Waterhemp was first found in Ontario about 20 years ago, 5 years ago in Quebec and only 3 years ago in Manitoba.2 This invasive weed is in the pigweed family and can be confused with other pigweed species. Waterhemp has the potential to become one of the most problematic weeds in Canada, as it is in the Corn Belt of the United States. It can be easily spread to new areas in seed lots, hay, on vehicles or equipment and by water and wildlife3. Once established, waterhemp thrives in agricultural production areas due to prolific seed production, extended emergence, high genetic diversity and herbicide resistance. Currently, there are waterhemp populations resistant to Groups 2 (ALS inhibitors), 5 (photosystem inhibitors), 9 (EPSP synthase inhibitor), and 14 (PPO inhibitors) herbicides in Ontario and Quebec and Groups 2 and 9 herbicides in Manitoba.4 Recently, group 27 (Carotenoid biosynthesis inhibitors resistance has been found in Quebec.5

Biology and Identification6

Waterhemp is an annual broadleaf weed that has botanical traits that contribute to it being difficult to manage.

  • Waterhemp emergence can occur throughout the growing season. This emergence pattern allows waterhemp to avoid many pre-emergence herbicides and often allows this weed to flourish after postemergence applications of non-residual herbicides.
  • A single female plant can produce up to 4.8 million seeds in a non-competitive environment, with an average of about 250,000 seeds per plant.
  • Waterhemp is dioecious (male and female plants), which leads to large genetic diversity that allows for the evolution of rapid herbicide resistance because of obligate outcrossing. Most other Amaranthus species are not dioecious.
  • Waterhemp is extremely competitive as the sheer number of plants can overtake other aggressive summer annual weeds. The growth rate of waterhemp ranges from 2.5 to almost 4 centimetres per day.
  • Waterhemp is extremely competitive as the sheer number of plants can overtake other aggressive summer annual weeds. The growth rate of waterhemp ranges from 2.5 to almost 4 centimetres per day.
  • Extreme season-long competition by waterhemp (more than 200 plants per meter) has been shown to reduce soybean yield by 44%. Late emerging waterhemp in relation to crop development (V5 soybean) reduced yields by 10%.
  • The seeds are small, about 3-mm, and can be easily transported by contaminated machinery, animals, and animal waste. Seeds remain viable in the soil for several years, but viability is greatly reduced after 4 years.
  • Waterhemp is extremely difficult to identify when small. However, there are a few key differences between other common Amaranthus species. Waterhemp cotyledons are often more egg-shaped than the longer, linear cotyledons of other Amaranthus species. The first true leaves of waterhemp are generally longer and more spear shaped than other pigweed species. Waterhemp seedlings and stems are hairless with leaves that look waxy or glossy (Figure 1).
  • Additional information to help with identifying waterhemp can be found at: https://extension.psu.edu/invasive-pigweeds-palmer-amaranth-and-waterhemp#section-10 and https://onvegetables.com/2020/04/14/waterhemp-a-spreading-invasive-weed-in-ontario-and-canada/.
Figure 1. A. Palmer amaranth female stem, B. Waterhemp, C. Green pigweed, D. Redroot pigweed, Image courtesy of C. Shropshire.
amaranth female stem

Managing Waterhemp7

  • Narrow row spacing (drilled or 38 centimetre row spacing) with optimum soybean seeding rates of 346,000 to 395,400 seeds per hectare (140,000 to 160,000 seeds per acre) can help increase crop competitiveness.
  • Deep tillage, burying the seed at depths that prevent emergence, can help reduce populations. Using deep tillage in combination with residual herbicides reduced emergence of pigweeds, including waterhemp, by 97%. However, deep tillage comes with its own set of drawbacks.
  • Fall-seeded cover crops, particularly cereal rye, can help reduce early-season waterhemp emergence the next spring.
  • Wheat, both spring and winter can be a useful crop to grow in rotation to help manage and reduce populations.
  • Residual herbicides, applied at a full rate (according to label guidelines for soil type and organic matter content) before or after planting, can help reduce waterhemp. Relying only on post-emergence herbicides can be a major factor in waterhemp developing herbicide resistance. Application of an effective, soil-applied residual herbicide such as a Group 15 product can help control early emerging waterhemp and reduce early season competition by delaying emergence until the crop can be more competitive.
  • Do not use reduced application rates as full rates will provide longer lasting control, thus delaying emergence of waterhemp as long as possible.
  • Depending on the herbicide resistance profile, effective soil-residual herbicides may include products in Groups 5, 15, and 27 for corn and Groups 5, 14 and 15 for soybeans.
  • In Roundup Ready 2 Xtend® soybeans and XtendFlex® soybeans, use of XtendiMax® with VaporGrip® Technology with other non-Group 4 products can help control waterhemp populations.
  • Scout fields after application and hand rogue plants that escaped the treatment.