Musk thistles in a pasture near Silt.

Thistle management

Cut thistles now

August 3, 2015

While a warm, wet spring has meant abundant growth of flowers in gardens and wildflowers in the high country, it has also led to an explosive growth of noxious weeds. In particular, invasive thistles – Scotch, musk, plumeless, bull and Canada – have proliferated in Garfield County.

Many of the biennial thistles – Scotch, musk, plumeless, and bull – have already flowered and are well on the way to seeding out or have already done so. Once the biennials start to seed out, they have completed their life cycle and spraying at this point is ineffective.

For flowering biennial thistles, it’s time to put up the sprayer for now and regroup, employing power weed eaters with blades or even an old-fashioned scythe.
Mowing or chopping is most effective on biennial thistles when the plants are at full bloom.

Be sure to properly dispose of flowering cut plants, since seeds can mature and become viable even after the plant is cut down.

After the cutting is completed, keep monitoring the site. Usually the biennials will germinate again in late summer and early fall as rosettes. Rosettes are the first year growth of biennial thistles, and consist of a taproot with a cluster of leaves on or near the ground surface.

These late summer rosettes will be the tall second year plants in 2016, so a fall herbicide treatment of rosettes will help manage the spread of the thistles once spring rolls around.

For more information about invasive thistles and how to effectively eliminate them, contact the Garfield County Vegetation Management Department, 970-945-1377, ext. 4305, or send an email.