Beware invasive, ‘frightening’ waterhemp

There’s a threat lurking on the edge of Manitoba farmers’ fields this fall that’s even scarier than the escalating trade war with their biggest customer.

Provincial extension advisers are warning farmers to watch for a weed known as “tall waterhemp,” which, true to its name, may already be towering above ready-to-harvest crops.

Those wispy tentacles waving in the breeze look pretty innocuous from a distance. But this highly invasive member of the pigweed family produces hundreds of thousands of seeds that are just itching for dispersal through farmers’ combines and across their fields.

Each of those tiny seeds — one small pill container filled halfway holds 44,000 — can produce plants with stems the width of saplings and heights of two to five metres. One or two plants can quickly turn into a patch. If waterhemp takes over a field, it can choke out the growing crop and cut yields by more than 90 per cent.

Provincial weed specialist Kim Brown doesn’t hold back when talking about its potential to cause widespread economic harm. “This is frightening,” she said. “This is a weed that is capable of taking out pretty much your entire crop.”

It’s her job to convince farmers to take it seriously.

Believed to have hitchhiked into the province from the U.S. on farm equipment and in bird droppings, waterhemp has gained ground quickly since it was first spotted in Manitoba in 2016. It’s now found in all municipalities on the eastern half of agro-Manitoba and has recently been identified as far north as the Rural Municipality of Dauphin and in the RM of Ellice-Archie, which is adjacent to the Manitoba-Saskatchewan border.

Waterhemp grows 50 to 70 per cent faster than other weeds, achieving daily spurts of a 2.54 centimetres or more. New plants germinate throughout the growing season in successive flushes from spring to freeze-up. It can produce viable seed within a month of pollination.

“I have seen them germinate at the end of September and still have seed on them by the end of October when we had a late freeze,” Brown told an extension webinar this week.

Plus, it can be difficult to identify, often mistaken for its cousins Palmer amaranth or redroot pigweed. “It’s a bit of a chameleon,” she said. “In corn, it gets really tall and skinny. In dry beans, it’s kind of short and fat.”

Like many weeds on Manitoba farms, these plants are rapidly developing resistance to herbicides farmers have in their toolbox. Farmers elsewhere have diversified crop rotations, changed the timing of weed control and now use different combinations of herbicides to keep it in check.

Slowing its spread starts with doing whatever is needed to avoid spreading seed via harvesting equipment. Tactics include mowing field edges and drains that are often the entry point for new weed species into a field.

Single plants or small clusters should be pulled out by hand or dug out with a spade before they go to seed, then kept in garbage bags until they rot.

“Keep garbage bags on the swathers, the combines, and the trucks,” Brown said. “Get that seed off the field.”

One of the strategies Ontario farmers are using to stop its spread is an attachment placed on their combine that pulverizes any weed seeds that make it through the harvesting process before they are deposited back onto the field. Although the technology is relatively new and not yet widely used, weeds such as waterhemp may speed up its adoption on Prairie farms.

Brown said farmers should also boost biosecurity protocols by cleaning machinery thoroughly before moving to a new field. Anyone entering the field on foot should don disposable booties and dispose of them afterwards.

All in all, this superweed is going to increase farmers’ costs without offering much potential for increased returns.

“We’re not going to stop the spread. We’re far too late for that. It was inevitable that it was going to get here and it was going to get moving around,” Brown said. “But we can slow it down.”

Laura Rance-Unger is editor emeritus for Glacier FarmMedia. She can be reached at lrance@farmmedia.com