Canada and Musk Thistle

CANADA THISTLE
Cirsium arvense

Canadian Thistle

MUSK THISTLE
Carduus nutans

Musk Thistle

Perennial:  Forb
Flowers/fruit: Small bristly clusters 1 to 5/branch. Size: ½ to ¾” wide. May produce 1,000 to 1,500 seeds/flowering shoot. Bracts are spineless.
Bloom: June–Aug./Sept.
Stem: Height to 4-6′ hollow, smooth around the top to hairy below, branching at the top.
Leaves: Variable
Root: Can spread 18′ wide and 15′ deep.
Reproduce: By seed & rhizome
Seed Viable:  20 yrs
Mechanical Control: Continuously stress plant by cutting at ground level every 10-21 days. DO NOT PULL! After blooms form, cut the heads and bag.
Chemical Control: Please go to the links at the bottom of this article for recommendations from Colorado Dept. of Agriculture and from Jefferson County.
Interesting Fact: A segment as small as 0.25 by 0.125″ can store enough energy to produce a new plant.
Note: According to US Dept. of Interior Land Management WY it disperses a chemical that may inhibit growth of other plants.
https://ag.colorado.gov/conservation/noxious-weeds/species-idhttps://www.jeffco.us/795/Invasive-Species-Management
Biennial, or winter annual: Forb
Flower/fruit: Solitary, terminal, showy. Size is 1½ – 3″ wide, flat, nodding. Blooms are  deep rose, violet or purple, rarely white, surrounded by many lance-shaped, spine-tipped bracts.
Bloom: May-July or June-Oct.
Stem: Hairless, can grow  up to 8′
Leaves: Lobed, wavy white outlined margins, dark green, light green midrib, hairless on both sides, long sharp spines.
Root: Taproot can extend to 2′ deep
Reproduce: By seed (1,200/flower). Approx. 1/3 viable .
Seed Viable:  10 yrs.
Mechanical Control: Pull or dig the Musk Thistle before going to seed to eradicate the plant for good. The root system does not extend beyond the individual plant. New plants may still evolve from viable seeds from previous years.
Chemical Control: Please go to the links at the bottom of this article for recommendations from Colorado Dept. of Agriculture and from Jefferson County.
Interesting Facts: The bract under the flower is a very distinctive identification feature. Colorado is home to 20 native thistles of which 8 are noxious.
https://ag.colorado.gov/conservation/noxious-weeds/species-idhttps://www.jeffco.us/795/Invasive-Species-Management