulls and found in dairy manure, producers are urged with great caution when importing feed additives from the South and importing manure from other farms. OSU Extension has a fact sheet showing impacted counties https://cpb-us-w2.wpmucdn.com/u.osu.edu/dist/7/3461/files/2018/12/2019-Palmer-overview-fact-23n4rw1.pdf

Management

It is resistant to glyphosate (group 9) and ALS inhibitors (group 2) and cannot be controlled by burndown or post emergence applications of glyphosate alone.

Plants with mature seed should be bagged and removed from field. Plants without mature seed (black) should be pulled out (uprooted) or cut off just below soil and removed from field, and then burned or buried at least a foot deep or composted.

Do not run the combine through Palmer patches. Use an appropriate herbicide program.

Other control methods:

Scout and destroy plants or remove from field before mature seed develop –when seed are absent or still green and soft
Seed heads with mature seed -turning dark and hard -should be bagged on site and removed
When purchasing used equipment, know where it has been previously. Avoid purchase of combines that come from Palmer-infested areas. Know where custom harvesting equipment has been previously.

Purdue Extension has a great fact sheet on palmer and its management https://www.extension.purdue.edu/extmedia/WS/WS-51-W.pdf