Invasive Species Spotlight: Canada Thistle

Taylor Wilson, Urban Conservation Technician
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Canada thistle (Cirsium arvense) might actually be everyone’s least favorite garden weed. Brought to North America in the 1600’s on accident, this weed has become a widespread problem throughout the USA and beyond. It infamously colonizes natural areas, roadsides, and is also an incredibly aggressive weed in lawns and landscaping beds. You might know it best by remembering running through the grass barefoot and stepping on thistles that were forcing their way through the lawn.
Canada thistle’s aggressive nature is due to its root structure. The root structure contains both vertical roots (taproots) and horizontal roots (rhizomes). Because of this, generally several Canada thistle shoots (the above ground portion) are actually part of the same massive plant connected by root system. So, usually when you pull a thistle, you’re just pulling out one part of a larger plant.
I once heard Canada thistle compared to a hydra—for every thistle you pull two more pop up. This is due to the resprouting response of the root system, and this is why manual control of Canada thistle can be really difficult. 

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Identification
This Cornell University page walks you through the identification of this plant. We also have several native thistles!

Management
As mentioned above, manual control of Canada thistle can be very difficult—but is possible with years of consistent work. The goal is to “starve” the root system by removing green growth consistently, taking away the plant’s ability to photosynthesize. Herbicide treatment is usually far more effective and works more quickly, but may not be ideal for some areas.
An in-depth explanation of Canada management can be found here at this Purdue Extension page.
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Noxious weeds
Canada thistle is one of the declared “noxious weeds” in Indiana and falls under the Destruction of Detrimental Plant law. Purdue Extension’s explanation of this law is listed below:
"Plants mentioned in this law are the noxious weeds listed in the code for county weed control boards. This law also empowers the township trustee to act to investigate and control the noxious weeds. A 48- hour notice can be issued notifying the property owner that the township trustee wishes to come on the property to investigate any noxious weed problem. Once a problem has been determined, similar to the weed board’s requirements, a five day notice has to be given to the property owner to start actions to control the noxious weed. If no action is taken the township trustee can either notify the county weed control board, if there is one, or initiate control measures themselves at the expense of the land owner. Once a bill has been sent to the property owner through registered mail or hand delivered, it has to be paid in 10 days. If the bill is not paid after 10 days it may appear on the land owner’s taxes that year."
 
Where to report
You can report instances of Canada thistle to the township trustee in the township the land is located in. Alternatively, you can always report it directly to the park, school, land manager, parcel owner, etc. To see who owns the parcel, you can utilize the county GIS program. On interstates and highways, you can report any invasive species via their online reporting system.

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