Amaranthus palmeri is a species of edible flowering plant in the amaranth genus. It has several common names, including Palmer's amaranth, Palmer amaranth, Palmer's pigweed, and carelessweed. It is native to most of the southern half of North America. Populations in the eastern United States are probably naturalized. It has also been introduced to Europe, Australia, and other areas. The plant is fast-growing and highly competitive.
Palmer amaranth is considered a threat most specifically to the production of genetically modified cotton and soybean crops in the southern United States because in many places, the plant has developed resistance to glyphosate, the active ingredient in the widely-used broadleaf herbicide Roundup. Glyphosate resistant pigweed not only dominates in cotton fields, it has wide ranging effects on other crops and productions as well. In the fall of 2005, North Carolina researchers collected Palmer pigweed by seed from 280 fields from the Virginia border to the South Carolina border. However, in 2001 Palmer amaranthus was found in the southern quarter of Illinois and appeared to be moving to northern Illinois in 2006.
Palmer amaranth may be the most aggressive pigweed species with respect to growth rate and competitive ability. Palmer Amaranth or Palmer pigweed in particular is highly competitive. It will out grow cotton and is much more efficient. Growers of crops have not been able to provide adequate moisture to offset temperatures staying above 85 degrees with 112 degree heat indexes. In these conditions, many herbicides break down over time and Palmer pigweed will keep thrusting.Palmer Pigweed can grow from 2 inches to 5 inches in 3 days or less. In only a few weeks Palmer pigweed can grow from 12 inches to 18 inches compared to cotton 5 to 8 inches.
It is most commonly spread by wind. The male produces the pollen and the female plant produces the seed.The wind carries the pollen from resistant male plants to female plants. In addition, it is spread by traditional means such as harvesting, lack of cleaning equipment and the spreading of infested materials.