Dandelions have been creeping up the provincial weed survey lists in the past 10 years to establish themselves as a persistent field crop weed, says a researcher who studied for her masters at the University of Manitoba on fall dandelion control.
“I looked at two different timings. We did a fall herbicide application which was post-harvest and we did a spring application, which was pre-seeding,” said Kristin Hacault, now a technical development specialist with BASF in Edmonton.
“We looked at three compounds at various rates. Glyphosate alone, in combination with fluorasulam, which is sold by Dow as PrePass, or tribenuron, which is Express.
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“We looked at those treatments in both the spring and fall, on dandelion control. We also did an in-crop herbicide application just to see what effect that would have on dandelion control.”
Hacault’s trials were done in spring wheat fields with AC Barrie under minimum tillage conditions.
“In terms of at least an 80 percent control and in terms of a statistical standpoint, we didn’t see any differences between the three-quarter litre rate of glyphosate and the labelled rates of either the PrePass or Express products, although we did see what I’d call enhanced control,” Hacault said.
“You did get a bit better burn down and you got some of the benefits those products provide: some residual control of some weed species. So I’m not saying you can’t use those, but in terms of a pure scientific standpoint, as long as you are applying in the fall, after harvest, the three-quarter litre per acre of glyphosate rate was performing the same as the labelled rates of the other products.”
Hacault said all the fall applications performed better than their spring counterparts. If you were going in the spring, it was better to add those top ups to get control.
“The fall applications work best for two reasons. First, there’s a theory that you have better herbicide translocation at the fall timing. Because it’s a perennial, it overwinters on its root stocks,” she said.
“All the carbohydrates and photoassimilates are being translocated down towards the root to prepare that plant to overwinter. The theory is you get enhanced translocation down that stream with those photoassimilates, carbohydrates and sugars. You’re getting more herbicide there faster.”
Hacault’s research also showed that when herbicides are applied in the fall, they are hitting all of dandelions’ physiological stages, at their most vulnerable time.
“You’re hitting the seedlings that were produced in the summer, you’re hitting the vegetative rosettes, which have the potential to flower the next year and you’re hitting those large rosettes that are reproductive and have flowered. In the spring, you’re missing the seedlings and in some cases, you’re not getting good enough translocation to kill some of the large rosettes,” she said.
“What I found with the spring applications is that you suppress the above ground matter and get above ground biomass control. But in the fall, when you look at that treatment with the spring application, you’re getting regrowth from the rootstock. You’re just getting suppression and top growth control during the season and it’s coming back in the fall. They wouldn’t flower that year, but if you left it uncontrolled it would be a problem.”
Hacault also examined the biology of how dandelions behaved in an annual cropping system.
“Dandelion is a simple perennial species, which means that although it reproduces from seed and pieces of rootstock, its sole source of population spread is seed,” she said.
“To control the plant, you have to control the seed or seedlings. Dandelion doesn’t form a persistent seed bank. The seeds germinate real quick.”
Hacault said dandelions can flower throughout the year, or only for short periods, depending on weather conditions.
“In 2003, we had one of the driest, hottest years on record. The dandelions I was working with flowered for about a two-week period, then senesced and that was it for the year,” she said.
“But in 2004, when it was cool and wet, we had multiple flowering periods. We had one peak in mid-June, then another peak at the end of August or early September. I think it’s that the conditions provided for the plant to keep on reproducing.”
While Hacault looked at fall versus spring applications, she didn’t get into fall timing. She waited until the crop was off and most of the dandelions had flowered but before a killing frost.
“There has been some research done on glyphosate that says it works better after a first frost, to a maximum of about -4 C. But I didn’t directly look at different times.”
In the first year of her master’s program, Hacault sprayed mid-September because the weather had become cooler and crops were off early. In the second year, it was cold and wet for much of the fall, so she couldn’t spray until October.
“I don’t think there was a huge difference in control. But one thing you want to watch is if you have plants that are flowering for a longer period into the fall, you might want to wait until they’re done flowering and setting their seed,” she said.
“You do want to control those seedlings. If they’ve set seed, you might want to wait a couple of weeks until the seedlings pop up and then you can hit it. But some of the residual products, like PrePass or Express, can help with that. You can go in earlier and still get seedling control in the fall.
“In my last year, I got about a week of frost. It wasn’t severe but it kind of purpled them up, and we still got decent control. It didn’t look good in the fall, but once we came in the spring, the treatments looked pretty good.”
Like most weeds, dandelions need a preferred list of weather conditions to flourish. Hacault performed her research in 2003 and 2004 in Manitoba.
“In 2003 we had a dry year and 2004 we had a wet year, but over the past decade, dandelion has been creeping up in the Manitoba weed survey. We’ve hypothesized that zero tillage seems to provide the conditions required by dandelion to thrive and survive,” she said.
“We’ve also found there’s been a reduction in post-harvest herbicide applications and that’s why dandelion is thriving. But most importantly, any cool, wet conditions, especially really wet falls, will mean a dandelion outbreak.
“In 2004 in Manitoba, we had one of the wettest, coolest summers on record and we had a population explosion of dandelion. Because it was so wet and harvest was delayed, a lot of post-harvest herbicide applications didn’t go down. That meant the problem was going to be worse in the spring of 2005.”
For the spring applications, Hacault applied herbicide as a spring burnoff. She couldn’t go in-crop because all the combinations had glyphosate and she was working with glyphosate-susceptible spring wheat.
“We found that farmers who think they’re going to control it with an in-crop herbicide application, first of all it wasn’t strong enough to kill some of the larger rosettes,” Hacault said.
“We were using things like Refine, Horizon and Buctril. The in-crop herbicide timing, which is at the three- to four-leaf stage of the wheat crop, did not coincide with (dandelion) seed production. The seedlings came on after, so we didn’t control them with an in-crop herbicide application. The plants flowered, they set their seed, but the in-crop herbicide application had no effect on those seedlings.”
Hacault’s trials were in farmer fields and part of her study included surveys with the landowners.
“They all had persistent dandelion problems. We did chemical histories and for a couple of the growers, for the past five years they had used spring herbicide applications to try and control dandelion and they said it was always a persistent problem. Once we switched them to fall applications, it really helped out.”
Hacault said with a fall dandelion spray application, she feels producers can achieve decent control for at least a couple of years.
“Because dandelion is a wind disseminated species, you can do all the control you want, but if there’s plants in your ditches or a neighbour’s fields that are left uncontrolled, you can still have a problem even though you do everything right.”
Hacault said farmers who are considering fall dandelion control might want to consider managing their trash cover to prevent it from impeding the herbicide reaching the plant.
“Dandelion has a prostrate growth habit. You might want to do some harrowing or move some trash around so you can get to the plant.”
While her study didn’t directly involve Canada thistle, Hacault took a look at that weed and found it reacts differently than dandelions.
“Other researchers have found pre-harvest applications work better than a post-harvest like you would with dandelion. The problem with a pre-harvest application with dandelion is that because it sits so low to the ground, you don’t get all the chemical down to the plant,” Hacault said.
“Canada thistle stands at the top of the canopy, can intercept the chemical and translocate it down, but because dandelion is primarily seed-spread, at the time of the pre-harvest herbicide application all the seedlings might not be up.
“While Canada thistle produces a lot of seed, most of it is unviable. Most of it is rhizomes that contribute to its spread, I think that’s why it’s more successful in the pre-harvest period.”
