How to control Canada thistle

Canada thistleThere’s no denying that Canada thistle is tough to control, but the first step in dealing with it is to understand it.

About Canada thistle

Canada thistle (Cirsium arvense) is a fast growing perennial plant that can grow to 1m or more in height. Its leaves are deeply serrated and spiny, and the stems have few, if any spines. Flowers are purple or pink, slightly feathery and seeds are easily dispersed by the wind. The plant develops from seed and also through its underground root system, or rhizomes that send out new shoots from the nodes. Rhizomes can extend outward to 6m (19 ft.) and to depths of 5m (16 ft.) or more. When a rhizome is split into pieces, each piece is capable of sending out new shoots — segments as small as 1.25cm (.5 in.) can start a new plant.


Controlling Canada thistle

One way to control the Canada thistle is to cut the stem at its base before the flower buds open. This will prevent seeds from spreading, and energy produced during photosynthesis in the stems and leaves from reaching the root system. It may take several years to eradicate Canada thistle but repeatedly cutting or mowing the plant will ‘wear it down’ and eventually you’ll win as its energy is gradually depleted.

Canada thistle grows best in open areas, the fringe of wetlands and in soil that is less fertile. It needs direct sunlight to thrive and is vigorous in land suited to annual crops, wastelands and pastures. So, aside from cutting it down and without using chemical herbicides, the only other way to control it is to make the area less inviting. Increase the fertility of your garden soil, plant grass seed on open patches and encourage your desired plants to drown out the thistle.

The Painted Lady butterfly and Canada thistle

Thistle caterpillars of the Painted Lady butterfly (Vanessa cardui) feed on the leaves of the Canada thistle. The butterflies migrate north from the southern states (Arizona and New Mexico areas) through the Western and Midwestern States and into Canada. They lay their pale green eggs on the leaves of host plants such as Canada thistle, sunflower and soybean, and the caterpillars feed for two to four weeks, then pupate.

Painted LadyA spring migration can include millions of Painted Lady butterflies when the southern states and northern Mexico receive high levels of precipitation during their winter months. While a large migration of the butterflies will do significant damage by defoliating the plants, energy stored in their rhizomes will generally allow Canada thistle to recover.

Naturally organic pre-emergent herbicides  

The use of organic herbicides is an important means of controlling aggressive weeds and the subject of ongoing study. The Montana State University is one of several universities that have been testing the herbicidal properties of corn gluten meal and wheat gluten as organic pre-emergent herbicides (inhibits seed germination). Of seventeen weed species recently tested for their tolerance to wheat gluten, twelve showed a 50% reduction in germination. Plants particular sensitive included Canada thistle, leafy spurge, pigweed, shepherd’s purse, quack grass and orchard grass.

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