Canada Thistle was introduced to Island County in the mid 1800’s probably via contaminated crop seeds.

• erect perennial
• rhizomatous roots
• can produce 1500-5300 seeds per year
• reproduces more commonly via creeping root system
• shoot production increases with nitrogen production
• within 3-4 days of full bloom the resulting seeds are potentially viable and can remain so for up to 20 years in the soil
• seedlings, once the are 20-30 days old with 4 true leaves become capable of reproducing through their root system
• root pieces 0.25” long by 0.125” in diameter are capable of developing into new plants even after going ~100 days without photosynthesis

Information on control methods

• to deplete the root system and actually kill C. thistle, you must prevent shoot growth for at least two years.
• mowing is effective if repeated at 7-28 day intervals for up to 4 years.
• tilling is effective when done repeatedly over several years – 10 cultivations over 1.5 growing seasons at 10-20cm below the soil surface
• Fall is the best time to apply herbicides – more is assimilated into the roots
• glyphosate (conc.=2.5%) is effective with good soil moisture – follow label directions
• no known effective biocontrol

If you have any questions or concerns, call Susan Horton, the program coordinator for the Island County Noxious Weed Control Board.

360-679-7327, or 360-321-5111, ext. 7211, or 360-629-4522

or email her at susanh@co.island.wa.us