Amaranthus Palmeri

Amaranthus palmeri, commonly known as Palmer amaranth, is a highly aggressive and fast-growing annual weed native to the southwestern United States and northern Mexico. Over the past few decades, it has spread rapidly across the southern and midwestern U.S., and more recently into parts of Canada, South America, and southern Europe. It thrives in hot, dry conditions and is especially problematic in large-scale row crop systems with reduced tillage practices.

Palmer amaranth is identifiable by its tall, erect growth (up to 2–3 meters), smooth hairless stems, and long, lance-shaped leaves. A key distinguishing feature is a prominent petiole (leaf stalk) that is often longer than the leaf blade itself, and female plants produce dense, spiny seed heads. The species is dioecious, meaning male and female flowers occur on separate plants. It belongs to a diverse genus, with close relatives like Amaranthus retroflexus (redroot pigweed) and Amaranthus tuberculatus (waterhemp), which differ in leaf shape, stem texture, and flowering habits.

Palmer amaranth is most damaging to crops like soybean, corn, cotton, and peanuts, where it competes fiercely for light, water, and nutrients. It grows extremely quickly—up to 5 cm per day—and produces hundreds of thousands of seeds per plant. Many populations have developed resistance to multiple herbicide classes, including glyphosate and ALS inhibitors, making it one of the most challenging weeds to manage. Integrated weed control strategies, including cover crops, crop rotation, and pre-emergence herbicides, are essential for limiting its spread.