Soybean School: Palmer amaranth confirmed in Ontario

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Palmer amaranth, the aggressive and highly competitive pigweed species that U.S. farmers have been tussling with for almost 30 years, has been found in Ontario.

Ontario Ministry of Agriculture, Food and Rural Affairs weed specialist Mike Cowbrough says DNA testing has confirmed that a plant found in a Wellington County field is palmer amaranth. He says the weed likely arrived as seed carried on machinery or feed moving north from the U.S. to Canada.

“Big picture is we found one plant, but are there more? Probably. But right now we’ve only found one. So this isn’t widespread by any means,” says Cowbrough. But the weed is “super difficult to control. So our best control is prevention.”

Cowbrough says early in the growing season, it’s difficult to distinguish Palmer amaranth from other pig weeds but the plant does become more distinctive as the season progresses.

“There’s not one feature that tells you automatically it’s palmer amaranth. There’s probably four to five that if you check off, there’s a high probability it’s palmer amaranth.” On this episode of the RealAgriculture Soybean School, Cowbrough uses the first plant found in Ontario to highlight five features that will help growers identify the weed.

First on the checklist is the length of the petiole, or stalk, that joins the leaf to the stem of the plant. In the video, he demonstrates how a plant with petioles that are longer than the leaf blade, could likely be palmer amaranth. Other distinguishing features include: a hairless stem; a larger stem than other pig weeds; a diamond-shaped leaf; and leaves that have a poinsettia look when observed in clusters. (Story continues after the video.)


When it comes to controlling the weed, Cowbrough notes that Ontario farmers will be able to draw on the work done on waterhemp by weed scientist Dr Peter Sikkema and his group at the University of Guelph.

“Practically speaking, your approach or your best practices to manage waterhemp are transferable to palmer amaranth,” says Cowbrough. On the herbicide end, effective soil-applied herbicides are the first line of defence, and then following those with effective post-emerge products. He notes that growers may have to utilize herbicide tolerant technology in both corn and soybeans to effectively incorporate this defence.

SEE RELATED: Going to war with Palmer amaranth — for $70 an acre

Long term, Cowbrough says “we’re not going to solve this out of the jug” — growers will have to incorporate other practices such as cover crops and tillage. Changing cropping systems and rotations to include crops where pig weeds don’t flourish, like winter wheat and winter canola, could also play a management role.

Are there things growers should be doing this fall when scouting and harvesting crops?

He says the plant should be easiest to identify during the fall in soybeans or at corn harvest. “If you find it, pull it out and destroy it so that it’s not setting seed and dispersing seeds so that the problem becomes bigger.”

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