Waterhemp difficult to distinguish from pigweed

Using herbicide programs effective on pigweed, should cover waterhemp too, so far

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Waterhemp and pigweed species are difficult to differentiate, until they become larger – often past the ideal stage to control the waterhemp species.

Mike Cowbrough, weed specialist with OMAFRA, says the herbicide programs identified by the University of Guelph Ridgetown campus, which are effective on waterhemp are equally effective on pigweed and other species.

“There is already benefit to using those programs and you’re also covered if you happen to have waterhemp.”

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Until you see it a number of times waterhemp is difficult to identify, and most people aren’t that confident in what they’re looking for, says Cowbrough.

Why it matters: Herbicide resistant waterhemp has been confirmed in seven counties across the province. Treating all amaranthaceace family weeds the same can help manage this problem weed.

It’s easier to pretend all amaranthacease family weeds farmers see are pigweeds.

“That was always one of my key messages, ‘do you have pigweed on your farm’, if the answer is ‘yes’, then let’s go at it like we are managing pigweed – a very competitive, prolific weed and if we happen to have waterhemp, we are covered.”

Pigweed is a common weed within Ontario, one of the first to emerge and is one of the most competitive. Farmers selecting a pre-applied residual herbicide, effective on common weeds, such as pigweed, lambs-quarters, and foxtail, but also on waterhemp will have a better weed management system, he says.

“This strategy will more often than not will have an economic benefit and will protect you from waterhemp, a species you won’t likely know you have until its too late.” says Cowbrough.

The most effective herbicides against these species are soil applied, with two modes of action minimum, in groups 14 and 15 as your foundation.

The Elora Research station, in both 2017 and 2018 saw a six bushel to the acre benefit adding a residual herbicide with glyphosate prior to soybean emergence.

It’s important to keep the fields clean during the critical weed free period; for soybeans – the first to third trifoliate, and corn – emergence to six to eight leaf stage.

Controlling it at any early stage is crucial. Waterhemp’s ability to produce tens of thousands seeds allows this weed to spread quickly.

If unable to control the herbicide resistant waterhemp early, growing crops which are herbicide resistant, allows farmers to use 2,4-D, dicamba and Liberty (glufosinate) post emergence.

Controlling the spread

If producers identify waterhemp, whether herbicide resistant or not, on their operations, controlling the spread is the next crucial management strategy.

Waterhemp is a cross-pollinating species – cross breeding and variability is more common through out these types of species, making it is easier for the resistance gene to spread.

When scouting fields, it’s important to clean socks and boots as they are an easy attachment site.

Cowbrough said last fall while scouting a field in Norfolk County, he was worried about spreading fleabane, so he checked over his boots for seeds.

“No fleabane in the bottom of my boots, but there was a lot of ‘pigweed’ seed, is it redroot, green or waterhemp – I’ll have no idea until it germinates. That’s how this stuff tends to spread, accidentally.”

Fields with these problem weeds are important to harvest last to be able to do a thorough clean out of them afterwards.

“It’s easy for me to tell you that after every field we should be washing equipment, but the likelihood of that happening is [low]. Farming in areas with waterhemp, there needs to be some thought process and plan so when done those fields a thorough decontamination can be completed.”

If farmers have a waterhemp issue on their farm year after year adding wheat to the rotation is an added benefit. The lack of amaranth species in winter wheat is due to the top growth presence when the weeds are wanting to germinate.

“By the time the pigweed species, like waterhemp, is wanting to germinate they are having to do so in an environment with not a lot of light, and they don’t like that.”

About the author


Jennifer Glenney

Jennifer lives on a farm in Cayuga, Ontario and has a lot of experience in the many aspects of agriculture.



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