HOLLAND, Man. –To control Canada thistle, people should go to the root of the problem, advises Rene Van Acker, an associate professor at the University of Manitoba’s plant sciences department.
The perennial weed is not a prolific seed producer and its seedlings are not competitive, said Van Acker during a municipal weed control meeting held in Holland on March 21.
What makes the plant difficult to control are its roots, which can spread up to six metres in one year.
New thistle shoots can start from root pieces as small as eight millimetres in length, according to Van Acker, and those pieces can survive up to 100 days. Canada thistle has been known to put down roots as deep as 4.5 metres.
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“Root spread can be very, very prolific,” he said.
That is why the focus for killing Canada thistle must be at its roots. Van Acker considers repeated applications of herbicides to be the best strategy for control.
“The products where we seem to get control or better control are systemic products …. They go down to the root reserve.”
In a handout provided at the weed control meeting, Van Acker said pre-harvest applications of glyphosate have proven effective at controlling Canada thistle. That’s because the timing coincides with the plant’s late flowering stage when its root reserves are low.
Pre-harvest applications have proven far more effective than post-harvest applications, he said.
In the handout, Van Acker discussed herbicides that can be used in combating Canada thistle.
- Picloram works because of its effects on root buds.
- Lontrel and products containing Lontrel, such as Curtail and Prevail, are consistently given superior control ratings for Canada thistle, especially when applied at the pre-bud stage.
Although Lontrel offers a good in-crop option for controlling Canada thistle, Van Acker said that approach is less effective than pre-harvest glyphosate applications.
- Group 4 products like 2,4-D, MCPA and Dicamba are also in-crop options. However, the ability of those products to control Canada thistle is below that of Lontrel, said Van Acker.
He said tillage can also give effective control if done intensively. Tillage helps deplete the root reserves, but needs to be done repeatedly through the growing season.
Mowing every three to four weeks can be effective. That may not be an option in crops like cereals, but Van Acker said it could work in forages, unless they are being grown for seed.
When mowing and in-crop tillage are not possible, post-harvest tilling can be a critical time for control of Canada thistle, he said. That is when the plant’s root reserves are low.
Van Acker focused on Canada thistle because of the weed’s prevalence in Manitoba.
The 1997 provincial weed survey found it is the fourth most common weed species in the province. It is also the most common perennial weed in Manitoba.
“It’s very abundant. It’s pretty well everywhere in the province.”
The number of Manitoba fields with Canada thistle has changed little during the past decade, Van Acker said. However, the weed becomes more of a menace during wet cycles, such as the one that began in 1997 and has continued to the present.
