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Waterhemp in Manitoba

Tammy Jones, Weed Specialist, Manitoba Agriculture and Resource Development

PIGWEED HAS BEEN an issue in Manitoba bean production (edibles and soys) in previous years with confirmation of more populations of Group 2 herbicide-resistant redroot pigweed and increasing occurrences of green pigweed/Powell amaranth. But another pigweed species caught many by surprise in 2019, even though the threat was imminent.

Waterhemp was confirmed in four new municipalities in the province this summer. While plants have been sighted in Manitoba in previous years, the severity of the infestation this year was a significant cause for concern. Previous discoveries in Manitoba involved small patches or individual plants, but this year’s detections involved a substantial number of acres and significant hours of roguing, mowing and spraying to destroy plant material. That will also mean years of monitoring and surveillance to ensure this Tier 1 noxious weed has been eliminated, as required by current legislation.


Waterhemp had been detected in the US Red River Valley in the 1990s, with the first confirmed herbicide-resistant populations discovered in Iowa and Illinois in 1993 to the Group 2 herbicide imazethapyr. The first Canadian confirmation of herbicide-resistant waterhemp (to that same active ingredient) occurred in 2002 in Ontario. And while the waterhemp populations were spreading in the U.S. and developing different combinations of herbicide resistance (to Groups 2, 3, 4, 5, 9, 14, 15 and 27), Manitoba had been relatively lucky to see a minimal number of detections. Media sources indicated that the threat was creeping closer, spreading across eastern North Dakota and being found in more counties in Ontario. So when the calls started coming in 2019, there should have been no surprise. As many have said, weeds do not respect borders.


Waterhemp has oval to lance or spearhead shaped waxy-looking leaves that grow three to six inches long with an alternate leaf arrangement on a hairless stem. It typically grows four to five feet tall but can grow to more than 10 feet tall. Waterhemp is dioecious (meaning there are distinct male and female plants) and many small green flowers form an inflorescence in July-September. The terminal inflorescence can be more than one foot long, with many thin lateral branches that produce, on average, 250,000 seeds per plant.

While the description of waterhemp does not sound at all like redroot pigweed, as small plants, they can look very similar. Two differentiators that are mainly used to identify waterhemp versus redroot pigweed are hairs on the stem and the seed head.


There is no easy answer to controlling waterhemp. It germinates throughout the summer when there is sufficient sunlight, which means that one herbicide application is unlikely to be effective for row crop situations. It is important to note that there are extremely limited post-emergent control options in edible beans and a few more options for soybeans as we see new herbicide- tolerant traits commercialized. Even with these options, weed control must be done when the weeds are small, less than 10 cm or four inches. “Rescue treatments” of herbicides on waterhemp result in poor control, at best. In addition, the Ohio State University Extension Agronomy team has indicated that the trend is for waterhemp to develop resistance to any new herbicide sites of action that are used in post-emergent treatments within about three cycles of use.

Crop canopy closure is key to reducing the germination and competition from waterhemp. That can be accomplished in a number of ways:

  • Use of soil residual herbicides
  • Narrow row spacings
  • Inter-row cultivation
  • Cover crops
  • Increased seeding rates

These strategies, in addition to crop rotation, machinery sanitation to prevent seed spread and destruction of escaped weed patches, will slow the spread of this significant weed. If a field becomes infested with waterhemp, Ontario counterparts estimate that weed control costs will increase by $45 to $60/acre. That is is why the eradication of waterhemp is the ultimate goal.