Waterhemp

Waterhemp (Amaranthus tuberculatus) is a summer annual broadleaf weed that is a member of the pigweed family. Originally from south central United States, waterhemp has been spreading north and west in agricultural fields. Waterhemp is an aggressive, fast-growing, and highly adaptable weed that poses significant challenges to crop production in Manitoba as it has developed resistance to most commonly used herbicide groups. Five way resistance to groups 9, 2, 5, 14, and 27 develops rapidly. Row crops such as soybeans, dry beans, corn and sunflowers are most impacted by waterhemp competition.

Biology

  • Summer annual, germinates in late spring and continues emerging throughout the growing season, even post harvest.
  • Can grow up to 2.5 cm per day under optimal conditions.
  • Dioecious (separate male and female plants), promoting genetic diversity and rapid adaptation.
  • Each plant can produce up to 4.8 million seeds per season under ideal conditions.
  • Seeds are small (less than 1 mm), facilitating spread via water, wind, animals, equipment, and contaminated seed or feed.

There are many different pigweeds in Manitoba. Some are common agricultural weeds such as red root pigweed, powell's or green pigweed and tumble pigweed. Waterhemp, smooth pigweed, and Palmer amaranth are more problematic species and are more difficult to control, so proper identification is important.

 Waterhemp Identification - See Waterhemp Factsheet 

 Pigweed Identification

 Waterhemp Distribution Map

 

Waterhemp Management

Waterhemp is currently classified as a Tier 1and Tier 2 Noxious weed in Manitoba depending on the Rural Municipality (RM) and when found requires immediate action for removal. For detailed RM information see Noxious Weeds list.

  • Pull or dig up plants with roots intact and remove from field.
  • Do not cut or put plants through combine.

Controlling Waterhemp requires using all the tools in the weed control toolbox. 

  • Diverse crop rotation with competitive crops in the mix.
  • Use of narrow row spacing.
  • Use of strategic tillage.
  • Use of pre -and post- emergent products with multiple modes of action.
  • Harvest weed seed control to prevent seed from spreading in the field through the use of chaff lining, chaff tramlining, bale direct, chaff wagons or weed seed destructors on combine.

Controlling Noxious Weeds FAQs

 

Additional Resources

Patrolling for Waterhemp and other herbicide resistant pigweeds

 

 For more information see Noxious Weeds in Manitoba