Eradication of alien invasive species in the soil with steam as an alternative to chemical fumigation may allow contaminated soil to be reused. We have investigated steam disinfestation of soil to combat invasive plant species in three experiments including different temperatures and exposure durations using a prototype stationary soil-steaming device. The experiments included effects on seed germination of bigleaf lupine (Lupinus polyphyllus Lindl.), ornamental jewelweed (Impatiens glandulifera Royle), and wild oat (Avena fatua L.; one population from Poland and one from Norway), as well as effects on sprouting rhizome fragments of Canada goldenrod (Solidago canadensis L.) and Bohemian knotweed (Reynoutria x bohemica Chrtek & Chrtková). In Experiment 1, we tested four different soil temperatures of 64, 75, 79, and 98 C with an exposure duration of 90 s. In Experiments 2 and 3, we tested exposure durations of 30, 90, and 180 s and 90, 180, and 540 s, respectively, at 98 C. Seed pretreatment of 14 d cooling for L. polyphyllus and I. glandulifera, no seed pretreatment and 12-h moistening for A. fatua populations, and 5- and 10-cm cutting size for R. x bohemica were applied. Our results showed germination/sprouting was inhibited at 75 C for I. glandulifera (for 90 s) and 98 C for the other species; however, longer exposure duration was needed for L. polyphyllus. While 30 s at 98 C was enough to kill A. fatua seeds and S. canadensis and R. x bohemica rhizome fragments, 180-s exposure duration was needed to kill L. polyphyllus seeds. The results showed promising control levels of invasive plant propagules in contaminated soil by steaming, supporting the steam treatment method as a potential way of disinfecting soil to prevent dispersal of invasive species.

Models have been developed to simulate the long-term effects of weed treatments across a landscape to determine effective management strategies, but those models might not be suitable for evaluating short-term action plans of weed treatments that are specific in time and place. In this study, we developed a simulation model to build and evaluate 5-yr weed treatment plan options in terms of their cost and effectiveness in minimizing total infestation areas over the short-term planning horizon. In an iterative, interactive process, 5-yr treatment plan options are developed based on user-defined weed treatment preferences, and evaluated in terms of total projected infestation areas at the end of the planning horizon. The simulation model was applied to a study area of 24,867 ha (61,447 ac) located in the Salmon River watershed in Idaho. Eight treatment plan options were developed using two treatment priority strategies and four increasing budget levels, and compared for their effectiveness. The application results showed that regardless of budget levels, site priority strategies were more cost-effective than the species priority strategies in reducing total infestation areas over time. This simulation model can provide weed managers with a useful tool to evaluate short-term treatment options, and thus support informed decision-making for effective weed management. Although the availability and quality of input data may be a practical limitation of using the simulation model, more data would become available and improved as more invasive species monitoring programs are implemented.

Japanese climbing fern (Lygodium japonicum) is an invasive vine that has been identified as an economic and ecological threat in forest ecosystems of the Southeast. In two separate studies, we examined the use of directed sprays of glyphosate, imazapyr, and metsulfuron-methyl herbicides, alone and in combination, for control of Japanese climbing fern and for impacts to associated vegetation in mixed bottomland hardwood–cypress forests at two first terrace sites of the Apalachicola River in north Florida. Two yr after treatment, various rates of glyphosate alone generally provided greater percent control (84 to 95%) than imazapyr alone (−25 to 86%) or metsulfuron-methyl alone (−5 to 53%). Combinations of 2% glyphosate and imazapyr and/or metsulfuron-methyl resulted in 81 to 97% control at 2 yr after treatment, with no significant differences among the combination treatments or 2% glyphosate alone. Change in percent cover of associated vegetation groups was not influenced by treatment except for one location where graminoid cover increased 1 yr after metsulfuron-methyl treatment but decreased after imazapyr or glyphosate application. Species richness of dominant understory vegetation remained nearly constant at both locations regardless of treatment. These data indicate that glyphosate alone or in combination with imazapyr or metsulfuron-methyl will provide effective, although not complete, control of Japanese climbing fern, and that directed herbicide sprays minimally impact nontarget vegetation.

Within highway rights-of-way, native perennial grasses provide desirable services to support natural and human constructed ecosystems. However, native perennial grass establishment in annual grass dominated roadsides of semiarid and Mediterranean climates of the western United States requires specific cultural and chemical management treatments to control weeds. In 2004, field studies were conducted in Sacramento Valley, California to determine the effect of herbicide, disc cultivation, and species selection on native perennial grass establishment and annual weed persistence. Perennial grass species mixes common to drier and wetter upland areas in northern California were drill seeded at two sites (I-5 North and I-5 South) that had been burned in 2003 and received weed control (i.e., herbicide, cultivation, mowing) in spring 2004. Herbicides were the most important treatments for native perennial grass establishment and weed reduction. Native perennial grass species persistence was largely unaffected by cultivation or native plant accessions at these sites. Native perennial grass density increased at I-5 North in the second year of growth (2006) resulting in a plant density totaled across all herbicide regimes of 3.9 plants m−1 compared to 2.5 plants m−1 at I-5 South. Vigorous native perennial grass growth in the more fertile and less droughty soils of I-5 North helped to limit annual weeds through competition, which is anticipated to reduce the need for chemical and mechanical control in years following early establishment.

Hoary alyssum [Berteroa incana (L.) DC.] is a nonnative invasive forb that is noxious in California, Idaho, Michigan, Montana, Oregon, Washington, Wyoming, Alberta, British Columbia, and Saskatchewan. Managing B. incana is difficult, because it has an extended flowering period, during which plants simultaneously flower and produce seeds. Consequently, poorly timed herbicide applications may kill B. incana flowers but not prevent viable seed production. We examined how different herbicide management practices used by invasive plant managers affected B. incana seed production and viability the year of application as well as population density 1 yr after application. Professional invasive plant managers sprayed B. incana with various herbicides as part of their current management practices at six sites in southwestern Montana in summer 2016. We collected B. incana plants at 4 wk postapplication for seed biology analyses. Across the six sites, nonsprayed B. incana produced 5 to 1,855 seeds plant−1 and averaged 429 seeds plant−1. Seed production was reduced by 64% to 99% with 7 of the 11 herbicide applications. Berteroa incana seed viability in nonsprayed areas averaged 53% and ranged from 36% to 73% across the sites. Nine of the 10 herbicide applications used by invasive plant managers reduced seed viability 49% to 100%. Few of the herbicide management practices reduced B. incana’s population density the following growing season, suggesting that managers should expect reoccurring infestations at least 1 yr after application. Our results show that invasive plant managers can reduce B. incana viable seed production even when spraying plants that have flowered and formed seed pods. However, sites may need to be monitored for additional years to treat reoccurring infestations.

Quinclorac will control leafy spurge and not injure many established native grasses and forbs. Seeding of desirable species is often required to reestablish native vegetation after an invasive weed-management program, but quinclorac residue may inhibit the reestablishment of native species. Greenhouse studies were conducted to estimate quinclorac dissipation rates in Northern Great Plains soils and the effect of residue on establishment of some native grass and broadleaf plants. Quinclorac 50% dissipation time (DT50) ranged from > 21 to 112 d in four soils from the Northern Great Plains. The quinclorac DT50 was dependent on several factors including soil type, moisture content, temperature, and especially organic matter (OM). Across four different soil textures, quinclorac dissipation generally increased as soil moisture content increased, but moisture had less of an impact in low OM soils. Quinclorac dissipation also increased as temperature increased in the four soils. The most rapid dissipation occurred in soils with higher OM (> 6%), with an average DT50 of < 38 d, at 45% moisture content, held at 16 C. Wild bergamot, purple coneflower, blanketflower, and stiff goldenrod seedling growth were all reduced by quinclorac residue at 6 μg kg−1, the lowest concentration evaluated in the study. The native grass species big bluestem, intermediate wheatgrass, and switchgrass generally were tolerant of quinclorac, but green needlegrass was sensitive, and seedling growth declined as quinclorac residue increased from 6 to 375 μg kg−1. Based on a quinclorac application of 840 kg ha−1 and 150 frost-free d, seeding of sensitive forbs and grasses should be delayed at least 12 mo after herbicide application.

Nonnative invasive plants (NNIP) have far-reaching effects on native ecosystems worldwide. Understanding the role of generalist seed dispersers in spreading NNIP across the landscape is important to the conservation of native ecosystems and to the management of NNIP. We studied white-tailed deer (Odocoileus virginianus) as a seed disperser in a mixed maritime pine (Pinus spp.) forests on Parris Island, SC, with particular interest in the dispersal of Chinese tallowtree [Triadica sebifera (L.) Small], a highly invasive tree species in the southeastern United States, which is a management concern on Parris Island, SC. We collected deer scat pellet groups along transects in two forest types: those that had recently been treated with silvicultural timber harvest (thinned) and those that have not been so treated (unthinned). Using two pellet-treatment methods, directly planting or rinsing and sorting, we determined that, out of 25 species grown under greenhouse conditions, 28% (n = 7) were nonnative, small-seeded, herbaceous species. However, T. sebifera was not identified in either of the two treatment methods. Recent forest thinning significantly affected the number of species determined in deer pellet groups (F = 8.37; df = 1; P < 0.01), with more native plant species identified in unthinned (x̄ = 25 ± 11) than in thinned (x̄ = 3 ± 10) forest stands (F = 5.33; df = 1; P = 0.02). Our results indicate that white-tailed deer are actively dispersing nonnative seeds but not those of T. sebifera or other woody NNIP.

Buffelgrass [Pennisetum ciliare (L.) Link] is an invasive C4 perennial bunchgrass that is a threat to biodiversity in aridlands in the Americas and Australia. Topography influences P. ciliare occurrence at large spatial scales, but further investigation into the relationship between local-scale topography and P. ciliare growth and reproduction would be beneficial. Further, density-dependent effects on P. ciliare growth and reproduction have been demonstrated in greenhouse experiments, but the extent to which density dependence influences P. ciliare in natural populations warrants further investigation. Here we present a study on the relationships between local-scale topography (aspect and slope gradient) and vegetation characteristics (shrub cover, P. ciliare cover, and P. ciliare density) and their interactions on individual P. ciliare plant size and reproduction. We measured slope gradient, aspect, shrub cover, P. ciliare cover, P. ciliare density, and the total number of live culms and reproductive culms of 10 P. ciliare plants in 33 4 by 4 m plots located in 11 transects at the Desert Laboratory at Tumamoc Hill, Tucson, AZ, USA. We modeled the relationships at the local scale of (1) P. ciliare cover and density with aspect and slope gradient and (2) P. ciliare size and reproduction with abiotic (slope gradient and aspect) and biotic (P. ciliare cover and density and native shrub and cacti cover) characteristics. Aspect and slope gradient were poor predictors of P. ciliare cover and density in already invaded sites at the scale of our plots. However, aspect had a significant relationship with P. ciliare plant size and reproduction. Pennisetum ciliare plants on south-facing aspects were larger and produced more reproductive culms than plants on other aspects. Further, we found no relationship between P. ciliare density and P. ciliare plant size and reproduction. Shrub cover was positively correlated with P. ciliare reproduction. South-facing aspects are likely most vulnerable to fast spread and infilling by new P. ciliare introductions.

Flowering rush (Butomus umbellatus L.) is an invasive aquatic and wetland plant capable of developing monotypic stands in emergent and submersed sites. This plant can rapidly outcompete native vegetation and impede human practices by reducing recreation (boating, fishing, and skiing) and disrupting agricultural use of water resources (irrigation canals). Mechanical removal practices occurring biweekly, monthly, bimonthly, and once per growing season were compared with chemical control with diquat applied sequentially at 0.19 ppmv ai for two consecutive months over 2 yr (2016 and 2017). Biweekly removal gave the most consistent control of B. umbellatus biomass and propagules. Diquat application along with monthly and bimonthly clippings gave varying degrees of B. umbellatus control. Clipping once per growing season did not control B. umbellatus when compared with reference plants, while clipping B. umbellatus every 2 wk (biweekly) controlled rush propagules most effectively. However, it is unlikely this method will be sufficient as a stand-alone control option due to the slow speed of harvester boats, the potential these boats have to spread B. umbellatus propagules to more sites, and the expense of mechanical operations. However, clipping could be used as part of an integrated strategy for B. umbellatus control.

Japanese hedgeparsley is a biennial plant that invades roadsides, rights-of way, and forested areas in the midwestern United States. Interest in managing populations by mechanical or hand-clipping techniques exists, but no information is available on the appropriate timing to maximize mortality and prevent the production of viable seed. To assess that, we applied clipping treatments at five periods throughout the summer to three Japanese hedgeparsley populations in southern Wisconsin and measured the number and viability of seeds produced by each plant during the year of treatment and the survival of plants clipped. Japanese hedgeparsley plants began producing seed by mid-July, but production was not maximized until early August. Viable seeds were not produced until early or mid-August, coinciding with the presence of ripened brown fruit. Clipping at any timing resulted in > 95% mortality by the fall of the treatment year. All plants that resprouted were in the vegetative stage when clipped, and no plants survived the following year. Results indicate that clipping Japanese hedgeparsley plants when they are in a reproductive phase before fruit turns brown is an effective management strategy for this invasive plant.

Invasive winter annual grasses, such as cheatgrass (Bromus tectorum L.) are considered serious threats to regional biodiversity. Pollinator populations that depend on the native flora are likely to be negatively impacted as these native species may be displaced by the invasive grass species. Colonization by cheatgrass is also predicted to increase risk of wildfires, as dead plant parts provide fuel in the already dry and arid regions of the western United States. Biocontrol, grazing, prescribed burning, or use of broad-spectrum nonselective herbicides have been suggested as possible means to control B. tectorum. Efficient control may facilitate regrowth of native flora that could in turn support other ecosystem functions. Reporting our findings as a case study, we describe here the results of the application of a preemergent herbicide, indaziflam, that limits germination of B. tectorum seeds. Herbicide was applied to the study locations during the months of December 2016, January 2017, and February 2017. The data reported here on the diversity of flowering plants were collected between May through September 2018. Herbicide-treated plots showed an increase in diversity and abundance of flowering plants compared to the untreated control within two seasons after cheatgrass control was implemented, suggesting that effective reduction of the population of the invasive annual cheatgrass may help facilitate the growth of native forbs. Further studies are necessary to understand mechanisms that facilitate reestablishment of native flowering species, the long-term consequences of reducing invasive annual grasses and to document any residual effects of the herbicide on ground-nesting pollinators.

There is limited documentation of cross-tenure collaborative weed management programs, and no consistent set of metrics for evaluating their performance. In this study, 12 weed management practitioners in southeast Australia participated in a qualitative social research project to discuss and document examples of cross-tenure collaborative weed management and critically reflect on whether existing metrics are suitable for evaluating the performance of their programs. Analysis of focus group discussions, project documentation, subsequent reflections, and review of the literature reveal that weed management practitioners, in Australia and elsewhere, mostly rely on metrics that measure weed management inputs, such as herbicides, labor, and costs. Metrics used to evaluate social outcomes focus on benefits for individuals rather than social relationships or achievement of equitable outcomes. Social research on collaborative governance and social science methods more broadly, such as social network analysis and collective narratives, could be used by weed management practitioners to better evaluate and explain social–ecological outcomes over time.

Japanese knotweed (Reynoutria japonica Houtt.) is an invasive Asian plant abundant along rivers in its introduced range. In riparian areas, floods and ice flows uproot the rhizomes, facilitating their dissemination downstream. Control of large, well-established R. japonica clones in riparian areas is difficult if the use of herbicides is prohibited. An alternative to controlling entrenched clones is the rapid detection and manual unearthing of rhizome fragments that have recently rooted after being deposited by floodwaters. We applied this strategy along a Canadian river where spring floods with abundant ice are recurrent. Two river stretches, with approximately 10 km of shoreline each, were selected for the fragment removal campaign. One of the stretches was heavily invaded by R. japonica, while the other was only sparsely invaded. In the heavily invaded stretch, 1,550 and 737 R. japonica rhizome fragments were unearthed in 2019 and 2020, respectively. Unearthed fragments had an average length of 27 to 32 cm. Only 21 fragments were found in the sparsely invaded stretch in 2020. Despite similar distances being surveyed, the detection and unearthing took 62% less time (overall) in the sparsely invaded than in the heavily invaded stretch. Along sparsely invaded riverbanks, a rapid response removal campaign for R. japonica cost, including transportation and labor, an estimated Can$142 (US$105) per aborted clone (i.e., fragment removed). A rapid response removal campaign is economically advantageous compared with the hypothetical eradication of large, well-established clones, but for it to be cost-effective, the time spent locating rhizome fragments must exceed the time spent unearthing them. The question is not whether rapid response unearthing is economically feasible—it is—but rather what invasion level renders the intervention practicable. In highly invaded river stretches generating thousands of fragments annually, finding and removing these fragments year after year would require a massive, unsustainable effort.

Nondestructive means for estimating air potato (also known as air yam; Dioscorea bulbifera L.) biomass will help gauge its management efficacy over time. We developed allometric equations to estimate total and fractional biomass components and densities of aerial bulbils and underground tubers of field-grown D. bulbifera in Florida. We selected four naturally infested sites representing its southern, central, and northern distribution in Florida and measured three independent variables (vine densities, stem diameters, and top heights) of 84 (21 site−1) discrete D. bulbifera patches during late October to early December of 2012. We destructively harvested D. bulbifera biomass, sorted by tubers, stems, leaves, and bulbils; counted units of bulbils and underground tubers (dependent variables); and dried to a constant weight. Mean percentages of tuber, stem, leaf, and bulbil fractions in total biomass were 42.0, 15.6, 26.0, and 16.4, respectively. We developed a parameterized multiplicative prediction model and regression equation for each dependent variable. Slopes of relationships among independent and dependent variables varied by biomass and density (bulbil and tuber) of plant components. Multiplied values of independent variables: all three for total, tuber, stem, and leaf biomass; two (vine base diameter*patch height) for bulbil biomass; two (vine density*patch height) for bulbil density; and only one (stem density) for tuber density provided best (R2-based) prediction values. These models will provide nondestructive methods for estimating biomass components and density of vegetative propagules of naturally growing D. bulbifera. Models are critical for understanding the performance of D. bulbifera in its exotic range, estimating biomass to project control costs, and comparing biomass components and bulbil/tuber densities during pre- and postmanagement periods to gauge control efficacy.

Although it has been demonstrated that environmental changes within a year can affect the reproduction, survival, and growth of invasive species, these factors have rarely been incorporated into demographic analyses. Therefore, we applied multistate demographic models (based on capture–recapture animal methods accounting for imperfect detectability of individuals in natural conditions) to evaluate the effects of reproductive phenology and rainy season on the survival and transition/retrogression rates among stage categories of black elderberry (Sambucus nigra L.)—an invasive tree species widely distributed in temperate forests of Europe and America. In the Abies religiosa temperate forest, Mexico City, a multistate demographic model of S. nigra was built using bimonthly censuses during a year. We selected the best-fitting model according to Akaike’s information criterion adjusted for small sample sizes (AICc). We determined the response of reproductive phenology of S. nigra to the rainy season for 2 yr through repeatability and phenotypic plasticity indexes. Our results showed that the reproductive phenology of S. nigra has a low repeatability index and a high phenotypic plasticity index. We demonstrated that additive and interactive effects of reproductive phenology and rainy season promote changes in survival and transition/retrogression rates among stage categories. During the rainy season, the survival probability of seedlings and transition probability toward the adult category increased. Therefore, our study represents a significant contribution to the knowledge of the demographic dynamics of invasive species on an intra-annual scale.

Black greasewood (greasewood) and gray rabbitbrush are important shrub components of several plant communities throughout western North America. Land managers may view these species differently depending on their goals. Greasewood and gray rabbitbrush shrub communities may be invaded by several invasive plants including Russian knapweed, perennial pepperweed, hoary cress, halogeton, and several annual mustards. Metsulfuron and chlorsulfuron are commonly used for controlling these and other invasive plants, but little is known regarding their impacts on greasewood and gray rabbitbrush. Our objective was to quantify the impact of these herbicides on greasewood and gray rabbitbrush communities from both an efficacy and nontarget impact perspective. Field studies were established in the spring of 2004 and repeated twice in 2005 near Laramie, WY, in a pasture with mixed stands of greasewood and gray rabbitbrush. Treatments included metsulfuron applied at 21, 42, 63, 84, 126, and 168 g ai/ha (0.3, 0.6, 0.9, 1.2, 1.8 and 2.4 oz ai/A), chlorsulfuron applied at 52, 105, and 157 g ai/ha (0.75, 1.5 and 2.25 oz ai/A), and an untreated control. All treatments contained methylated seed oil at 2% v/v. Treatments were applied in mid-June to 3.3 by 9-m (10 by 30 ft) plots with a handheld broadcast sprayer delivering 187 L/ha (20 gal/ac) in a randomized complete block, with three blocks per study. Plots were sampled 12 and 24 mo after treatment (MAT), utilizing visual control estimates and point frame sampling for vegetative cover of greasewood, gray rabbitbrush, perennial grasses, and bare ground. Metsulfuron at 42 g/ha and chlorsulfuron at 105 g/ha provided > 75% visual control of greasewood 24 MAT. For gray rabbitbrush, metsulfuron at 63 g/ha provided approximately 60% control 24 MAT, while chlorsulfuron provided negligible control at any rate. These results suggest differential impacts of these herbicides on greasewood and gray rabbitbrush, and provide land managers with a decision tool for noxious and invasive weed control when managing for or against greasewood and gray rabbitbrush.

Eastern redcedar (Juniperus virginiana L. var. virginiana; hereafter ERC) is a native species currently invading open areas and grasslands outside of its original range in the United States. We studied ERC’s invasion patterns in the Lakeside Daisy State Nature Preserve (LDSNP), a short grass prairie located on the Marblehead Peninsula in Ohio, examining the changes in the genetic diversity and structure of the encroaching population. We investigated the relative importance of long-distance dispersal versus diffusion in the invasion of this short grass prairie by ERC. We use eight microsatellite marker loci to infer gene flow from external sources versus within-population recruitment. We found that the older trees in this preserve were less than 50 yr old, indicating that the population was established between 1970 and 1980. When we grouped trees into five age categories of 10-yr increments, we found that the allelic diversity, as indicated by the average number of alleles per locus, increased as the age of the trees decreased. We also found that not all loci were in Hardy-Weinberg equilibrium, probably due to the arrival of new variants in the preserve. Moreover, heterozygosity remained high, with an excess of heterozygotes in all age groups (F = −0.163 ± 0.046). Principal coordinate analysis showed two distinct groups of trees in the LDSNP. Analysis of the cryptic population structure of the ERC trees using STRUCTURE revealed four ancestral clusters in the ERC population. All ancestral clusters are present in all age groups, suggesting that all trees sampled are derived from an admixed population. Furthermore, the high observed heterozygosity and lack of inbreeding in this dioecious species maintained all ancestral clusters over time. Overall, our findings indicate that ERC encroachment of the LDSNP results from multiple and reiterated gene flow events from the edge of the range through animal-mediated seed dispersal.

In 1994, a general population survey was conducted to evaluate Montanans’ knowledge about noxious weeds, and results from that survey launched an ongoing statewide education campaign. In 2019, we conducted another general population survey to assess the views and behaviors of Montanans as they relate to noxious weeds and to identify new approaches for disseminating noxious weed information. We also asked questions to evaluate changes over the 25-yr period, although our ability to make direct comparisons is subject to limitations. We implemented a mail-based survey in March through May 2019. The response rate was 18%, with 830 responses from 4,582 valid mailing addresses. Just under half (48%) of respondents report “little” or “no” knowledge about noxious weeds, which would constitute a 19-point improvement since 1994. A large majority (68%) of respondents indicate that noxious weeds are a “serious” or “very serious” problem, and appreciation for the range of negative impacts associated with noxious weeds is considerable. Most respondents (61%) identify humans as contributing “a lot” to noxious weed spread, and respondents report that their behaviors to prevent the spread of noxious weeds have increased over time. While the 1994 respondents rated television, newspaper, and radio as the best ways to disseminate information about noxious weeds, respondents now also recognize methods such as websites and social media pages as effective. Our survey identifies a need for increased educational messaging for women and people in the 18 to 39 age group. Overall, our results indicate that individuals who have seen different forms of advertisements and have participated in educational programs are more likely to consider noxious weeds a serious problem and to engage in behaviors to stop their spread.

When exotic species invade a region, it becomes important to assess their invasiveness in adjacent uninvaded regions to determine if weed prevention measures are needed. Leafy spurge and knapweed species are absent from the vast majority of eastern Montana, but the region is surrounded by regions heavily invaded by these species. To assess invasiveness of leafy spurge and Russian and spotted knapweed in common eastern Montana grassland sites, I introduced these species to three sites as seeds (120 live seeds plot−1) and seedlings (6 plot−1). I assessed how common grazing regimes influenced invasiveness by imposing cattle, sheep, mixed grazing (i.e., cattle plus sheep), and grazing exclusion treatments for 7 yr. Invader survival did not appear to differ greatly among sheep, cattle, and mixed grazing treatments, but excluding grazing lowered probabilities that plots maintained invaders for the entire study period at two of three sites. At these same sites, grazing exclusion increased growth rates of those invaders that did survive, at least in the case of leafy spurge. Regardless of grazing treatment or site, however, large proportions of plots did not maintain invaders through the end of the study period. At one heavy clay site, only one small leafy spurge plant persisted through the end of the study. In the seventh study year, the plots with the most leafy spurge and Russian knapweed produced 222 and 112 stems, respectively, and the stems remained mostly confined to the 2- by 2-m plots. These findings suggest that, barring intense disturbance, leafy spurge and spotted and Russian knapweed might be incapable of invading some grasslands of eastern Montana, particularly upland sites with high clay content. Any upland sites in the region these species are capable of invading will likely be invaded only very slowly.