The Painful Problem Of Thistle

Canada Thistle (Carduus arvensis) is one of the toughest (and most painful) weeds to get rid of in the garden. Not only does it have spiny thistles which makes it difficult (even painful in some cases) to grasp and pull out, it has roots that can grow 3’ deep and horizontal. If the root snaps off, the plant will grow back.

Identifying Canada Thistle

What distinguishes Canada Thistle from other thistles is that it has a long, creeping root system. This helps it spread quickly and one of the major reasons why it is important to get it under control fast.

The stem is green and has grooves and many branches. It can grow up to 4’ tall. The stem itself does not have the spiny thistles.

The leaves are a light olive green color, oblong with irregular lobes and spines along the edges. The underside of the leaves are “hairy”.

The flower is actually quite pretty. The flower head is made up of as many as 100 purple (lavender) tubular flowers clustered together. The flower head is ¼” to ¾” wide. The may be produced in single heads or in groups of two to five. The flowers are produced June to August.

Controlling Canada Thistle

Canada Thistle is a perennial weed, meaning, if not eradicated, it will come back year after year, often stronger because it has had more time to develop a robust root system.

Hand pulling Canada Thistle is only effective if one can get the entire root.

It is possible to wear out Canada Thistle by repeatably cutting the stems off at ground level. This must be repeated every week or so until the plant just withers away due to a lack of chlorophyll production. This process could take a very long time.

If a problem area is not being used for a while, covering it with a heavy, dark tarp can also suffocate the plants. Place bricks or wood boards on the tarp to wear it down.

Lastly, there are several organic and non-organic herbicides that are effective at eradicating Canada Thistle. These are non-selective herbicides so be careful when using them as they will kill all plants they come in contact. As with all pesticides, read the entire label before use.

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