Species spotlight: thistle control


Of all the weeds in Archuleta County, thistle is the reigning champion of invading and degrading landscapes. 

There are five thistle species on the Colorado Noxious Weed List, of which four occur here. Not all thistles are bad, however, as we have many native species that serve a role in our ecosystem. The invasive thistles often occupy the same environment and pose a risk to the survival of our native species. 

Our native thistle species typically sport a white, yellow or light pink bloom, have a gray underside of the leaf, and do not form monocultures. The invasive thistles usually have purple flowers, though sometimes you will see white Canada or musk thistle blossoms, the leaves are greener and they form monocultures.

To control thistle, a shovel works fine on most species. Musk, bull and Scotch thistle are all biennial and severing the root crown 4 inches below the soil surface will ensure that the plant will not return. 

For plants that have already started to bloom, deadhead the flowers and bag them to ensure that no viable seed is produced and then cut down the rest of the plant so that it can’t form new flowers. 

Mechanical control will not work well on Canada thistle, as it is a perennial species and spreads through a creeping root system that can generate up to 300 feet of root mass in a single growing season. Repeated mowing can exhaust these root reserves if done with diligence. Tillage should be avoided as each root fragment will form a new plant. Solarization can be effective in controlling Canada thistle, though it must be done right. Solarization is the practice of using the sun’s heat to sterilize the soil by placing a black or clear piece of plastic over the ground. The edges of the tarp should be staked the entire way around, leaving no gaps for air exchange or heat to escape. The plastic should also be replaced if it gets torn. This process should be done during the hottest months of the year.

Biological control agents are available for musk thistle and Canada thistle. Musk thistle can be controlled using a crown-feeding weevil and Canada thistle can be controlled using a rust fungus. Both of these controls are host-specific and will not damage another plant species, even close relatives of their target species. Biocontrols will not eradicate a population of weeds, but will help reduce their spread and density. The musk thistle crown weevil is available from the Palisade Insectary. The Canada thistle rust fungus is not available at this time.

When treating large swaths of thistle, herbicides can be a cost-effective and time-saving option. When using herbicides, read the label. Herbicides have different regulated use sites, and a different herbicide may be needed to treat your hay meadow versus your open range or turf grass lawn. If you need help making an herbicide selection, please call the Archuleta County Weed and Pest Office. 

Archuleta County Weed and Pest is your local resource for managing noxious weed populations and controlling other pests.

Mother’s Day event

On May 11, the Archuleta County Master Gardeners will be hosting a Paint It, Plant It program for kids at Terry’s Ace. Come out and paint a pot, plant it and give it to Mom on Mother’s Day.

Certified seed potato sale

Mark your calendars to pick up seed potatoes from the Colorado State University (CSU) Extension on May 15 and 16. 

Bring your bags to the CSU Extension Office between 9 a.m. and 6 p.m. and purchase your certified potato seed for $2 per pound. There are six different varieties. Potatoes are fun and easy to grow.