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  • Integrated Pest Management
  • Protecting Pollinators
  • Invasive Species
  • Reducing Pesticides
  • Mosquito Control Program

Noxious Weed Ecology

Noxious Weed Ecology

A weed is a plant growing in a place where it is not wanted. The more difficult a plant is to remove from a site, the more likely it is to be considered a weed. If it threatens local agriculture or a native ecosystem, then it is considered a noxious or invasive species.

Noxious weeds can threaten the economy and environment of a region by:

  • Outcompeting native species for water, sunlight, and soil nutrients;
  • Converting a diverse plant community to a monoculture of weeds;
  • Displacing habitat and food sources for insects and wildlife, including threatened or endangered species;
  • Changing or reducing water availability;
  • Causing erosion;
  • Blocking water channels and creating flood hazards;
  • Changing fire regimes - some weeds can increase the risk of wildfires;
  • Competing for pollinators; and/or
  • Inhibiting native species by releasing chemicals into the soil that prevent other plants from growing.

Many North American weed species arrived here as stowaways, such as Cheatgrass (Bromus tectorum). A great number of weeds were deliberately introduced as ornamentals, such as Purple Loosestrife (Lythrum salicaria), or to provide medicine, forage, erosion control, or rapid tree cover.  Some of these nonnatives were able to proliferate without the herbivores, parasites, diseases and competing plants that are present in their native range. This allowed them to become established over large areas and to continue to spread.

Not all nonnative (or exotic) plants are noxious weeds. Of the approximately 4,000 species of nonnative plants in the United States, about 25 percent are considered a threat to natural areas.

Colorado’s lists of noxious weeds were developed by the Colorado Department of Agriculture. Colorado’s "List A" weeds are rare weeds that are targeted for mandated state-wide eradication. Populations of these weeds are small enough that eradication is feasible. "List B" weeds are those whose further spread should be halted. "List C" weeds are recommended for control by local jurisdictions. "Watch List" weeds are intended for identification and reporting.

To provide feedback on the proposed City Manager's rule, contact Jennifer Riley at 303-441-1877.

Noxious Weed Photo Gallery

Russian OliveScotch ThistleBladder SennaSoapwort or Bouncing BetSulphur CinquefoilPerennial SweetpeaTamarisk

See full photo set in the Photo Gallery

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