June’s Huerfano County Weed of the Month is Scotch thistle (Onopordum acanthium). This exceptionally large biennial noxious species that originates from Eurasia is capable of reaching heights upwards of 8 feet and can produce up to 40,000 seeds per plant which remain viable up to forty years within the soil seed reserve. Scotch thistle’s rapid growth allows it to overtake our slower developing native plant species, resulting in altered ecosystems that negatively impact nearly all forms of wildlife. Given Scotch thistle’s prolific seed production, infestations often come in the form of large monocultures (a colony of plants comprised of one species) which form “living fences” that oftentimes prevent access by humans, livestock and wildlife alike into infested areas.
Reproducing only by seed, this species can be controlled by mechanical methods through interrupting seed production. Plants should be severed around three inches below the soil surface to prevent regeneration from growth nodes that lie just below the soil surface. Plants can also be “dead headed”, which is a process where the flower buds are snipped off and disposed of. Properly timed mowing (prior to seed set) can be effective at limiting seed production, but it will not kill the plant and small flowers capable of producing seed may still form just below mower deck height. Herbicides can be very effective for controlling larger sized patches and depending upon the product used, can even prevent the germination of seedlings- all while not impacting desirable grasses. Whichever control methods are chosen, they should be performed prior to seed development. For a site specific herbicide recommendation please contact the Huerfano County Noxious Weed Department.
Key Identifiers
- Leaves have a bluish-green color caused by fine cobweb-like hairs that cover the leaf surface
- The stem has vertical leaf growth upon bolting\flowering, unlike other common thistle types
- Flowers June-September
- Seedlings are exceptionally cold tolerant, having been observed growing all twelve months of the year in Huerfano County. If you see a small thistle before March, there is a high likelihood that it is Scotch thistle rather than other noxious or native species that tend to favor warmer temperatures and longer days.
- With few exceptions (mainly musk thistle) any exceptionally tall thistles you may see (over 2.5 feet) are likely to be Scotch thistle. It is worth noting though that Scotch thistle plants have been observed flowering at all heights, from one inch to ten feet!
For more information related to control options, noxious plant species identification or any other weed or herbicide related topics, please contact the Huerfano County Noxious Weed Department by phone or text @ 719-989-1353 or email us at [email protected]. Being that June is a particularly busy month for our department and we are often in areas of poor phone service, please be sure to leave a message or email if we don’t answer.
