Palmer Amaranth

Amaranthus palmeri

Palmer amaranth growing in a pot
Palmer amaranth growing in a pot. Photo courtesy of Bruce Ackley. Licensed under CC BY 3.0.
Palmer amaranth is a summer annual broadleaf plant in the pigweed family that is considered to be an invasive species. It is highly competitive with crops, as a single plant can produce up to 600,000 seeds.
Emergence of palmer amaranth begins in late spring and continues late into growing season for crops. The cotyledons of the palmer amaranth are relatively long, followed by the first true leaves that are ovate or egg-shaped and have a notched tip. Stems are smooth along with the leaves, which grow alternately along the stem. The underside of the leaves have distinct white veins that are fairly prominent. As the plant matures, the leaves will begin to look more poinsettia-like.
Palmer amaranth seedling
Palmer amaranth seedling. Photo courtesy of Bruce Ackley. Licensed under CC BY 3.0.
Palmer amaranth has a smooth stem and unlike other pigweed species, is generally hairless. A mature plant can grow up to 8 feet. It has a leaf petiole that is longer than the leaf itself. The first true leaves of the plant may have a single hair at the very tip, with some leaves also having a white "fingerprint" located on its center. Its shape is ovate to diamond-shaped. One can easily distinguish the amarath from waterhemp by noticing that waterhemp has linear, lance-shaped leaves.
Mature palmer amaranth plant
Mature palmer amaranth plant. Photo courtesy of Pompilid. Licensed under CC BY 3.0.
There are both male and female plants of this species. The males have a more "feathery" appearance, while females have stiffer bracts that tend to be sharp. In the presence of competition, the female plant will produce at least 100,000 seeds, whereas an undisputed plant can produce as many as half a million seeds.
Palmer amaranth inflorescence
Palmer amaranth inflorescence. Photo courtesy of Howard F. Schwartz. Licensed under CC BY 3.0.
Due to its fast growth rate, palmer amaranth is highly competitive with agricultural crops, causing yield losses of up to 70-90%. It is particularly dangerous to cotton and soybean crops. During extremely hot periods of time when crops slow their growth, palmer amaranth will continue to thrive. It was selected as the "weed-of-the-year" in 2014 due to its devestating effects.
Control the palmer amaranth mechanically can be very effective if done properly. Deep tillage (moldboard plow) can reduce the emergence of palmer amaranth by 50 percent. Buried seed remains viable for 5 years, so only deep till once. A well-managed cereal rye crop can help suppress Palmer amaranth well. In severe infestations, it has become necessary to hand weed and burn or compost the weed. Plants laid on the soil will reroot. It is essential that an investation be handled before the plants mature. It is better to leave mature amaranth undisturbed in order to avoid further spread of seed. Be sure to flag these areas for intense scouting and management implementation the following year.
Several herbicides are labelled for use on Palmer amaranth. One should consider doing a double pass for more effective control of the weed. However, palmer amaranth quickly develops resistance to herbicides and has already shown resistance to Glyphosate and ALS. Control efforts should include a post-control inspection to ensure that the weeds were killed. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.