Canada thistle is not Canada’s friend. Nor is it Canadian. Originally from Europe and Asia, Canada thistle crossed the ocean in the 1600s and quickly spread to most parts of North America.
In Ontario, Canada thistle is considered a noxious weed. It enjoys that honour because it is an invasive non-native plant that threatens agriculture crops and harms the growth and spread of native plants.
In my backyard, thistles are merely a nuisance. They are not plentiful, and Cheryl and I are able to keep them in check by weeding with a traditional hoe or simply pulling them out. Thistles never grow in solitude, but rather grow in colonies or clusters. They are distantly related to asters, which explains why they have a growth spurt in autumn.
Thistles are easy to identify. When young, they resemble dandelions but have sharp barbs along the leaf edges. The sharp leaf edges act as their own built-in defence mechanism. As they mature, thistles form a central leader growing upright with leaves growing alternatively up the stem. The weed normally grows to about four feet, but if left alone can grow up to six feet high.
Flowers of thistle are rather pretty in white to pink to purple. Flower heads grow up to an inch in diameter and are found in clusters at the top of the plant. Flowers quickly fade to straw colour or brown.
Each thistle plant has the capacity to produce more than 1,000 tiny seeds, which are mostly spread by wind. Moving water, animals, clothing and vehicles can also disperse seed. Thistle seed is favoured by many birds, especially finches.
Thistle weeds have a tap root which is easier to pull out than a dandelion’s. Unfortunately, when pulling out thistles, the gardener will likely leave a few root remnants behind, which will invariably establish themselves and produce new plants. If there are no flowers on young thistles, they can be dug up and tossed on the compost heap. But once thistles have flowers, it is highly recommended to put them in a bag and set them at the curb for fear of spreading seeds.
Canada thistle will grow freely wherever it wants. It does not need good soil and can often be seen growing at the side of highways or along railway lines. Thistles grow best in rich soil and can appear almost overnight in the lawn or at the edge of the veggie garden. Thistles will not grow in shade and will not grow in waterlogged soil.
For all the hatred we might have for thistles, it is only fair to recognize their positive attributes. Birds feast on their seeds. Some thistle varieties are highly ornamental with large, long-lasting blooms. If you roll the leaves and smash them to destroy the spines, the leaves can be eaten like spinach. Or pick young leaves before they produce spines for a highly nutritious and tasty addition to sandwiches or stir fries.