Waterhemp gains a competitive advantage over several more aggressive summer annual weeds through the sheer number of plants that can infest an area. Season-long competition by waterhemp (more than 20 plants per square foot) has been shown to reduce soybean yield by 44%. Waterhemp that emerged as late as V5 soybean reduced yields up to 10%.
Waterhemp grows more rapidly than most weeds or crops – typically about 1 to 1.25 inches per day during the growing season. This allows waterhemp seedlings to acquire more sunlight than other weeds.
This species emerges throughout the growing season, and a higher percentage of plants can emerge later in the season than is typical with most other summer annual weeds. This emergence pattern allows waterhemp to avoid many pre-emergence herbicides and often allows this weed to flourish after post-emergence applications of non-residual herbicides like glyphosate.
Waterhemp is a prolific seed producer and able to produce as many as 1.5 times more seeds than most other pigweed species. Waterhemp plants generally produce about 250,000 seeds per plant, although some plants can produce 1 million or more seeds under optimal conditions in noncompetitive environments.
The seeds are small (approximately 3-mm in length) and can easily be transported by contaminated machinery, by waterfowl, through the spread of poultry litter as fertilizer, etc.
Like most weeds, waterhemp seeds remain viable in the soil for several years. Research has shown that only 1 to 12 percent of waterhemp seeds remain viable in the soil seedbank after four years.
Waterhemp has a remarkable ability to adapt to control tactics and has evolved resistance to many different classes of herbicides. To date, waterhemp has evolved resistance to herbicides from six classes, including 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).
Many populations in the Midwest now exhibit multiple herbicide resistances that include herbicides from several families. For example, Group 2 and 9 (e.g., ALS and glyphosate) resistance in waterhemp is fairly common, and in many states resistance to as many as three, four, or five groups now occurs. In 2017, a population with resistance to herbicides from six commonly-used herbicide groups was confirmed. It should be noted that Group 14 PPO-inhibitor herbicides with residual activity are likely to have utility in controlling PPO-resistant waterhemp when applied pre-emergence.
The focus of this section is predominantly chemical control. However, given the extent of herbicide-resistant waterhemp populations, cultural and mechanical options, such as the following, should be considered.
The most effective strategies to reduce herbicide-resistant waterhemp populations will integrate cultural and mechanical techniques with chemical control.
Scout the field within seven to 14 days after the initial post-emergence application to determine treatment effectiveness. If there are still surviving plants present, rogue these plants from the field before they reach a reproductive stage of growth.
Photo: Aaron Hager, USDA Agricultural Research Service, Bugwood.org