Waterhemp is one of the most common weeds found in the Midwest and is actually a member of the Amaranth family. It is easier to distinguish it from other weeds in the pigweed family as it mature.
Although other summer annuals may be more competitive, waterhemp has an advantage in the number of plants infesting an area. And it has a high relative growth rate – can be 1 to 1 ¼ inches per day.
Because waterhemp is dioecious, meaning there are male and female flowers on separate plants, two plants mix genes when reproduction occurs. This leads to an increase in a population’s genetic diversity and thus increases the potential for spreading herbicide resistance traits even more so than other agronomic weeds.
Waterhemp has evolved to have resistance to 6 herbicide classes to date. Group 5 (e.g., triazines like atrazine and simazine), Group 2 (e.g., ALS-inhibiting herbicides like Pursuit® and Classic®), Group 14 (e.g., PPO-inhibiting herbicides like Ultra Blazer®, Cobra® and Flexstar®), Group 9 (e.g., glyphosate), Group 27 (e.g., HPPD-inhibiting herbicides like Callisto®, Laudis® and Impact®) and Group 4 (e.g., 2,4-D). (Take Action – Waterhemp Management in Soybeans publication; technical editor Kevin Bradley, Ph.D. University of Missouri, 2013).
And waterhemp was one of the first weeds in the U.S. identified with multiple herbicide resistance. Both ALS-inhibitor and glyphosate resistance is common for waterhemp in the Midwest and many populations have shown resistance of up to five herbicide groups.
Check out the whitepaper, Biology and Management of Waterhemp written by Dawn Nordby, University of Illinois; Bob Hartzler, Iowa State Univeristy; and Kevin Bradley, University of Missouri for more information and tips for controlling waterhemp: