Propagation
Waterhemp is a dioecious plant, meaning it has male and female plants that cross-pollinate. This allows for more genetic variability and therefore can adapt quickly to different environments or management changes.
Waterhemp emerges throughout the growing season. A higher percentage of plants germinate later in the growing season than most other annual weeds. It is very prolific, and can produce over 1 million seeds per plant in a non-competitive environment. Waterhemp plants that emerge after crop establishment produce smaller plants and significantly fewer seeds than plants that emerge with the crop..
Viable seeds remain in the soil for several years, however after 4 years there is a rapid decline in viable seeds. Because the seed is small, it can only emerge from shallow in the soil profile. Therefore waterhemp emergence is greater in no-till and minimum till systems than conventional tillage systems.
Similar species
Waterhemp can be easily misdiagnosed for redroot or green pigweed.
Management
Waterhemp management requires a diversified integrated weed management plan. This should include a diverse crop rotation, multiple herbicide modes of action every year on every field, cover crops, and strategic use of tillage. Cultural practices that increase the competitiveness of the crop such as planting rates and row widths contribute to effective management of waterhemp.
A two-pass program provides the most effective and consistent control. Given the prolific nature and extreme weed pressure waterhemp can produce, one should strive for near-perfect weed control to reduce the weed seed return to the soil. Do not rely on a one-pass herbicide program. Using a pre-emergence residual herbicide is critical to manage waterhemp due to the potentially extreme pressure of waterhemp, how fast it can grow (up to 1” per day), and the extended window of emergence through the growing season. Plan to follow up in-crop with an effective herbicide treatment.
Tillage – Multiple studies by Refsell and Hartzler (2009) show that waterhemp emergence is greater in no-till than conventional tillage systems and germination continues longer into the growing season in no-till.
Winter Wheat – No research has been done in Ontario for control of waterhemp in winter wheat as it is not typically a problem weed in this crop. Because winter wheat is competitive in the spring, having it in rotation is a good part of a weed control management strategy for waterhemp.
Soybeans – Boundary LQD, Fierce, Authority Supreme, and Triactor are effective herbicides for waterhemp control. If a follow up post emerge application is needed, Reflex + Turbocharge or Blazer are excellent options, providing about 80% control.
Corn – Most HPPD inhibitors are effective in controlling waterhemp in corn, however there are biotypes resistant to group 27 herbicides in the US so it’s critical to use multiple modes of action. Acuron pre-emerge gives effective and consistent control, as does Lumax EZ. Other effective pre-emerge options include Callisto + AAtrex, and Converge XT. Post-emerge treatments include Acuron , Lumax, Callisto + Atrazine, Converge or Armezon + AAtrex. If a group 27 herbicide is used pre-emergence and a post-emergence application is required, dicamba is a good option. If waterhemp is at 6-8 leaf at the time of application, then the high rate of dicamba should be used.
