Weeds: Canada Thistle

This is the fourth in a series of blogs, shared with permission, about weeds in the garden. Each blog will discuss a common and specific weed found in Maine, its history, uses, what story it tells about the soil it grows in, and how to get rid of it. The original blogs, along with other gardening related blogs, can be found here.

Canada thistle, or Cirsium arvense is not actually from Canada. It is a native of Europe and Asia, and in that area it is typically called creeping thistle. Other names include lettuce from hell thiste, prickly thistle, way thistle, small-flower thistle, cursed thistle, field thistle, hard thistle, green thistle and perennial thistle. It is a plant considered to have originated during the Mesozoic and Paleozoic eras, preferring areas with moderate sunlight and temperatures, plenty of carbon dioxide and a great deal of ground water. The plant spreads through strong, deep root structures as well as through seeds.

You may want to just keep this plant around if you aren’t afraid to keep cutting it back. Although it is a weed, the Canada thistle’s roots can delve up to 20 feet deep – and bring all the nutrients back up to the surface. Additionally, this ground breaking work makes it easier for less vigorous plants to survive in a space.

Birds love the seeds of the thistle, especially goldfinches; and it is at least as good as alfalfa for livestock. As a bee lover (and a food lover) the best reason to keep Canada thistle around is because it is considered a fantastic pollinator. One 2016 British study found it was considered the second best producer of nectar sugar. However, you can also eat them. In Portugal for instance, thistles are still collected and sold in markets. Thistles are considered to be higher in fiber, protein, phosphorus, magnesium, calcium, copper, zinc and other nutrients than many other commonly eaten vegetables.

What does it mean?

Thistle is typically an indication your soil needs more acid. Correct this problem with soil amendments like ferrous sulfate or aluminum sulfate. Another way to get rid of thistles with is to use root secretions from tall marigolds Tagetes minuta or dahlias can kill it.

The easiest way to get rid of them is to have too much heat or too little water. Alternately preventing any sunlight from reaching the space after seedlings emerge will kill off the plants as well.  Even a small fragment of the root will spread to create new plants.  Avoid trying to dig up the plant, and instead chop it down before the flowers turn into seeds. If you do this several times, you will drain the root reserves and the plants will die.

Want to learn more?

If you would like to learn more about weeds, why they grow, and what that means about your garden, consider the following texts:

Weeds and Why they Grow, by Jay L. McCaman

Weeds and What They Tell Us, by Ehrenfried E. Pfeiffer