Weed of the Month

Learn about noxious weeds and steps you can take to manage or eradicate.  Each month we will bring you a new profile and helpful resources.

Canada Thistle Flower

October Weed Profile 

Canada Thistle

Cirsium arvense

Canada thistle is a perennial plant that is part of the Asteraceae family. These plants can grow anywhere from 2 to 5 feet tall. Canada thistle reproduces by root propagation and seeds which results in the colony formation of this plant. Roots can grow 15 feet or more horizontally and 6 to15 feet vertically.

Leaves are 4 to 8 inches long and dark green with spiny, serrated edges. Canada thistle flowers from June to August with purple, pink or white flowers.

Canada thistle can be found in fields, riparian areas, pastures, roadsides, and lawns and gardens. Due to growing behaviors and root systems this noxious weed is one of the most widespread noxious weeds in Colorado.

Canada thistle is best controlled with mowing June through September and then treating with herbicide in the fall. Canada thistle rust is a new biological control method that is showing good success in parts of Colorado.  It is available from the Palisade Insectary.


  1. Identify nearby populations that may spread to your property
  2. Limit disturbances on your property that can allow Canada thistle to encroach
  3. Educate your neighbors on Canada thistle so they also understand why it is important to control this noxious weed

What you can do

  1. Treat your Canada thistle populations and help your neighbors if they allow it
  2. Report any populations you locate to your county weed coordinator

Clean your gear before and after hikes to limit spread.

Items of Interest

Weed of the Month

January Weed Profile – Cheatgrass

February Weed Profile – Myrtle Spurge

March Weed Profile – Hoary Cress

April Weed Profile – Leafy Spurge

May Weed Profile – Scotch Thistle

June Weed Profile – Dyer’s Woad

July Weed Profile – Orange Hawkweed

August Weed Profile – Purple Loosestrife

September Weed Profile – Teasel

Canada Thistle Seed
Canada Thistle Rosette
Canada Thistle Rust