Scientific Name: Cirsium arvense L.
Description. Canada thistle is a rhizomatous perennial often with deep roots. Plants can be 1-5 feet tall. Leaves are variable, but tend to be 5-8 inches long, not very dissected, tend to be a sage color in our area, and are always prickly. Flowers are purple and small, about 1/2 inch in diameter. It can be difficult to tell native thistles from invasive thistles. In our area, if flowers are white or light pink, it is probably a native thistle. The native bees love all the thistles. They are nectar rich.
Methods that work.
1. Spot-spraying with Milestone. Milestone is a very mild herbicide that kills some plants. It works on thistles and it works really really well in our high mountain environments. It is not commonly available in stores, but it can be purchased online and some municipalities may make it available. A small amount goes a long way. Be sure to read the label. We use milestone to control Canada thistle. In fact, the plant is nearly eradicated on our property.
Methods that don’t work.
1. Hand-Pulling. If you pull up Canada thistle, it grows right back again from the roots.
2. Biocontrol. In the past, a weevil was introduced to control Canada thistle, but the weevil ended up preferring native thistles instead. This is a common problem with biocontrol agents. Many imported insect herbivores do more damage to native plants, than to the invasives they are supposed to control.