New cases of Tall waterhemp found in Manitoba

This is a Tier 1 noxious weed that Manitoba Agriculture wants to prevent from spreading

Manitoba Agriculture has recently confirmed more cases of Tall waterhemp in the province. It’s a tier-one noxious weed that must be destroyed no matter where it’s found.

Tall Waterhemp has been confirmed in four new Manitoba fields and there are rumours of more, Manitoba Agriculture weed specialist Tammy Jones said in an interview Aug. 2.

Tall Waterhemp is a Tier one noxious weed that must be destroyed no matter where it’s found, but that can include hand weeding within crops where practical, Jones said.

“One or two plants one year can be thousands of plants the next year…” she said. “It’s really aggressive. Its terrifying.”

The latest cases are in the R.M.s of Rhineland, where the weed was found in a different field two years ago, Reynolds and Whitemouth.

There was a previous case in the R.M. of Ste Anne.

Two cases of Tall Waterhemp were found in Manitoba in 2017. One plant was found last year.

Manitoba Agriculture will monitor the four fields and test the weeds to see what herbicides they are resistant to (click here for a distribution map of waterhemp in Manitoba).

Most Tall waterhemp in the United States is resistant to glyphosate.

Jones is urging farmers to report any weeds they suspect are Tall waterhemp. She worries some might be reluctant to do so, but if caught early the weed can be more easily controlled, she said.

Tall Waterhemp is a prolific seed producer with a rapid growth rate that outcompetes many crops. photo: Tammy Jones, Manitoba Agriculture

“I think a lot of people may let it go when they have one or two suspect plants,” she said. “In a couple of years that becomes a huge patch and that’s what has happened in at least two of these cases. It was tiny to begin with.

“There are so many unknowns it’s hard to figure out a management plan, but Manitoba Agriculture can help with that. If we can catch it early we have a positive story, but if we let it go it’s such a negative story.”

Manitoba Agriculture has information on Tall Waterhemp on its website.

Here’s what it says:

‘Tall Waterhemp has been detected in at least three municipalities in Manitoba in 2019. Tall waterhemp is a significant weed of corn and soybean production in the Midwest United States and has been confirmed in several counties in Ontario. While plants have been sited in Manitoba in previous years (see map), the severity of the infestations this year is 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, significant hours of rogueing, mowing and spraying to destroy plant material, and will require years of monitoring and surveillance to ensure the Tier 1 Noxious Weed has been eliminated.’


Tall Waterhemp has oval to lance or spearhead shaped leaves, 3-6 inches long, and:

  • alternate leaf arrangement
  • petiole is shorter than the length of the leaf blade
  • hairless stem
  • typically grows 4-5 ft. tall but can grow to more than 10 ft tall
  • dioecious – meaning there are distinct male and female plants
  • many small green flowers form an inflorescence in July-September
  • the terminal inflorescence can be more than 1 ft long, with many wiry lateral branches

Tall waterhemp emerges throughout the growing season (April -August), typically after most other summer annual weeds have been sprayed.

It will flourish when there is sunshine, as is often the case in row crops. Season-long competition by waterhemp (more than 20 plants per square foot) has been shown to reduce soybean yield by 44 per cent.

Waterhemp that emerged as late as the V5 stage in soybeans can reduce yields up to 10 per cent.

Tall waterhemp is a prolific seed producer, generally producing about 250,000 seeds per plant, although individual plants can produce more than one million seeds under optimal conditions.

Like most weeds, waterhemp seeds remain viable in the soil for several years.

Tall waterhemp has documented herbicide resistance to many different classes of herbicides. In the U.S.

Tall waterhemp has evolved resistance to at least seven herbicide classes, including Group 2, 4, 5, 9, 14, 15 & 27.

Many populations exhibit multiple herbicide resistances that include several herbicide families. For example, Group 2 and 9 (e.g., ALS inhibitors and glyphosate, respectively) resistance in Tall Waterhemp is fairly common.

Tall waterhemp populations detected in Manitoba are suspected to be resistant to glyphosate, along with one or two other herbicide classes.

About the author


Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.



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