CO-Horts Blog

Monday, May 17, 2021

This is Colorado Noxious Weed Awareness week!


Colorado Governor Jared Polis recently designated this week, May 15-22nd, as Noxious Weed Awareness Week in Colorado. For anyone who, like me, considers themselves a Weed Warrior, this proclamation helps highlight the need for everyone in our state to take noxious weed control seriously. And while many landowners are very familiar with the identification of noxious weeds, many home gardeners are not and may inadvertently be harboring these weeds in their gardens.

As a little reminder, noxious weeds aren’t just annoyances in your yard or property: they are non-native, invasive weeds that have a competitive advantage over our vegetation and displace our native plants. Many of them are from other continents entirely and don’t have insects or disease they may have had in their native land that kept them from becoming unruly. Noxious weeds are ones that have been determined by state or local governments to be a threat to local environments and have been placed on a Noxious Weed List. In Colorado, there exist three lists of noxious weeds, and corresponding controls are needed for each.

List A species are weeds that must be eradicated whenever detected in order to protect neighboring communities and the state as a whole. These weeds may or may not be in the state yet, or may be in adjacent states and infestation is probable. Eradicating them as soon as they are found helps keep them from getting a foothold in our state.

A few plants that may be planted in gardens can be found on List A. Myrtle Spurge, Orange Hawkweed, and Purple Loosestrife are all plants that someone thought would make wonderful ornamentals in the yard, but showed quickly that they are aggressive, dominating species that quickly take over our native vegetation when they escape the confines of a garden.

Oxeye Daisies taking over fields
near Steamboat Lake

List B
species are those that are present in the state, but the Colorado Department of Agriculture, in consultation with the Colorado Noxious Weed Advisory Committee, has developed a plan to keep these weeds from spreading. Plants that were once considered ornamentals that have now found themselves on List B due to their aggressive tendencies include Common Tansy, Dalmatian Toadflax, Dames
Rocket, Oxeye Daisy, Russian Olive, and Yellow Toadflax.

List C species are weeds that the CDA will help support efforts by local county and city governments to educate people on their threat and to help use management methods to limit spread. This list includes many common ‘garden weeds,’ like Field Bindweed and Redstem Filaree. List C also includes common mullein, which is spreading along roads, railroads, and trails across the state.

Dalmatian Toadflax on a hillside

Controlling noxious weeds is not only the right thing to do if you’re a gardener or native plant advocate, but also because it is the law. Many of these weeds can be controlled with mechanical, cultural, or chemical means, and using an integrated approach to control using all of these methods is most effective. Of course, keeping the ground covered with healthy vegetation and not giving weeds a place to grow is the best solution, and also gives our native pollinators the best chance at thriving, too.


  1. Do you have a link to a complete list of these weeds?