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Canada Thistle: A Worthy Foe

To kill thistle, we must focus on its extensive underground network of roots. Now is the time for action!

Canada thistle
Autumn is the best time to spray Canada thistle. The plant will accidentally send the herbicide down into its root system, causing maximum damage.

Have you ever tried to pull out a thistle? Ouch! Those spines are vicious.

You can pull a thistle or hoe it, again and again. The weed still keeps coming back. That’s because the root system of thistle is amazing.

One Canada thistle (Cirsium arvense) plant can grow 300 feet of roots in one summer! Wow! These roots grow 6 feet or deeper and generate dozens of new shoots. One thistle plant can eventually produce a network of thousands of plants connected underground.

This weed is a worthy foe for any gardener.

To win the battle, we must focus on its underground network. We must destroy its roots and exhaust them of their energy. 

Cultivate or mow the weed as close to the ground as possible. The best time to do this is in summer after its flower buds begin to show. The roots use much of their energy to produce these flowers and have little food in reserve at that time. 

This cultivation helps, but a more effective strategy will include the poisoning of the roots. Autumn is the best time to attack. Now!

The nights are getting longer and the thistle in your yard knows winter is coming. It is beginning to channel its sugars down into its roots to store them over winter. That’s perfect for us. If we spray a thistle with a systemic herbicide now, the thistle will accidentally channel it along with its sugars down into its roots. Gotcha!

Products containing dicamba are recommended to control thistle on lawns in autumn. A spot spray of glyphosate works well and would be the best option if spraying under trees or in a garden. Organic products are not very effective on established thistles.  

Herbicides can be sprayed in summer if needed when flower buds appear. These chemicals will burn the shoots but cause less damage to roots.  

If you have thistle in your yard, now is the time to attack it. The battle may continue for a few years, but with perseverance you can prevail.

Written by Tom Kalb, Extension Horticulturist, North Dakota State University. 
Beck, K.G. 2013. Canada thistle. Fact Sheet 3.108. Colorado State University Extension.
Gover, A., J. Johnson and J. Sellmer. 2007. Managing Canada thistle. Conservation Reserve Enhancement Program Technical Assistance Series. Factsheet 1. Penn State University.
Nadeau, L. B. and W.H. Vandenborn. 1989. The root system of Canada thistle. Can. J. Plant Sci. 69: 1199–1206.
Photo courtesy of Rob Routledge, Sault College, Bugwood.org.

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