A Close-up View of the
"Canada Thistle"

  Cirsium arvense

by Brian Johnston   (Canada)

This very aggressive weed has many names: corn thistle, field thistle, creeping thistle, perennial thistle, Californian thistle and Canada thistle.  Since it is native to southeastern Eurasia, it seems strange that the last two names were given to a plant that reached North America in the late 18th century.  In fact, most farmers consider it a very unpleasant, and difficult to eradicate invader of their land.  Having this pernicious weed named after their country or state, may be a dubious honour.

The examples photographed for this article came from a typical infestation located at the edge of a park near my home.  An area of about five metres by three metres has so many Canada thistle plants that it would be simply impossible for man or beast to cross the patch.  Each plant is chest high, and so many have grown in the space that the area is a paradise for butterflies - one of the few admirers of the species.

Canada thistle is different from other thistles in that male and female flowers exist on separate plants.  Such plants are called dioecious.  Normally, all flowers in a particular patch are of one sex.  (Research has shown that the reality is not quite as simple as I have indicated.  Canada thistle is considered to be “imperfectly dioecious”.  Some of the flowers are self-fertile hermaphrodites having both male stamens, and female pistils.)

The two images that follow show the general appearance of the plant during the summer season.  Notice that although the leaf edges are ringed by sharp spines, there are none on the stems.

The leaf spines are extremely unpleasant, as they are very sharp, and thus easily penetrate skin and clothing.

Red buds are almost spherical in early stages of development.  Notice in the fourth image, the spider-web like fine white filaments that enshroud the bud.  The bull thistle and nodding thistle discussed in an earlier article share this characteristic.

As the buds develop, they become longer and more ellipsoidal in shape.

The Canada thistle bloom (up to about three centimetres in diameter), is quite striking in appearance.  The pinkish-purple composite flowerhead is composed of either staminate (male) flowers, pistillate (female) flowers, or (unusually) hermaphrodite flowers.  In the bloom shown below, the flowers are all female since no bright yellow pollen is present anywhere.  The greenish-purple layered bracts, (modified leaves) making up the calyx of the flowerhead are sharply pointed, but not needle-like as are those of other species.

The aster family (Asteraceae) to which the thistles belong has flowerheads that may possess both ray and disk flowers.  In the case of thistles, only disk flowers are present.  The pink projections that can be seen below are the individual pistils of the disk flowers.

Under the microscope, the bi-lobed stigma (female pollen accepting organ) can be seen to be held aloft by the white style

With higher magnification, stubby projections can be seen that help the pollen grains adhere to the stigma.

At the base of the style, you can see the tiny “petals” of the flower that are normally buried deep within the flowerhead, and are thus not visible.

Canada thistle flowerheads are extremely rich in nectar, and are clearly one of the favourite haunts of butterflies, moths and bees.  The patch from which these plants were obtained has the highest density of butterflies on any given day, that I have ever seen. 

In the late summer and early fall, the plants die, and dry out to an attractive light brown colour.  The feathery white pappus may help to carry the seed to other locations with the aid of the wind.  Usually, however, the pappus breaks off, leaving the seed attached to the base of the dried flowerhead, where it may fall to the ground, or be carried away by animals.  In a particular patch of plants, vegetative reproduction from the root system accounts for most of the local growth.

Whenever I photograph thistles, I have scars as trophies of my efforts!  Even so, this unusual plant is worth the amateur naturalist’s attention.

Photographic Equipment

Two-thirds of the photographs in the article were taken with an eight megapixel Sony CyberShot DSC-F 828 equipped with achromatic close-up lenses (Canon 250D, Nikon 5T, 6T, Sony VCL-M3358, and shorter focal length achromat) used singly or in combination. The lenses screw into the 58 mm filter threads of the camera lens.  (These produce a magnification of from 0.5X to 10X for a 4x6 inch image.)  Still higher magnifications were obtained by using a macro coupler (which has two male threads) to attach a reversed 50 mm focal length f 1.4 Olympus SLR lens to the F 828.  (The magnification here is about 14X for a 4x6 inch image.) The remainder of the photographs were taken with an eight megapixel Canon 20D DSLR equipped with a Canon EF 100 mm f 2.8 Macro lens.  The photomicrographs were taken with a Leitz SM-Pol microscope (using a dark ground condenser), and the Coolpix 4500.


The following references have been found to be valuable in the identification of wildflowers, and they are also a good source of information about them.

 All comments to the author Brian Johnston are welcomed.

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