Common Name: Palmer Amaranth
Alternate Names: Palmer pigweed, careless weed
Scientific Name: Amaranthus palmeri S. Watson
Legal Status: Prohibited - Eradicate
All above and below ground parts of the plant must be destroyed. Additionally, no transportation, propagation, or sale of this plants is allowed. Failure to comply may result in an enforcement action by the county or local municipality. Minnesota Noxious Weed Law.
Palmer amaranth is an annual plant native to the arid southwestern United States and northwestern Mexico. It is a traditional food of Native Americans including the Navajo, Pima, Yuma and Mohave. Its life-cycle is adapted to desert conditions; it will germinate and grow to quickly produce abundant seed when water is available. Palmer amaranth was accidentally introduced to the southeastern US.
- 1915 - First reported in Virginia but was not considered problematic for some time.
- 1989 - Documented in a weed survey in South Carolina.
- 1995 - Ranked the most troublesome weed in cotton in the Carolinas.
- 2009 - Ranked the most troublesome weed in cotton in the southern US.
Palmer amaranth developed resistance to multiple classes of herbicides and their different modes of action making it challenging to control. Palmer amaranth seed was accidentally moved, perhaps with cotton meal, and continues to spread. It is causing extensive corn and soybean crop losses in many areas.
- Palmer amaranth is a summer annual that commonly reaches heights of 6-8 feet but can reach 10 feet or more.
- The green leaves are smooth and arranged in an alternate pattern that grows symmetrically around the stem. The leaves are oval to diamond-shaped. There is a small, sharp spine at the leaf tip.
- The leaves of some Palmer amaranth plants have a whitish V-shaped mark on them. Not all Palmer amaranth plants display this characteristic.
- There are separate male and female plants.
- Palmer amaranth looks similar to our native pigweeds such as tall waterhemp (A. tuberculatus), redroot and smooth pigweeds (A. retroflexus and A. hybridus respectively). Here are some distinguishing characteristics:
- Redroot and smooth pigweeds have fine hairs on their stems and leaves. Palmer amaranth and waterhemp do not have these hairs.
- The petiole (stalk connecting a leaf to the stem) is longer than the length of the leaf. For tall waterhemp, the petiole will be only half the length of the leaf.
- Seedhead spikes on female Palmer amaranth plants are much taller (up to three feet long) and more prickly than waterhemp or redroot and smooth pigweed spikes.
Palmer amaranth is adapted to the arid habitat of the desert southwest. Outside of its native range, Palmer amaranth has been documented in annual row crop fields and disturbed, sunny areas.
Means of spread and distribution
Seed is the means of spread and female plants are prolific seed producers. A study in Missouri documented more than 250,000 seeds produced per plant. Seed can be spread in water movement, by wildlife and via agricultural practices such as plowing, harvesting and spreading manure.
Palmer amaranth is documented in 28 states including neighboring South Dakota, Iowa and Wisconsin. It has been documented in Douglas, Lyon, Todd and Yellow Medicine Counties in Minnesota.
Palmer amaranth competes aggressively with crops. It has a fast growth rate of 2-3 inches per day and commonly reaches heights of 6-8 feet, greatly inhibiting crop growth. Yield losses have been up to 91% in corn and 79% in soybean.
Prevention and management
- Be proactive and prevent Palmer amaranth establishment. Familiarize yourself with Palmer amaranth identification and actively look for it in crop fields, borders, ditches and around dairies. Palmer amaranth is difficult to control because it can be resistant to multiple classes of herbicides and their different modes of action. Populations of Palmer amaranth have been documented with resistance to one or more of the following classes: Dinitroanilines, triazines, ALS (acetolactate synthase) inhibitors, glyphosate and HPPD (4-hydroxyphenylpyruvate dioxygenase) inhibitor herbicide.
- Mowing alone is not as effective as cultivation, as Palmer amaranth plants are usually not killed by mowing. They can regrow from cut stalks and set seed close to the ground. Mowing must therefore be done in conjunction with other methods of control like herbicide application, prescribed fire or propane weed torching to be effective.
- Prevent all Palmer amaranth plants from producing seed if possible. Plants can be hand weeded and removed for disposal. If hand weeding is not feasible, contact your local University of Minnesota Extension agent, co-op, or certified landscape care expert for a specific herbicide recommendation.
- Always clean vehicles and equipment after exiting infested areas. If seed was produced, deep tillage will reduce the quantity of seeds that can readily germinate.
- A cereal rye cover crop can reduce Palmer amaranth germination and growth.
Palmer amaranth can be toxic to livestock if the concentration of nitrates in the leaves is high.