Palmer amaranth ( _Amaranthus palmeri_ ) is an annual broadleaf weed that significantly impacts agricultural systems. Native to the southwestern United States and northern Mexico, it has spread widely, becoming a troublesome weed in various crops across the U.S. Its rapid growth, high seed production, and ability to develop herbicide resistance make it a major concern for farmers and land managers. Understanding its characteristics and implementing effective management strategies helps mitigate its impact on agricultural productivity and costs.
Identifying Palmer Amaranth
Palmer amaranth has distinct physical characteristics. Its leaves are ovate to diamond-shaped, hairless, and arranged alternately along the stem. The petiole, which connects the leaf to the stem, is often as long as or longer than the leaf blade. Some plants may have a white or V-shaped watermark on the leaves.
Stems are smooth and can be green to red. When viewed from above, the leaf arrangement can create a symmetrical, poinsettia-like appearance. Mature plants commonly reach 6 to 8 feet, sometimes up to 10 feet. Female plants produce long, rope-like seed heads, up to 3 feet, with stiff, sharp bracts that are prickly to the touch.
Why Palmer Amaranth is a Threat
Palmer amaranth is a significant threat due to its biological advantages. It grows aggressively, up to 2 to 3 inches per day under favorable conditions. This rapid growth allows it to outcompete crops for sunlight, water, and nutrients, leading to significant yield reductions. Yield losses can be as high as 91% in corn and 79% in soybeans when uncontrolled.
Its prolific seed production also contributes to its invasiveness. A single female plant can produce 100,000 to 500,000 seeds, potentially reaching 1 million to 1.8 million in non-competitive environments. These small seeds disperse easily by wind, water, animals, and farm equipment, spreading rapidly. Seeds remain viable in the soil for several years, creating a persistent seed bank. Palmer amaranth is dioecious, with separate male and female plants, promoting outcrossing and increasing genetic diversity. This diversity enables the weed to adapt quickly and develop resistance to multiple herbicides, making chemical control challenging.
Strategies for Managing Palmer Amaranth
Effective management of Palmer amaranth requires an integrated approach, combining various control methods to prevent its establishment and spread. Prevention involves using certified weed-free crop seeds and thoroughly cleaning farm equipment when moving between fields to prevent seed transfer. Controlling Palmer amaranth in non-crop areas, like field edges and ditches, also reduces seed production and dispersal.
Cultural practices enhance crop competitiveness and suppress weed growth. Diverse crop rotations, including those with different herbicide modes of action or earlier harvest times, can disrupt the Palmer amaranth life cycle. Narrower row spacing for crops leads to faster canopy closure, shading out later-emerging Palmer amaranth seedlings. Cover crops, especially high biomass varieties, suppress emergence and growth by providing shade and competing for resources.
Mechanical control methods are effective for small infestations or as part of a larger strategy. Timely cultivation and hand-weeding manage small plants, ideally before they exceed 3-4 inches. If hand-pulling larger plants, remove them entirely from the field, as Palmer amaranth can re-root and produce seeds. Deep tillage, such as moldboard plowing, can bury seeds below their preferred emergence depth, reducing initial emergence.
Chemical control is an important tool, but it requires careful planning to combat herbicide resistance. Assume Palmer amaranth populations may be resistant to common herbicides like glyphosate and ALS inhibitors. Apply pre-emergence herbicides with residual activity, followed by timely post-emergence applications. To manage resistance, use multiple effective modes of action in herbicide tank mixes for both pre- and post-emergence applications. Regularly scout fields to detect new flushes and apply herbicides when plants are small (under 4 inches) for effective control.
Distinguishing Palmer Amaranth from Look-Alikes
Palmer amaranth is often confused with other common amaranth species, including common waterhemp (_Amaranthus rudis_ or _Amaranthus tuberculatus_), redroot pigweed (_Amaranthus retroflexus_), and smooth pigweed (_Amaranthus hybridus_). Differentiating these species is important for accurate management. A distinguishing feature for Palmer amaranth is its petiole length; the petiole on mature leaves is typically as long as or longer than the leaf blade. In contrast, common waterhemp usually has petioles shorter than its lance-shaped leaves.
Another reliable characteristic is the presence or absence of hair. Palmer amaranth and common waterhemp are generally hairless on all plant parts. Redroot pigweed and smooth pigweed, however, have fine hairs, particularly on their stems and sometimes on leaf margins. Seed heads also offer clues: female Palmer amaranth plants have long, spiny seed heads with stiff, sharp bracts that are painful to the touch. Waterhemp seed heads are typically less prickly, and redroot/smooth pigweed heads are often more branched and less spiny. While some Palmer amaranth plants may have a single hair in the leaf tip notch or a whitish V-shaped watermark, these traits are variable and should not be the sole basis for identification.