The tenacious dandelion, as everybody knows, is a hard foe to battle and stubbornly re-occurs time and again despite the best efforts to eradicate it. What makes dandelion particularly hard to get rid of are its two growth periods throughout the year: early in the spring and again in the fall. Dandelions are largely dormant throughout the growing season, when the crop canopy deprives them of essential light and moisture.
“The best time to control dandelions is during their growth periods,” says Ken Sapsford, a research assistant at the University of Saskatchewan. He’s involved in university’s four-year dandelion control study. “So if you haven’t done anything in the fall, then spring application is the time to hit them.”
Initial trials that compared spring versus fall applications of different herbicides found that a fall application was generally more effective, but in both cases the earlier the application could be made, the better.
“We found we got a little better control when they were in the flowering stage,” says Sapsford. “But if we waited until that stage, we got lower crop yields because dandelions can suck so much moisture out of the soil surface that it will affect the growing crop. So our best timing for control of the dandelions in the spring was as early as we can get out there.”
In fall, a September application was much more reliable than an October one, largely because of weather risks. “September gave us more consistent control, and that is because by the end of October some years we have had a killing frost so the products we were applying were not able to trans-locate into the crown and control it,” explains Sapsford.
Which herbicides were most effective depended on the crop to follow. Sapsford says the trials used applications of PrePass, Express with a tank mix of glyphosate, or straight glyphosate at a rate of one litre per acre.
“PrePass was one of the best ones,” says Sapsford. “The residual control did a good job of controlling dandelions in spring application.”
PrePass, however, can only be used if cereals are being seeded as the next crop. It cannot be used before oilseeds or pulses. “But all the treatments did control a majority of the dandelions that year,” adds Sapsford.
These rules of thumb have been further refined using data from the study, which compared not just timing and different weed control products but the intensity of applications, too.
“We are looking at more of a systems approach for controlling dandelions because we have found out that any one-shot treatment does not control and remove the dandelions completely,” says Sapsford. “They come back either because you have not controlled all the perennial plants, so they keep growing from the crown, or they re-germinate. We all know that dandelions produce hundreds if not thousands of seeds.”
On a four-crop rotation of wheat, barley, canola and peas, the team experimented with the three herbicide treatments, as well as fall versus spring applications and six different levels of intensity.
The lowest intensity was one fall application and then no further treatments for three years. “That is quite often what will happen on the farm,” says Sapsford. In this instance the dandelion population in three years was back up to the same level as a check plot that had received no herbicides to control dandelion. “So that one shot does not work,” says Sapsford. “Three years later they are back.”
Other plots were treated every second or third year at different times, and the highest level of intensity involved an application every fall for three consecutive years.
“Our best treatment is where we’ve hit it in the fall a minimum of two years back to back,” Sapsford concludes. “And this is where we would do the treatment of PrePass in the fall if we are coming in with cereals the next year. Or we use just the equivalent of a full litre per acre of glyphosate in the fall if we are coming in with the peas or canola.”
Spring application can also have an effect on crop yields, particularly in years where moisture is a concern, because the dandelions can quickly dry out the soil in the spring says Sapsford. But at the end of the day, if dandelions are present, a spring application is preferable to doing nothing.
“You might be losing a little bit of yield but you’ve got to do something to control those dandelions that year,” says Sapsford. “My recommendation would be if you have a dandelion problem this spring, come in and treat it, but do a follow up treatment in the fall. Then follow up that treatment for at least two years, with fall treatments, to better remove the dandelions from your field.”
Angela Lovell writes from Manitou, Man.