Weed of the Week: Canada thistle

One of Canada’s nastier pests originally emigrated from Europe and then made its way west.

Its European name describes it perfectly: creeping thistle.

But Canadians have adopted it and call it their own — Canada thistle.

It likely arrived as seeds, but once started, it laid down roots and started to colonize.

Root buds will spread up to six metres, creating the term thistle patch.

But the real success of the weed comes from what its heavy roots can do when it comes to hunting for resources, especially water. The pest is able to send down its white, ropy roots as far as three metres, allowing it to grow through otherwise terrible drought conditions.

There was an old-fashioned cure for Canada Thistle, but it involved excessive tillage and, for smaller patches, this can still be an acceptable method of control.

Cultivation should begin in fall. It often takes several passes and should be done with an eye to keeping the plants from growing more than five to eight centimetres tall.

If they grow more than this the weeds will be able to store energy in their roots and survive the winter.

The problem with tillage is that Canada Thistle can reproduce from small root pieces, so the practice can also help spread the pest.

The weed can grow up to 1.5 metres high if its allowed to reach maturity and set seed, which can happen in as little as two weeks from emergence.

A mature plant often produces about 1,500 seeds, but can reach 5,000.

Flowers range from purple to pink or white.

The tufted, airborne seeds often germinate within a year but are capable of lying dormant, buried in soil for up to 21 years, waiting for tillage that will bring them to the field’s surface.

Control can be frustrating — early spraying of established patch often only slows it.

The weed is most vulnerable to herbicides during the early bud stage. After a short time of herbicide ingestion at this point, tillage can be effectively used. Several broadleaf herbicides suppress the weed through the growing season and glyphosate can be used in the fall to starve the plants out and allow winter to finish them off.

Applications of dicamba with mecoprop, DyVel DSp, or dicamba with MCPA are good bets. High rates of 2,4-D or mixes of clopyralid and imazethapyr, Lontrel and Odyssey, can give extended control but can also affect choices of the following year’s crop.

Curtail, clopyralid and MCPA can be used in barley, oats, spring wheat, canaryseed, flax and timothy hay. When the weed has recovered after harvest, glyphosate can be effective.

Farmers can use Prestige, a mix of fluroxypyr, clopyralid and MCPA are effective in spring cereals, canaryseed, forage grasses and timothy. On the forage grasses, when used for seed, and canaryseed producers should recognize this falls under minor-crop use permits and can only be used for seed production, haying and grazing are not permitted.

Thifensulfuron and tribenuron, known as Refine SG, Deploy or Nimble, is another set of chemicals that can be used in cereals and some grasses. A variety of tank mixes can be used with this combination.

Pastures and hayland are not immune to Canada thistle.

Registered for suppression in pasture are Tordon 22K, which is picloram, on its own or with 2,4-D, that pairing is called Grazon. In either case manure from animals having eaten this combination should not be spread on cropland. A mix of 2,4-D and aminopyralid, called Restore can also be used, however the land should not be broken up and replanted to broadleaf crops for a minimum of three years.

Pre-harvest applications of glyphosate are effective, even more than if done in the fall. Producers need to remain vigilant about avoiding its use in cereal crops until these have passed below 30 percent moisture.

Herbicide tolerant crops can be used effectively to control the weed.

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