Dandelions are among the toughest of weeds. Deep-rooted, perennial and robust, they are a particular problem right now due to a combination of factors. “The last three seasons have been next to ideal for dandelions,” says Gilles Quesnel, field crop integrated pest management program lead at the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). “We had the third wet June in a row in 2010, and if seeds blow on to wet soil in June, they will establish. If it’s dry, there’s less chance of germination. The crop will also have covered the ground, shading late-emerging dandelion seedlings. It all comes back to moisture in June.”
Brian Woolley, sales manager for row crops at Bayer CropScience, agrees that dandelions are definitely a growing issue in Ontario. However, he sees this development as being more of a function of management practices, principally the continued decrease in the use of deep tillage and a significant increase in the amount of glyphosate used on an annual basis at annual weed control rates. He adds that, “It’s natural that increased dandelion populations will create more seed for the seed-bank, and thus, the problem will continue to at least maintain itself.”
In winter wheat, Woolley believes that increased dandelion presence is resulting from seeding primarily in a no-till fashion almost immediately after soybean harvest. “A large number of the dandelions seen in the spring in the winter wheat crop were already present in the field at soybean harvest or very soon thereafter. They begin to grow dramatically when the soybean canopy is removed,” Woolley says. “They can be quite large by the time winter arrives.”
The only option left for growers in this situation, says Woolley, is a spring-applied post-emergence herbicide such as Infinity or Estaprop. “But this isn’t a silver bullet,” he cautions. “Dandelions are very hard to kill in the spring because they’re drawing on large taproots to push up fast and produce flowers.”
Excellent top-growth control is still possible if the weeds are not too big, he says, but control of the whole plant to the point that it does not regrow and reflower is not possible. “This is mainly because we’re not getting herbicide to the root system,” he notes. “Even glyphosate does not do its best job in the spring on dandelions because insufficient product gets to the roots.”
Farmers facing large dandelions in the spring should therefore use two times the standard Roundup rate, says Mike Cowbrough, OMAFRA weed management field crop program lead. “You’ll also need to use a residual to deal with germination of new seedlings in corn and soybean,” he stresses.
Fell dandelions in the fall
It is strongly recommended instead to control dandelions before winter sets in. “In the fall, dandelions are building their root system to create as much energy reserve as possible,” says Woolley. “At this time, herbicide is taken deep into the root system and the plant is killed. So I suggest holding off on planting wheat, letting the dandelions green up after soybean harvest and hitting them with a significant dosage of glyphosate.”
Cowbrough agrees. “The only chance of penetrating the taproot is in the fall,” he adds. Cowbrough says that based on all the work that Dr. Peter Sikkema, professor of field crop weed management at the University of Guelph’s Ridgetown Campus and his colleagues have done, the most effective means of controlling dandelion are fall applications of glyphosate applied at 1350 to 1800 gai/ha (which equals 1 to 1.5 L/ac of the “old” 360 g/L glyphosate concentrations).
Woolley concludes that applying a high rate of glyphosate will result in a much cleaner winter wheat crop during the over-wintering period and allow spring post-emergence herbicides to be far more effective because the majority of annual weeds will be easier to kill. “However,” he advises, “application should occur prior to freezing temperatures so that dandelions are still actively growing.”