Getting rid of Canadian thistle - Ask Extension
I have a large amount of Canadian thistle in an area of my garden. It is mixed in between a patch of day lilies and spreads under my old pine tree and...
Knowledgebase
Getting rid of Canadian thistle #878073
Asked July 20, 2024, 10:32 PM EDT
I have a large amount of Canadian thistle in an area of my garden. It is mixed in between a patch of day lilies and spreads under my old pine tree and has little sprouts creeping into my lawn.
I read an article about solarization having some success to smother thistle. But I’m curious your suggestions. Currently I put down a sheet of clear plastic on one area and am also trying cardboard in another area. But I’m not sure if what to do under the tree and around other shrubs.
What is the best method of getting rid of Canadian thistle? Can I use the plastic sheet method under tree and near other shrubs? What’s the difference between using black plastic, clear plastic or cardboard? And how long should I leave it? Also I’m not sure exactly what to do after it’s done or what is best to plant after this process is complete.
Thank you for your help!
Hennepin County Minnesota
Expert Response
To start with, there aren't a lot of good methods for dealing with Canadian thistle. Hand removal of small areas, mowing to reduce the seed load, and herbicides in the fall:
https://extension.umn.edu/identify-invasive-species/canada-thistle
As for solarization, you are confusing solarization with occultation. Solarization heats up the plants, effectively cooking them. Clear plastic is used. Unfortunately, it is not especially effective in our climate. Occultation cuts off light to the plant and may be more effective here, but not as quick. Black plastic or cardboard or even newspaper (but several layers) can be used. Here is a good description of both methods:
https://extension.umn.edu/planting-and-growing-guides/solarization-occultation
https://extension.umn.edu/identify-invasive-species/canada-thistle
As for solarization, you are confusing solarization with occultation. Solarization heats up the plants, effectively cooking them. Clear plastic is used. Unfortunately, it is not especially effective in our climate. Occultation cuts off light to the plant and may be more effective here, but not as quick. Black plastic or cardboard or even newspaper (but several layers) can be used. Here is a good description of both methods:
https://extension.umn.edu/planting-and-growing-guides/solarization-occultation
