Canada Thistle

The subject of my previous post, the red admiral butterfly, was photographed as it, along with several other butterflies, visited Canada thistles (Cirsium arvense) in the McArthur Swamp (Shasta County CA).

Canada thistle is native to Southeastern Eurasia where it is commonly called a creeping thistle. In the late 1700s C. arvense was introduced to Canada in crop seeds and has since spread and become well established throughout most of North America, thus its colloquial name. Other familiar names for this member of the sunflower/aster family are field thistle, cursed thistle, Canadian thistle and small-flowered thistle.

A perennial with deep roots and rhizomes, Canada thistle forms large colonies. Found in open, disturbed areas, this aggressive thistle will rapidly overrun pastures and fields and is difficult to eradicate. Breaking up Canada thistle roots by chopping or plowing only serves to increase the number of plants, since new vegetative growth can arise from portions of the root.

Lance-shaped, irregularly-lobed, bright green leaves without petioles (stalks) alternate up the ridged Canada thistle stem. The leaves are well-armed with spines, are hairless above and may have white hairs on the underside. The leaves do not extend down the stem as “wings”.

Canada thistle is the only thistle that has male and female flowers on different plants. The male flower heads are “showier” than the female heads, however both look very similar to my eye when casually observed. Several to many heads form the inflorescence (flower cluster). The pink purple Canada thistle flower is composed solely of disk (tubular) flowers. The involucre bracts surrounding the disk flowers are tipped with weak prickles. The flower head is 1/2 to 3/4 inch in diameter and is small compared to many thistles. Occasionally the Canada thistle flower is white.

Canada thistle achenes (small, dry fruits containing one seed) can remain viable for more than 20 years. The seeds of this thistle require full sunlight for development.

The peeled stems and roots of Canada thistle are edible. However, care must be taken not to eat excessive amounts of thistle since they do contain alkaloids that may cause adverse reactions in quantity. Years ago I was quite adept at peeling thistles without drawing blood (a skill lost with disuse) and would often nibble the core along the trail. I always compare thistle cores to celery – rather bland and crunchy. The leaves of Canada thistle are also edible, however I never cared to deal with the spines. Many species of butterflies and other insects visit thistle flowers for nectar while birds, especially finches, enjoy the seeds.

The species name, arvense, comes from the Greek and means “of planted fields” – very appropriate.

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