Canada thistle’s ability to thrive in most prairie field conditions makes it a tough problem for producers.
The good news is that despite its prevalence across the West, this member of the sunflower family is often confined to patches where it can be targeted for control.
Creeping thistle, as it is known in some parts of Europe, starts from seed but becomes a serious issue as a perennial plant.
American farmers blamed Canadian traders for bringing it into the New England colonies, hence the name, but it likely migrated from the Mediterranean to both countries at about the same time during colonization.
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This unwelcome European immigrant might start from a seed, but colonies often form from root buds that spread up to five metres from the parent plant.
Canada thistle can resist drought by sending down roots as deep as three metres in search of water and nutrients. It is easily spotted thriving in drought conditions while the crop around it fails.
Plants grow as high as 1.5 metres and sprout flowers that are often purple but can also be pink or white.
The tufted, airborne seeds generally germinate within a year, but are capable of lying dormant, buried in soil for up to 20 years, waiting for a tillage operation that will bring them to the surface.
Black summerfallow was the old-fashioned cure for Canada thistle. In the days before reduced tillage, the pests would hang around field edges, fence lines and ditches, where the steel didn’t reach.
Cultivation should begin in fall with multiple passes to prevent the plants from growing more than five to eight centimetres tall. Plants that are allowed to develop taller than that will replenish their energy stores in their roots and survive the winter.
The plant is most vulnerable to control during the early bud stage. This is when chemicals have the greatest impact and can be used in combination with tillage. A variety of broadleaf herbicides can provide top growth suppression.
Producers should think about systemic herbicides with this pest because it recovers from top growth loss quickly.
Saskatchewan Agriculture suggests several methods of control. The rosette method combines tillage and a systemic herbicide.
The ministry said tilling or mowing thistles will drain the root of more reserves than if the first flowering stem was left to grow.
New shoot growth that emerges during periods when day length is less than 15 hours will form rosettes and continue to produce carbohydrates for storage in the roots.
Tillage or mowing that will generate rosettes can take place south of Regina after July 25, Regina to Saskatoon July 29, Saskatoon to Prince Albert July 31 and north of Prince Albert after Aug. 2. The same guidelines apply in Alberta and Manitoba.
Tillage should be done to five cm, and producers should then wait four to six weeks to allow maximum rosette emergence before spraying with a systemic herbicide. Rosettes should be five to eight cm across at treatment.
The combination of defoliation and harassment culminates with the ingestion of the herbicide into its sap and roots.
A post harvest application of glyphosate effectively starves the plant over the winter and is the best strategy for killing the pest without tillage.
Applications of dicamba with mecoprop, DyVel DSp or dicamba with MCPA can be effective. Higher rates of 2,4-D or mixes of clopyralid and imazethapyr (Lontrel and Odyssey) provide longer-term control in the soil but can also affect timing of the following crop.
Curtail (clopyralid) and MCPA can be used for season long control in barley, oats, spring wheat, canaryseed, flax and timothy hay. The weed will likely show signs of growth in fall and can then be killed with glyphosate.
Prestige (fluroxypyr, clopyralid and MCPA) are effective in spring cereals, grasses and timothy.
Thifensulfuron and tribenuron (Refine SG, Deploy or Nimble) is another combination used in most cereals and some grasses. A variety of tank mixes can be used with this combination. Refine SG is registered for aerial applications.
Other products registered for suppression include Tordon 22K (picloram) on its own or with 2,4-D (Grazon) and a mix of 2,4-D and aminopyralid (Restore) for control in pasture.
michael.raine@producer.com
