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Idaho has about 800 of the nations 2,000 weed species, most of which are alien to the state. Idaho's noxious weeds have been introduced from other regions. We do not have the natural systems to keep them in check. Importing natural controls is not possible for all weeds, and it takes decades to and vast resources to introduce naturals controls. As a result, noxious weeds overwhelm native plant communities and disturbed areas, spreading steadily year by year. Several species that were previously unreported have been found each year over the last 10 years. Recognition of invaders is the first step in dealing with them.

Noxious weeds spread rapidly as if a cancer in the natural system. They reproduce, multiply, and spread as an explosion of environmental pollution, in slow motion, While many pollutants tend to decay and become dilutes, weeds become concentrated and spread to new locations. Once the habitat becomes infested it may never recover to a native community even with intensive management

Prevention and inspection are the most economical management system for reducing the rate of noxious weed spread. Once noxious weeds have become established expect to spend $30 to $70 per acre for management on your land or in higher taxes for their management on public land.

What is a noxious weed?

Noxious weeds are plant species that have been designated "noxious" by law. The word "noxious" simply means deleterious, and all listed weeds are deleterious by definition. There are hundreds of weed species in Idaho; however, only 35 are designated noxious by Idaho law as of 1977. There are hundreds of weeds species in Idaho; however, only 35 are designated noxious by Idaho Law as of 1997. Currently, the Idaho Department of Agriculture uses the following criteria for designation of a noxious weed:

  1. It must be present in but not native to Idaho.

  2. It must be potentially more harmful than beneficial to Idaho.

  3. Eradication must be economically physically feasible.

  4. The potential adverse impact of the weed must exceed the cost of control

The purpose of the Idaho Noxious Weed Law is to protect lands within  the state from invasion by noxious weeds. The Idaho Noxious Weed Law requires landowners to eradicate noxious weeds on their land, except in special management zones.

Managing Noxious Weeds

The procedure to control noxious weeds depends on the species of weed, the habitat, the surrounding environment, and the availability of equipment, materials and personnel. Eradication and restoration require that weeds be killed. Their very nature make that difficult, for they are invaders by nature and do not succumb to control except at great cost.

Guidelines to help lessen the spread of noxious weeds in Idaho

  • Avoid driving in noxious weed infested areas. Seeds can become stuck in tire treads or mud on the vehicle and be carried to unaffected areas.

  • Don't transport flowering plants that you cannot identify.

  • If you find a small number of isolated noxious weeds that have no flowers or seeds, pull the weeds and leave them where you found them to dry out.

  • If you find noxious weeds and they have flowers or seeds, pull them, place them in a plastic bag or container to avoid spreading seeds, and either burn them or dispose of them in a sanitary landfill.

  • Report newly-found noxious weeds to the county weed superintendent or county extension office. If you need help in identifying new weeds here is the procedure.

Noxious Weeds
of Idaho

Black Henbane


Canada Thistle

Common Crupina

Dalmatian Toadflax

Diffuse Knapweed

Dyer's Woad

Field Bindweed

Hoary Cress


Jointed Goatgrass

Leafy Spurge


Meadow Hawkweed

Meadow Knapweed


Musk Thistle

Orange Hawkweed

Perennial Pepperweed

Perennial Sowthistle

Poison Hemlock


Purple Loosestrife

Rush Skeletonweed

Russian Knapweed

Scotch Broom

Scotch Thistle

Silverleaf Nightshade

Skeletonleaf Bursage

Spotted Knapweed

Syrian Beancaper

Tansy Ragwort

Toothed Spurge

Yellow Starthistle

Yellow Toadflax

1999 University of Idaho: Text and photographs for these pages from Idaho's Noxious Weeds, by Robert H. Callihan and Timothy W. Miller (revised by Don W. Morishita and Larry W. Lass).

Please contact: Ag Publishing, University of Idaho, Moscow, Idaho 83844-2240; 208 882-7982;; or visit the Resources for Idaho website at, for more information about this or other publications.


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