Today, waterhemp is known as one of the worst weeds Midwest farmers have to tangle with each season, but it wasn’t always this way. Native to North America, waterhemp was just a plant that kept more to marshy areas than farm fields. In the last 25 to 30 years, however, waterhemp populations have exploded.
- Common names: Waterhemp, Rough-fruit amaranth, tall waterhemp
- Scientific name: Amaranthus tuberculatus
- Cotyledons: Egg- to ovate-shaped
- Leaf shape: Lanceolate
- Stems: Hairless
- Flowers: Individual plants produce either staminate flowers (male) or pistillate flowers (female). Both types of flowers are less than 1/8-inch long. Each staminate flower consists of five sepals, five stamens and no petals; it is surrounded by one to three narrow bracts with pointed tips.
- Reproduction: Dioecious (male and female flowers on separate plants)
- 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 optimum conditions in noncompetitive environments.1
- A joint study written by professors from the University of Illinois, the University of Missouri and Iowa State University reports waterhemp can reduce soybean yield by up to 44% and corn yield by up to 15%.
- Cross-pollination allows for healthier populations. If either the male or female plant develop resistance, the offspring will carry the resistant trait moving forward.
- Waterhemp has an extended emergence period, which allows waterhemp plants to surface late in the growing season.
The chances of waterhemp evolving resistance to herbicides that utilize a single mode of action is very high. That’s why it’s important to implement a program approach that incorporates multiple modes of action.
In addition to using a strategic combination of herbicides, there are several cultural practices to help control waterhemp, such as:
- Planting soybeans in narrow rows to promote earlier row shading and discourage the growth of waterhemp.
- Deep tillage to reduce the amount of waterhemp seeds that germinate by burying them at unfavorable depths.
- Planting fall-seeded cover crops like cereal rye.
1 United Soybean Board. 2020. Waterhemp. https://iwilltakeaction.com/weed/common-waterhemp.
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