Weed of the week: Canada thistle

Canada thistle has more than one way or reproducing, making it tougher to control

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It takes more than one field action to control Canada thistle – a prolific weed with a couple tricks up its sleeve.

Also called creeping thistle and field thistle, Canada thistle (Cirsium arvense) is a perennial weed that reproduces both by seed and by horizontal roots. Dense patches of new shoots can form from these new root buds, making control difficult with only single herbicide applications or other in-field actions.

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Why it matters: Canada thistle can spread from seed, as well as creeping roots. This makes repeated control efforts critical.

Crop impact

As described by OMAFRA, Canada thistle grows throughout southern Canada and is most often a problem in perennial crops, as well as areas where reduced tillage is practised. According to Manitoba Agriculture, it’s also a strong competitor for light, moisture, and nutrients. This can translate to substantial crop yield losses, while green matter in harvested grain can increase drying and dockage fees.

OMAFRA also indicates Canada thistle is similar to Japanese barberry, and does not normally act as a host for rust fungus. However, hybrids of the two species can act as a disease vector.

Identifying Canada thistle

Mature Canada thistle plants feature erect, hollow stems between 30 and 120 cm (1-4 feet) high, with branching and flowers (purple, pink, or white) occurring at the top and ends of stalks. Leaves are alternate, oblong, or lance shaped, with sharp spines protruding from irregularly cut segments. Each thistle flower produces about 700 seeds per plant. Scouting for Canada thistle means scanning field edges, shelterbelts, sloughs, and other low areas.

Canada thistle can be confused with other weeds of the species. This includes bull thistle (distinguished by the absences of spines from the surface of the leaf blades), and sow thistle (which does not excrete a milk-coloured fluid from its leaves and stems). It can also be mistaken for biennial thistles, though biennial varieties generally have a more slender physique, as well as an absence of creeping underground roots during non-flowering stages.

Control with multiple applications

OMAFRA resources say Canada thistle can be controlled with dicamba and glyphosate products. These should be applied at the early bud stage, or in early fall, though repeated applications are required in order to kill all root buds.

University of Guelph research indicates 540 grams per litre of glyphosate, applied at 1.34 litres per acre, brings between 85 and 100 per cent control. 2,4-D alone only provides top-growth control.

Additionally, OMAFRA lists repeated mowing (at 21-day intervals) as an effective way of controlling “light to medium infestations.”

A bio-control for Canada thistle and other broadleaf weeds is also currently listed as in-development by Agriculture and Agri-Food Canada.

Phoma macrostoma is a fungus that naturally infects Canada thistle, causing plants to turn white and die from a lack of chlorophyll. The fungus has been isolated and purified from plants growing in Saskatchewan and other provinces by AAFC scientists.

About the author

Contributor

Matt McIntosh

Matt is a freelance writer based between Essex County and Chatham-Kent. He is interested in all things scientific, as well as rock n' roll, hunting and history. He also works with his parents on their sixth-generation family farm.

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