Weed of the Month: Canada Thistle (Circium arvense)

What is Canada Thistle?

Canada thistle is a perennial that reproduces through underground rhizomes and wind dispersed seeds.  Pink to purple flowers are present throughout the summer from June to August.  Leaf margins have sharp spines, but the stems are for the most part smooth.  Other thistles, such as bull thistle, have spiny stems and hairs on the upper leaf surface.  Canada thistle can grow to 1 to 4 feet in height.

Managing Canda Thistle

Canada thistle management is problematic since it is perennial and grows from rhizomes that can extend many feet underground. It easily recovers from mechanical or chemical control attempts because that extensive root system contains large reserves of nutrients. Avoid tilling, because that just breaks the rhizomes into small pieces and a new plant will sprout from every piece.

Management should focus on exhausting those root reserves – repeated cutting and/or pulling before it goes to seed and herbicide applications are the main control options. Herbicide treatments in landscape beds must be carefully targeted spot treatments to avoid damaging desirable plants. Bentazon (Basagran), clopyralid (Lontrel), glufosinate-ammonium (Finale), and glyphosate (Round Up, many others) are most effective. Bentazon should be applied when Canada thistle is eight inches tall with a repeat application in seven to ten days. Clopyralid, glufosinate-ammonium and glyphosate should be applied when Canada thistle is actively growing. Repeat applications will be necessary for complete control.

Canada thistle can be controlled in lawns with applications of selective herbicides such as 2, 4-D, MCPP, MCPA, dicamba, triclopyr, and carfentrazone.