How does weather impact the type of weeds farmers see in their fields each spring?
'Every year is different' is a phrase that's often applied to agriculture and warm versus cool spring conditions can certainly impact weed pressure and weed control challenges, says recently retired University of Guelph weed scientist Dr. Peter Sikkema.
At a Ridgetown agribusiness breakfast meeting this week, Sikkema said he is surprised by the impact that weather can have on weed species competition. At a farm where's he's conducted research trials for 35 years, he noted that one year he would see the field dominated by ragweed, but 12 months later it would be overpowered by lamb's quarters. "It's the same seedbed, and the only difference is weather from one year to the next and it amazes me the impact of weather on weed species and composition."
Sikkema says the key driver is early-season temperatures that can favour one species over another. "Once the one species gets established, it outcompetes the others. And the seedbank, I don't think, changes appreciably from one year to another."
This weed behaviour is really driven by temperature, says University of Guelph weed scientist Dr. Isabelle Aicklen. She says the early-season temperatures in which different annual weeds will prosper does vary. "If there's a prolonged window where temperatures are lower, then some weeds will be more likely to germinate sooner." She notes that pig weeds and waterhemp tend to prefer warmer early-season temperatures.
Ontario growers have been wresting with waterhemp since glyphosate-resistant plants were first discovered in Lambton County in 2014. Since that time, the resistant weed has spread across the province and into Quebec. Many growers worry that weed control will get even tougher with the pending arrival of palmer amaranth, the aggressive and highly competitive pigweed species that U.S. farmers have been tussling with for almost 30 years,
Palmer amaranth was first identified in the province in a Wellington County field in 2023, but Sikkema says those cooler conditions that cause different species to dominate in different years could play a key role in preventing the weed from gaining a foothold in Ontario.
"Palmer is the number one weed in the southern U.S. It's been in the Corn Belt, the 'I' states for many years," says Sikkema. But the weed has not set down significant roots in the northern tier states. "It's been in Michigan for 15 years, but it's not the issue — waterhemp is the issue."
Sikkema says waterhemp is a challenge for Ontario growers because of its ability to adapt to northern growing conditions, a trait that palmer amaranth has yet to display in a significant manner.
