3. Weed Control action plan for Palmer amaranth

Palmer amaranth (Figure 1) and waterhemp (Figure 5) do not have hairs on the stems. Redroot pigweed (Figure 9) has short, dense hairs, while Powell amaranth (Figure 12) can be sparsely to densely hairy. Always look for hairs at the top of the plant near the newest growth. Pull older, more mature leaves of suspected Palmer amaranth or waterhemp plants to examine petiole length. Palmer amaranth (Figure 4) has petioles as long as or longer than the leaf blade. Waterhemp (Figure 8) petioles are shorter than the leaf blades. Palmer amaranth female plants are easily distinguished at maturity from other pigweeds. There are spiny bracts (Figure 15) at each leaf axil, and the seed head (Figure 16) is prickly and rough to handle. Male plants have smooth inflorescence that can be confused with other pigweeds. Palmer amaranth and waterhemp have smooth stems at maturity. Palmer amaranth (Figure 2, Figure 3) has the longest inflorescence/seed heads of the pigweeds. The long terminal seed head often can reach 2 to 3 feet long by late summer. Waterhemp (Figure 6, Figure 7) tends to have thin inflorescence/seed heads compared to the other pigweeds. Redroot pigweed and Powell amaranth have hairy stems near the inflorescence/seed heads. The seed heads of redroot pigweed and Powell amaranth are compact and branched. Redroot pigweed (Figure 10, Figure 11) has short branches, while Powell amaranth (Figure 13, Figure 14) has longer branches. Biology of Palmer amaranth and waterhemp n Genetic Diversity Both Palmer amaranth and waterhemp easily adapt to new locations. These weeds are both dioecious, meaning they have separate male and female plants. This means that plants must outcross to pollinate and produce seed, which leads to increased genetic diversity. Research has shown that herbicide-resistance traits can transfer on pollen, which quickly can expand geographic areas of resistant populations. n Prolific Seed Production Both Palmer amaranth and waterhemp can produce up to 100,000 seeds per female plant in direct competition with crops. In the absence of competition, plants can produce more than 1 million seeds per plant. This prolific seed production allows for populations to quickly take over fields if plants are not controlled and are allowed to produce seed. n Small Seed Size The seeds of all pigweed species are very small, which allows seed to be easily transported through various means. Palmer amaranth and waterhemp have been introduced to new areas through contaminated straw, hay and other animal feed. Equipment, especially combines, can carry seed long distances. Contamination of Conservation Reserve Program seed and cover crop seed have led to new introductions in the Corn Belt. Wildlife and migratory birds also can spread seed. n Extended Germination Period Both Palmer amaranth and waterhemp can germinate throughout the growing season. Germination typically begins mid-May in North Dakota, but can continue throughout the summer into September. This leads to management challenges in crops that struggle to canopy and could be a challenge after harvesting early maturing crops. n Competitiveness Palmer amaranth and waterhemp can be aggressive competitors under ideal growth conditions. These plants can grow 2 to 3 inches per day if conditions are favorable. Postemergence herbicides are most effective when the plants are smaller than 4 inches. Their rapid growth can lead to compressed spray application windows for optimal control. Weeds larger than 4 inches likely cannot be controlled by any post-emergent herbicide, including dicamba or 2,4-D. n Herbicide Resistance Resistance to the ALS-inhibiting (Group 2) herbicides and glyphosate (Group 9) is so widespread in both plants that it is safe to assume resistance to both modes of action. Protoporphyrinogen oxidase (PPO)-inhibiting (Group 14) herbicide resistance is becoming more common. Populations also have been identified with resistance to dinitroanilines (Group 3), growth regulators (Group 4), triazines (Group 5), chloroacetamides (Group 15), and 4-Hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitors (Group 27). 2 Identification, Biology and Control of Palmer Amaranth and Waterhemp in North Dakota (W1916) | www.ag.ndsu.edu/extension