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Management of Palmer amaranth and waterhemp
<br />All photos by Joseph Ikley, Brian Jenks and Tom Peters,
<br />North Dakota State University
<br />n Crop Rotation
<br />Palmer amaranth and waterhemp are very competitive
<br />against most summer annual crops, but the availability of
<br />effective herbicides for corn and soybeans provide more
<br />options for chemical weed control. Small grains offer a
<br />competetive advantage compared to row crops due to early
<br />row closure that helps prevent late-season germination.
<br />Limited post-emergent herbicides for pulse crops, flax,
<br />canola, sunflowers and other minor crops make Palmer
<br />amaranth and waterhemp control almost a no-win situation
<br />in these crops.
<br />Effective crop and chemical rotation can help preserve
<br />effective herbicides and reduce the chances of Palmer
<br />amaranth and waterhemp developing resistance to
<br />herbicides. Due to constantly evolving herbicide products
<br />and the wide variety of crops across the state, specific
<br />herbicide programs are updated regularly at https://www.
<br />ag.ndsu.edu/weeds.
<br />n Overlapping Residual Herbicides
<br />The extended germination period of Palmer amaranth and
<br />waterhemp make these two weeds more difficult to control
<br />than traditional pigweeds. In addition to the use of multiple
<br />effective modes of action, multiple applications of residual
<br />herbicides should be the foundation of a herbicide program
<br />focused on Palmer amaranth or waterhemp control.
<br />Research has shown that effective herbicide programs
<br />include multiple applications of residual herbicides. Apply
<br />residual herbicides at planting to control early emerging
<br />weeds. An in-crop application of effective post emergence
<br />herbicide, plus an additional residual herbicide about 28
<br />days after planting, can help reduce weed pressure until the
<br />crop can canopy.
<br />For most crops, the most effective residual herbicides
<br />are the Group 15 herbicides, which help control the later-
<br />emerging plants.
<br />n Good Agronomic Practices
<br />Season-long control of Palmer amaranth and waterhemp
<br />relies on the ability to get a crop canopy. This means that
<br />good agronomic practices are an important component of
<br />a management program. Problems such as soybean cyst
<br />nematode (SCN) and iron-deficiency chlorosis (IDC) should
<br />be addressed because these problems can delay or prevent
<br />the formation of a full crop canopy.
<br />n Tillage
<br />Thorough tillage controls emerged plants prior to planting. Inter-
<br />row cultivation also can control emerged plants in-crop. Deep
<br />tillage can help control areas with a large population of emerged
<br />plants.
<br />The small seed size of Palmer amaranth and waterhemp restricts
<br />germination to the top one-half inch of the soil profile. If seed
<br />is buried deeper than a few inches, Palmer and waterhemp
<br />cannot emerge. Deep tillage should be a one-time event to avoid
<br />bringing buried seed back near the surface. Pigweed seed can be
<br />expected to remain viable at least 3 to 5 years on the soil surface.
<br />Research has shown that buried seed can remain viable for up to
<br />16 years.
<br />n Cereal Rye Cover Crop
<br />Research in southern areas of the U.S. has shown promise with
<br />using a cereal rye cover crop to suppress Palmer amaranth and
<br />waterhemp. About 1,500 pounds of cereal rye biomass per acre
<br />at termination can delay the time it takes Palmer amaranth to
<br />grow to 4 inches, by about 2 weeks. Producing enough biomass
<br />for complete weed control in North Dakota is unlikely, but it can
<br />help widen the window to make a post emergence herbicide
<br />application.
<br />n Hand Weed
<br />Hand weeding is time and labor intensive, but it can be very
<br />effective in a total weed management program. A zero-tolerance
<br />policy for seed production can help with management of Palmer
<br />amaranth and waterhemp due to their abundant seed production.
<br />If you suspect a pigweed is Palmer amaranth or waterhemp, pull
<br />the plants to help prevent a seed rain and avoid major issues in
<br />future years.
<br />n Field Edges, Borders, Ditches and Fence Rows
<br />Palmer amaranth and waterhemp can live and produce seed in
<br />areas beyond crop fields. Field edges and borders between fields
<br />are important areas to monitor for weeds. Pigweeds in these
<br />areas can contribute to spreading resistant populations since
<br />herbicide resistance can spread via pollen.
<br />Communication with neighbors helps prevent pollen shed and
<br />seed production in these areas. Weeds also can thrive in ditches
<br />and fencerows. Pasture and right-of-way herbicides can be
<br />effective against Palmer amaranth and waterhemp.
<br />n Sanitation
<br />Equipment is one of the easiest ways to move pigweed seed
<br />from one area to another. If you have infested fields, farm these
<br />fields last. Combines are especially capable of spreading seed, so
<br />harvest infested fields last. If you run a combine through a patch
<br />of Palmer amaranth or waterhemp, you will spread the seed to
<br />other fields. Since the seed size is very small, thoroughly clean
<br />equipment to ensure seed is not spread to additional fields.
<br />For more information on this and other topics, see www.ag.ndsu.edu/extension
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<br />4 Identification, Biology and Control of Palmer Amaranth and Waterhemp in North Dakota (W1916) | www.ag.ndsu.edu/extension
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