Canada Thistle spreads easily by seed and rhizomes (basically it's root system). Canada Thistle is a perennial plant that will easily grow in disturbed sites, but also out compete grasses in your hay fields. This is one reason that I typically recommend someone interested in planting a native wildflower/grass meadow to let the meadow establish before installing the wildflowers. This allows you to gain complete control of the thistle without harming the broad leaf flowers planted in the meadow.
It can be very difficult to control, and a variety of methods are needed to completely control the plant. If you keep your meadow regularly mowed, it will not set seed, so it will not be able to spread past your field, but the repeated mowing does not kill the root system. You can mow it early in the year to set it back, and then again while flowering but before seed when most of the plant's nutrients are in the shoots and not the roots. This method will keep the plant under control, but it will never completely eradicate it from the property. It will just not set seed and spread beyond your property. After great persistence with mowing, you should be able to obtain adequate control.
There are fairly effective chemical methods. If you have a large infestation you will need repeated applications throughout the year. I would recommend using a clopyralid based herbicide to control the plant. This active ingredient is specific to broad leaves so it will not harm the grasses in a meadow like a non-selective glyphosate based herbicide. You have a few options for chemical control. The first few years of controlling the plant you should either control it with chemicals to keep it from setting seed in the spring, and then repeating the process on any last hurrah's from the roots in the fall. The other option would be to should mow it early in the year to keep it from setting seed and then spray in the fall to kill the plant in it's entirety. If you have decided to spray the thistle, you should do this sooner than later as it will be setting seed before mid-June.
|Unhappy Thistle at Stateline Woods|
If you only have the Canada Thistle in a small section, or in your flower beds, there is a third option which only has to be repeated once, and will certainly eradicate the plant. It is not cost nor time effective for a large patch. I would cut the budding head off of the plant and paint it with a straight glyphosate solution using my trusty mustard bottle. This will kill the plant, and not allow any re-sprouts, but is very time intensive.
Persistence is the main key in controlling Canada Thistle, but it should be controlled to the best of your ability. If you do not have the capability to either spray or mow, you should consider hiring a contractor for the job. I would be happy to pass along recommendations if you are interested.