Residents in Stillwater County, MT, used a weed bounty program as an educational tool to promote awareness, detection, and control of spotted knapweed (Centaurea maculosa Lam. # CENMA) on range and noncropland. Many people became involved with the weed control effort, weed awareness was increased, spotted knapweed was controlled, and the program was cost effective. The success of a weed bounty program is influenced by 1) the enthusiasm and innovations of the county extension agent, weed supervisor, or other key individual or group in the community; 2) the choice of the weeds selected; and 3) good media coverage of the bounty program. With proper planning and organization, weed bounty programs effectively promote awareness and action in other areas of public concern.

Competition between bluebunch wheatgrass and spotted knapweed was quantified using three addition series experiments in an environmental chamber. Using two density matrices, spotted knapweed plants were thinned by either 45 or 90% of their original densities 40 d after emergence. Another matrix of density combinations was not thinned. All plant material was harvested 90 d after thinning. Spotted knapweed was about four times more competitive than bluebunch wheatgrass. Reducing spotted knapweed by 45% did not alter the competitive relationship between the two species. We believe remaining knapweed individuals captured the majority of the newly available resources. Ninety percent reduction was necessary to shift the competitive relationship in favor of bluebunch wheatgrass. Successful integrated spotted knapweed management must exploit key mechanisms and processes directing plant community dynamics, in conjunction with weed density reduction, if communities are to be shifted toward those that are desired.

Diffuse knapweed, a biennial or short-lived perennial, and spotted knapweed, a perennial, are taprooted Eurasian weeds invading rangeland in the western United States and Canada. Knapweed (Centaurea spp.) invasion is associated with reductions in biodiversity, wildlife, and livestock forage, and increased erosion. Spotted knapweed grows to about 1 m and usually has purple flowers, whereas diffuse knapweed is slightly shorter, usually with white flowers. Persistent flower bracts on diffuse knapweed bear a rigid terminal spine about 8 mm long with four or five pairs of shorter lateral spines. Bracts on spotted knapweed have dark spotted tips. Knapweed management involves a combination of containing infestations and control efforts. Hand pulling in areas with small infestations can be effective for controlling spotted and diffuse knapweeds. Picloram applied at 0.28 kg ha−1 provides control for about 3 yr. Effective long-term control of knapweeds requires periodic applications of picloram, which are only cost-effective on highly productive range sites with a residual grass understory. About 12 insect species have been released for knapweed biocontrol. Seed production has been reduced by 46% by insects feeding in the flower heads. Although insects have not reduced spotted knapweed densities, they may stress the weed and shift the competitive balance to associated species. Sheep grazing reduces the density of very young seedlings and may limit seedling recruitment into the population. In areas without a residual understory of desired plant species, revegetation of knapweed-infested rangeland is required. Components of any integrated weed management program are sustained effort, constant evaluation, and the adoption of improved strategies.

The influence of spotted knapweed on surface runoff and sediment yield was determined under simulated rainfall conditions near Garrison, MT. Comparisons were made in 12 paired plots: one of each pair dominated by bunchgrass and the other dominated by spotted knapweed. Runoff and sediment yield were 56% and 192% higher, respectively, for the spotted knapweed, rather than the bunchgrass vegetation types. Spotted knapweed invasion onto bunchgrass range of western Montana is thus detrimental to the protection of soil and water resources.

Soil characteristics, elevation, annual precipitation, potential evapotranspiration, length of frost-free season, and mean maximum July temperature were estimated for 116 established infestations of spotted knapweed (Centaurea maculosa Lam. # CENMA) in Montana using basic land resource maps. Areas potentially vulnerable to invasion by the plant were delineated on the basis of representative edaphic and climatic characteristics. No single environmental variable was an effective predictor of sites vulnerable to invasion by spotted knapweed. Only a combination of variables was effective, indicating that the factors that regulate adaptability of this plant are complex. This technique provides a first approximation map of the regions most similar environmentally to infested sites and, therefore, most vulnerable to further invasion. This weed migration prediction technique shows promise for predicting suitable habitats of other invader species.

Experiments were established in 1982 to study the loss of viability of spotted knapweed seeds (achenes) in soil. Greater than 50 and 25% of buried seeds remained viable but dormant after 5 and 8 yr at two locations, respectively. Decline in seed viability was also measured in two natural seedbanks. Although the soil seed reserve decreased by 95% over a 7-yr period, approximately 400 000 viable seeds per ha remained, indicating that spotted knapweed seeds will last for many years in soil.

A survey was conducted at 88 sites in Montana in 1984 and 1985 to determine the distribution of Urophora affinis Frauenfeld and U. quadrifasciata (Meigen), two seed head flies released on spotted knapweed (Centaurea maculosa Lam. # CENMA) in North America. U. affinis, released in Montana during 1973 to 1977, was found at 40 of the 88 sites. The fly was concentrated within a short radius (about 5 to 8 km) of release sites, even at sites having well-established populations. U. quadrifasciata, released in British Columbia in 1972, but not in the United States, was found at 84 sites. The data suggest that the fly entered the state in the northwest corner and dispersed in a southeasterly direction. There was a strong positive relationship between mean number of galls per seed head and percent infestation for both fly species.

The Chemical Movement through Layered Soils (CMLS) model was modified and combined with the USDA-SCS State Soil Geographic Data Base (STATSGO) and Montana Agricultural Potentials System (MAPS) digital databases to assess the likelihood of groundwater contamination from selected herbicides in Teton County, MT. The STATSGO and MAPS databases were overlaid to produce polygons with unique soil and climate characteristics and attribute tables containing only those data needed by the CMLS model. The Weather Generator (WGEN) computer simulation model was modified and used to generate daily precipitation and evapotranspiration values. A new algorithm was developed to estimate soil carbon as a function of soil depth. The depth of movement of the applied chemicals at the end of the growing season was estimated with CMLS for each of the soil series in the STATSGO soil mapping units and the results were entered into ARC/INFO to produce the final hazard maps showing best, weighted average, and worst case results for every unique combination (polygon) of soil mapping unit and climate. County weed infestation maps for leafy spurge and spotted knapweed were digitized and overlaid in ARC/INFO with the CMLS model results for picloram to illustrate how the results might be used to evaluate the threat to groundwater posed by current herbicide applications.

Seed response to grinding in a hammer mill or extrusion through a pellet die was determined. Less than 1% of spotted knapweed, sulfur cinquefoil, timothy, and alfalfa seeds were intact after grinding to pass through a 1-mm screen. Intact seeds of all tested species except sulfur cinquefoil were able to germinate after grinding. Extruding unground alfalfa seed through a die to make hay pellets decreased viability 60 to 63%. After grinding and extrusion through a grain pellet die, 53% of intact alfalfa seeds in the pellets were viable. These data demonstrate that feed pellets can contain viable weed seeds.

Spotted knapweed (Centaurea maculosa Lam. # CENMA) communities were sampled to determine the relationship between age and the number of root rings, and the population age structure. Spotted knapweed taproots add one ring of secondary xylem annually. In 1984, populations were expanding with high densities of individuals in the early age classes, followed by a steady decline in the older classes. In 1985, the majority of the individuals in knapweed populations were in the older age classes. This change in the population age structure was attributed to high mortality among the young age classes due to a drought in 1985. The maximum age class at the sites ranged from 5 to 9 yr. The percentage of plants with floral stalks increased with age to a peak of 75% in the fifth year in 1984 and in the seventh year in 1985.

Rangeland and pastures comprise about 42% of the total land area of the United States. About three-quarters of all domestic livestock depend upon grazing lands for survival. Many ranges have had domestic stock grazing for more than 100 years and, as a result, the plant composition has changed greatly from the original ecosystems. Western rangelands previously dominated by perennial bunchgrasses have been converted, primarily through overgrazing, to annual grasslands that are susceptible to invasion by introduced dicots. Today there are more than 300 rangeland weeds in the United States. Some of the most problematic include Bromus tectorum, Euphorbia esula, Centaurea solstitialis, C. diffusa, C. maculosa, and a number of other Centaurea species. In total, weeds in rangeland cause an estimated loss of $2 billion annually in the United States, which is more than all other pests combined. They impact the livestock industry by lowering yield and quality of forage, interfering with grazing, poisoning animals, increasing costs of managing and producing livestock, and reducing land value. They also impact wildlife habitat and forage, deplete soil and water resources, and reduce plant and animal diversity. Numerous mechanical and cultural control options have been developed to manage noxious rangeland weeds, including mowing, prescribed burning, timely grazing, and perennial grass reseeding or interseeding. In addition, several herbicides are registered for use on rangelands and most biological control programs focus on noxious rangeland weed control. Successful management of noxious weeds on rangeland will require the development of a long-term strategic plan incorporating prevention programs, education materials and activities, and economical and sustainable multi-year integrated approaches that improve degraded rangeland communities, enhance the utility of the ecosystem, and prevent reinvasion or encroachment by other noxious weed species.

To identify the most commonly regulated weedy plants in the United States and southern Canada, we compiled a database of noxious weed lists obtained from the 48 continental states and six bordering provinces. The 10 most frequently listed weeds are Cirsium arvense, Carduus nutans, Lythrum spp. (includes purple loosestrife), Convolvulus arvensis, Euphorbia esula, Acroptilon repens, Sorghum spp. (includes johnsongrass and shattercane), Cardaria spp. (includes hoary cress, also called whitetop), Centaurea maculosa, and Sonchus arvensis. When genera are ranked, the top genus is Centaurea, which includes C. maculosa, C. diffusa, and C. solstitalis. Biological control programs have targeted many of the top dicotyledonous weeds of national concern, but none of the weedy grasses and sedges. We recommend that exploratory studies be initiated to determine the feasibility of developing biological control agents for the latter species. The complete database of noxious weed lists is available on the Internet at http://invader.dbs.umt.edu. This information may be useful to resource managers and regulatory officials in assessing which weeds are problematic in adjacent geographic areas and by researchers to help select which weeds to target with new management strategies.