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Canada thistle still on ‘least wanted’ list

Early and thorough control is essential to keep Canada thistle under control

Canada thistle

Canada thistle has been a thorn in farmers’ sides since before Confederation.

In 1865, when the Canada Thistle Act of Upper Canada was signed into law, Her Majesty required all citizens of Upper Canada “to cut, or to cause to be cut down all the Canada thistles growing theron.”

The Act continued, “If any owner, possessor, or occupier of land shall knowingly suffer any Canada thistles to grow theron and the seed to ripen so as to cause or endanger the spread thereof, he shall upon conviction be liable to a fine of not less than $2 nor more than $10 for every such offence.”

The value of the dollar has gone up since 1865, but producers are used to counting the cost of Canada thistles.

“The Canada Thistle Act started legislation for weeds,” says Nicole Kimmel, a weed specialist at the Pest Surveillance Branch of Alberta Agriculture and Forestry. “It’s been listed ever since.”

The reason for Canada thistle’s remarkable persistence is twofold — the plant is allelopathic, meaning it has an inhibitory effect on other plants, and it quickly regenerates from cut root pieces. In fact, only 10 per cent of Canada thistle comes from spreading seeds, Kimmel says. The majority spreads through root fragments.

“All you need is a root piece that’s a quarter of an inch by an eighth of an inch round. That’s enough to start a new Canada thistle plant,” says Kimmel. “And those roots can extend down six meters. So Canada thistle is tapping into nutrients and water that little else reaches.”

As Canada thistle has such a great amount of biomass in the root system, it can only be controlled with aggressive weed control measures.

Control options

“The worst thing you can do is go out and spray Canada thistle one year, assume it’s controlled, forget about it next year and go back to a herbicide with only top growth suppression,” says Bill May, a crop management agronomist with Agriculture and Agri-Food Canada.

“For control, you need two years of concerted efforts, either in-crop with clopyralid or pre-harvest or post-harvest glyphosate, depending on the growing season, to start to knock down the Canada thistle density,” he says.

Good to excellent control of Canada thistle in canola and flax can be achieved with the Group 4 herbicide cloypyralid (Lontrel) added into a tank mix, he adds. “Pulses are the big problem and with the current price of lentils I expect producers are going to end up with lentils on fields that are not as low in perennial weeds as they would want,” he says.

May was involved in a four-year study of herbicide timing and intensity in the control of Canada thistle from 2004 to 2008. He says pulses suffered the largest yield decrease from Canada thistle, but over two to three years it could be controlled with the use of herbicides. However, it only took a couple of years for Canada thistle to re-establish in fields once herbicides used in those fields shifted away from those that controlled the weed.

“There’s a huge amount of biomass in the root system of Canada thistle. You need to select a herbicide that doesn’t just burn off the tops, but translocates into the roots,” says Kimmel.

Kimmel says tillage used to be one possible control strategy for growers battling the noxious weed — but the measure was labour intensive, requiring repeated passes over summer fallowed fields. Since the zero-till movement, which emphasizes minimal soil disturbance, and virtually eliminates summer fallowing, growers are turning to chemical controls.

“If no-till growers can do a good job of cleaning up fields from Canada thistle prior to moving to no-till, they have a lower risk of Canada thistle re-establishing because they’re doing minimal disturbance,” she says.

For growers who do not yet have Canada thistle in their fields, Kimmel recommends staving the weed off with aggressive preventative measures.

“Clean equipment between fields, especially if equipment comes into contact with Canada thistle,” she says.

“If you do have Canada thistle problems, broadleaf crops and hay and pasture offer challenges in thistle control. Any herbicides that you use to control Canada thistle will kill alfalfa and clover. There’s no herbicide that selects specifically for Canada thistle.”

May emphasizes that the use of good agronomic practices to promote crop growth can help growers combat weed pressure. The list includes the use of appropriate nitrogen fertilizer and other nutrient rates, as well as appropriate seeding rates — high enough to get a good canopy and quick closure.

Despite the high biomass of Canada thistle, Kimmel says cows prefer not to eat it, so even its value as a viable feed crop is low.

“It’s tenacious and aggressive. As a weed, it’s done very well for itself. If we could get crops to grow the way Canada thistle does, we’d be laughing,” she says.

About the author


Julienne Isaacs

Julienne Isaacs is a Winnipeg-based freelance writer and editor. Contact her at [email protected]

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