Preventing Palmer Amaranth from Taking Over Your Field

Mark Loux, an OSU Extension weed specialist, in a field overrun with Palmer amaranth. (Photo: Ken Chamberlain, CFAES)

COLUMBUS, Ohio – With the corn and soybean planting season right around the corner, farmers need to know how to prevent Palmer amaranth from getting into fields and what to do if the fast-growing weed is already growing.

Palmer amaranth is a rapidly spreading type of pigweed that has become one of the most detrimental and invasive weeds in the southern and midwestern United States, said Bruce Ackley, an Ohio State University Extension program specialist in weed science. OSU Extension is the outreach arm of The Ohio State University’s College of Food, Agricultural, and Environmental Sciences (CFAES).

The fast-growing plant, which emerges anytime between early May and August, is extremely aggressive and can outcompete crops on almost every level. This has caused substantial losses in crop yield and farm income across the country, as well as a permanent increase in the cost of herbicide programs.

According to Mark Loux, an OSU Extension weed specialist, research has shown the need for a zero-tolerance threshold on Palmer amaranth. The plant so far has been found in 18 Ohio counties, and the majority of the populations are resistant to both glyphosate and acetolactate synthase (ALS)-inhibiting herbicides. Postemergence herbicide application timing is especially critical and must be applied when the Palmer plants are less than 3 inches tall.

Palmer amaranth can be introduced to a crop field by farm equipment and from the manure of animals that have eaten cotton-derived feed products containing Palmer seed from the South, said Loux. The best way to avoid this type of spread is by thoroughly cleaning out equipment between entering different fields, checking that suppliers have checked for Palmer seed in feed products, and by storing manure in pits for a short time to reduce the seed viability.

Palmer amaranth can also be introduced through the use of seed for cover crops or conservation and wildlife plantings that is contaminated with Palmer amaranth seed. Contact the Ohio Department of Agriculture (ODA) for screening of seed for the presence of noxious weeds, including Palmer amaranth. ODA will screen any type of seed used for cover crops or conservation and wildlife seedings. Loux recommends having this done prior to planting. Contact the ODA Grain, Feed, and Seed Program at 614-728-6410. The seed must be picked up by ODA representatives; it cannot be mailed or dropped off.

To stop an infestation from occurring, experts recommend that farmers start scouting soybean fields in mid-July for Palmer plants that have escaped prior herbicide treatments. Any Palmer plants found should first be checked for mature seeds, which are small black seeds found when shaking or crushing the seed heads. If there are none, cut the plants off just below the soil line, remove them from the field, and burn or compost. If the plant does have mature seeds, cut plants off as before, but bag seed heads on site prior to removing the plants from the field.

When in doubt, get help with identification to avoid contaminating combines and other equipment with Palmer seed. Contact the OSU Extension agronomic crops team at at any time for help with identification or management advice.

The agronomic crops team is a multidisciplinary group of educators, state specialists and researchers from OSU Extension and from the Ohio Agricultural Research and Development Center, which is CFAES’s research arm.

Additional information can be found at

CFAES News Team
For more information, contact: 

Mark Loux

Bruce Ackley