Noxious Weeds

Noxious weeds are non-native plants that displace native plants or out-compete native plants for soil moisture and degrade natural habitat. Many noxious weeds are undesirable to cattle and wildlife, and can even be toxic, thereby reducing available forage habitat. It is important to manage noxious weeds in our area through identification, prevention and control of infestations.  

Noxious weeds can be introduced through seeding, as ornamental escapes or can establish in disturbed areas (roadsides, overgrazed pastures, construction areas) as well as riparian corridors and wetlands. To control or prevent infestations and educate landowners, the state-wide Colorado Noxious Weed Act (35-5.5 CRS) was established to identify a noxious weed list and prioritized management goals. The state’s noxious weeds are grouped into three different lists, A, B and C. For a current list of those plants, please click here. This categorical listing of a noxious weed plant as A, B or C will dictate whether it is to be eradicated, contained or controlled.

Diffuse Knapweed 1

Top five noxious weeds in Castle Rock

Several types of noxious weeds have been identified as the most undesirable, unwanted or priority for control in Douglas County.

Diffuse Knapweed

  • List B – to be eliminated or suppressed depending on the infestation
  • Native to the Mediterrean area
  • Tends to invade disturbed or overgrazed areas and roadsides
  • One plant can produce 18,000 seeds that may remain dormant for years but still be viable
  • Also known to outcompete native plants by releasing a chemical in the soil that suppresses other plant growth

Learn more about Diffuse Knapweed

Diffuse Knapweed 2

Musk Thistle

  • List B – to be eliminated or suppressed depending on the infestation
  • Spreads only by seed but one plant can produce up to 20,000 seeds
  • Prefers open degraded habitat and pastures
  • Can grow up to 6 feet tall

Learn more about Musk Thistle

Musk Thistle

Dalmatian Toadflax

  • List B – to be eliminated or suppressed depending on the infestation
  • Native to the Mediterrean area introduces as ornamental plant
  • Tall (3ft) plant that reproduces by seed and creeping underground roots
  • One plant can produce 500,000 seeds but does not spread far
  • Control best accomplished by early detection and eradication or multiple management strategies

Learn more about Dalmation Toadflax

Dalmatioan Toadflax

Leafy Spurge

  • List B – to be eliminated or suppressed depending on the infestation
  • Native to central and southern Europe
  • Long-living creeping perennial that spreads by seed but also has extensive root system that makes control difficult
  • A management scheme that combines control methods over four to five years is recommended

Learn more about Leafy Spurge

Leafy Spurge

Myrtle Spurge

  • List A – must be eradicated on all property
  • also called "donkey tail" or "creeping spurge"
  • drought-tolerant perennial native to Eurasia, first introduced to North America as an ornamental
  • considered a noxious weed because it is aggressive and proliferates easily, outcompetes native plants, and has toxic milky sap

Replacements for Myrtle Spurge

  • Colorado Four o'clock (Mirabilis multiflora)
  • Sulphur-flower Buckwheat (Eriogonum umbellatum)
  • Kinnikinnick (Arctostaphylos uva-ursi)
  • Creeping Barberry (Mahonia repens)
  • Four-nerve Daisy (Tetraneuris acaulis)
  • Tufted Evening Primrose (Oenothera caespitosa)

Learn more about Myrtle Spurge

Myrtle Spurge


Noxious weeds can be managed by using a combination of control methods including mechanical, cultural, biological, preventive and chemical, but since they grow or spread differently, not all methods will be effective on all weeds. Refer to the Colorado Department of Agriculture for required controls. Consult with Douglas County Weed Control for specific recommendations for eradication, containment or control. It is important to properly identify noxious weeds and use diverse control methods to reduce or eradicate infestations.

The Town Parks and Recreation Department, Public Works and Castle Rock Water utilizes integrated management techniques to actively manage noxious weeds on Town-owned properties, including parks, trails, open space areas, well sites and public right of ways. Mowing, hand pulling, native grass restoration, herbicides and biological control agents are all examples of integrated management techniques used by the Town.

The Town does not spray herbicides indiscriminately but rather uses a spot spray approach for heavy infestations of weeds and only use low-toxicity chemicals that are not restricted. Additionally, the Town has found success utilizing releases of biological control agents, seedhead and root-boring weevils, to control large infestations of Diffuse Knapweed on Town-owned open space properties.

Castle Rock residents are encouraged, and required by both Town Municipal Code and state statute, to control noxious weeds on their property.

What You Can Do To Help

  • Prevent – Eradicate - Control
  • Learn about noxious weeds in Colorado
  • Familiarize yourself with the noxious weeds in your area 
  • Work with your neighbors to identify the extent of weed populations
  • Landscape with plants native to your area