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  • Integrated Pest Management
  • Protecting Pollinators
  • Noxious Weed Removal
  • Pesticide Applications
  • Mosquito Control Program

Noxious Weeds - Purple Loosestrife

Noxious Weeds - Purple Loosestrife

Purple Loosestrife (Lythrum salicaria) is a hardy perennial weed with spikes of eye-catching rose-purple flowers. It is on Colorado’s “List A” of noxious weeds, which requires  you to remove it from your property. Native to a swath of Eurasia from Great Britain to southeast Asia, Purple Loosestrife was introduced to North America in the 19th century as an ornamental and as a medicinal plant to treat bacterial infections and intestinal maladies.

Purple Loosestrife  is a " List A" Noxious Weed under the Colorado Noxious Weed Act (35-5.5 CRS) and must be eradicated on all property.

Why Is It Noxious?

One of the most disruptive weeds in American wetlands, Purple Loosestrife has quickly adapted to North American habitats, displacing native species, forming dense monocultures and restricting waterways. As plants mature, they produce progressively more shoots to create dense populations.

Butterflies, waterfowl, and other wildlife that depend on native species lose their food sources, nesting sites, and other habitat in infested areas. Purple Loosestrife flowers for most of the growing season, each plant producing up to three million seeds annually that are viable for up to 20 years. The weed can also reproduce vegetatively from its substantial taproot.

Identifying Purple Loosestrife

Clusters of pink to magenta flowers, each with five to seven petals, grow on spikes that range from 4 to 10 feet tall. Leaves are two to five inches long, lance-shaped and rounded at the base, and have no stems.

Mature plants can have 30 to 50 spikes growing from a single rootstock. Although it is thought of as a wetland herb growing along moist to wet habitats, it is also found in landscaping plans and along right of ways.

Purple Loosestrife Gallery

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See all related photos in the Photo Gallery

Controlling Purple Loosestrife

  • Early detection is key when infestations are small and fairly easy to control, and before seed numbers have become overwhelming. Be sure that you have identified Purple Loosestrife properly, as native Spotted Gayfeather (Liatris punctata) may be confused for loosestrife.
  • Manual control - Pull and cut Purple Loosestrife before it produces seed to prevent seed distribution. Try to pull out the entire rootstock. A hand cultivator or shovel will be helpful, especially for older plants and those in deeper soils. Purple Loosestrife can spread vegetatively from stem cuttings and rootstock fragments, so rake the area that has been treated and bag all fragments. Follow-up treatments for Purple Loosestrife are recommended for at least three years after initial removal.
  • Curb fertilizer use - Studies show that abundant phosphates, nitrates and ammonia support Purple Loosestrife growth.

* Please do not compost noxious weeds as this will spread the weeds to other locations.

Plant Native Replacements

Spotted Gayfeather (Liatris punctata) also has showy pinkish-purple flowering spikes and is a significant source of nectar for native butterflies and other pollinators. Other ideas are Rocky Mountain penstemon (Penstemon strictus), beebalm (Monarda fistulosa), coneflower (Echinacea spp.) and columbine (Aquilegia spp.). Curlytop knotweed (Polygonum lapathifolium) can outcompete purple loosestrife.

More Information

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

For a full list of noxious weeds contact the Colorado Department of Agriculture at 303-239-4100.

Purple Loosestife Information From

  • Colorado Department of Agriculture
  • Plant Conservation Alliance’s Alien Plant Working Group
  • PLANTS Database
  • Weber, W.A. and R.C. Wittmann (2001) Colorado Flora: Eastern Slope, 3rd Ed. University Press of Colorado.


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