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This rapidly growing and evolving weed has become resistant to multiple herbicides in some states, including glyphosate and ALS and HPPD inhibitors, and has spread as far north as Michigan, Wisconsin and Illinois.
A late-spring through late-summer emergence window requires ongoing management. Growers who let Palmer go to seed can lose a field within 3 years.
“Palmer amaranth has many similar biological characteristics as waterhemp and we’ve had to learn that lesson with herbicide resistance time and time again,” says Aaron Hager, weed specialist for the University of Illinois Extension.
Palmer amaranth can grow up to 3 inches per day and reach heights over 6 feet tall.
Leaf petioles are long, or longer than the leaf blade. The thick plant has minimal branching and can be recognized by its V-shaped notch at the tip of the leaves, or by the symmetrical arrangement of leaves that resemble a poinsettia.
A dioecious species, male and female plants are required for reproduction. Female plants can produce upward of 500,000-1 million seeds, which are very small, comparable to poppy or radish seeds. Despite mass production, their longevity is limited and about 80% die within 3 years.
SCOUT CLOSELY. Early identification of Palmer amaranth is crucial to controlling its spread, but that can be troublesome because Palmer closely resembles some other weed species. Above are true leaves of young Palmer (top) and waterhemp.