What classifies a noxious weed
A weed is defined as any plant that interferes with management objectives for a given area of land (or body of water) at a given point in time. Once a plant has been classified as a weed, it attains a “noxious” status by Rule as described in the Montana County Weed Control Act. Noxious weeds are defined as “plants of foreign origin that can directly or indirectly injure agriculture, navigation, fish or wildlife, or public health”. Currently there are 27 weeds on a statewide noxious weed list that infest about 8.2 million acres. An additional five will likely be added in 2007. Although there are native and intentionally introduced non-native plants that have invasive characteristics, The State Weed Management Plan focuses on state-listed noxious weeds. Noxious weeds are reducing economic productivity and ecological integrity of Montana’s lands and waters. The rate of introduction and spread of noxious weeds has increased dramatically over the past 150 years with increases in human activities, trade, and commerce. For example, spotted knapweed was first recorded in the state in the early 1920’s. Since that time, it has spread to infest about 3.8 million acres in the state. (Montana Weed Management Plan)

Impacts of noxious weeds
The  ecological and economic impacts caused by noxious weeds in Montana are numerous; the following information describes some of these effects. Water quality and long-term production potential of land can be reduced when tap-rooted species such as spotted and diffuse knapweed invade grasslands. In western Montana, surface runoff was 56% higher and sediment yield was 192% higher on spotted knapweed infested sites compared to those dominated by native bunchgrass (Lacey et al 1989).Exotic species can also alter hydrologic cycles, sediment deposition, erosion, and other ecosystem processes causing serious ecological damage. Saltcedar, a relatively new invader in Montana, impacts wetland and riparian areas by lowering water tables and changing soil properties. This reduces or eliminates surface water habitats required by native plants and animals. Saltcedar infestations also trap more sediment than stands of native vegetation, thus altering the shape, carrying capacity and flooding cycle of water courses (McDaniel et al. 2005).

Noxious weeds are recognized as serious problems on lands managed for wilderness or wildland values by federal, state, and private entities in Montana. When weeds invade and expand into a wilderness environment, the “naturalness” of the area is degraded and scientific values of once biologically diverse landscapes are impaired. Examples include leafy spurge infestations at Pine Butte Swamp Preserve and the remote Danaher Creek area of the Bob Marshall Wilderness, and spotted knapweed invasions in most wilderness areas and National Parks.

The introduction of exotic plants influences wildlife by displacing forage species, modifying habitat structure—such as changing grassland to a forb-dominated community—or changing species interactions within the ecosystem. Leafy spurge reduced habitat utilization by bison (Bos bison), deer (Odocoileus spp.), and elk (Cervus elaphus) (Trammell and Butler 1995) in western North Dakota. Grasshopper sparrow (Ammodramus savanarum) and savannah sparrow (Passerculus sandwichensis) densities were lower on high (>60%) leafy spurge cover than on medium (20 to 60%) or low (0 to 20%) cover (Scheiman et al. 2003). Spotted knapweed was shown to influence elk and deer foraging behavior and population distribution in western Montana. Elk use increased an average of 266% after knapweed was removed from a winter range site (Thompson 1996). Although knapweed is common on most winter ranges in western Montana, studies indicate that the plant is not a major component of mule deer diet
Noxious weeds also impact small birds and mammals. Purple loosestrife, a weed that infests wetlands, was first reported in Montana in 1980 and by 2004 had been reported in 10 counties in the state but has been eradicated in seven of the 10 counties. The weed forms dense infestations that reduce desirable plants, such as cattails, that are preferred habitats for muskrats and long-billed marsh wrens. Waterfowl broods are also more susceptible to predation because dense stands of purple loosestrife reduce access from water to nesting sites (Brown 2005). Changes in bird species have been reported on sites dominated by non-native weed species such as leafy spurge. Russian knapweed has been shown to reduce small mammal populations (mice) by altering species diversity (Kurz 1995).

Economic losses caused by leafy spurge and spotted knapweed have been calculated for Montana. The cost of leafy spurge to grazing lands and wildlands in the upper Great Plains including the states of Montana, North and South Dakota, and Wyoming is estimated at $129.5 million annually and represents a potential loss of 1,433 jobs (Leitch et al. 1994).

 Knapweeds in Montana cost an estimated $42 million annually, money that could support 518 full time jobs in the state. If spotted knapweed invaded 34 million vulnerable acres in Montana, loss to the livestock industry alone is estimated at $155 million (Hirsch and Leitch 1996).

Although significant progress was made in weed management since 2000 (D. Burch personal communication), inadequate financial and manpower resources are available to effectively manage noxious weeds in Montana. Increased funding to private land managers, county weed districts, federal, and state agencies, and improving efficiency and organization of grass-roots efforts are needed to move Montana forward in effective weed management. (Montana Weed Management Plan)