Medusahead is among the most invasive grasses in the western United States. Selective control of this noxious winter annual grass is difficult in California grasslands, as many other desirable annual grasses and both native and nonnative broadleaf forbs are also important components of the rangeland system. Intensive grazing management using sheep is one control option. This study was designed to determine the optimal timing for sheep grazing on heavily infested medusahead sites, and to evaluate the changes in species composition with different grazing regimes. Midspring (April/May) grazing reduced medusahead cover by 86 to 100% relative to ungrazed plots, regardless of whether it was used in combination with early spring or fall grazing. Early spring (March) or fall (October to November) grazing, alone or in combination, was ineffective for control of medusahead. In addition, midspring grazing increased forb cover, native forb species richness, and overall plant diversity. At the midspring grazing timing, medusahead was in the “boot” stage, just prior to exposure of the inflorescences. The success of this timely grazing system required high animal densities for short periods. Although this approach may be effective in some areas, the timing window is fairly narrow and the animal stocking rates are high. Thus, sheep grazing is unlikely to be a practical solution for management of large medusahead infestations

Giant reed is an invasive plant of riparian habitats throughout California and the United States. Glyphosate is approved for controlling giant reed in California. Sources indicate that 1.5% to 5% glyphosate solutions are effective at controlling giant reed. There is little published data on the relative efficacy of different concentrations of glyphosate which can be used to select an appropriate application regime for California habitats. We conducted two field experiments to test the hypothesis that glyphosate concentrations of 1.5%, 3%, and 5% applied as foliar sprays were equally effective at killing giant reed plants. Leaf chlorophyll content and the proportion of living stems declined significantly following treatment with 1.5% or greater solutions of glyphosate. New stems were observed the spring following treatment for plants treated with 1.5% glyphosate. No new stems were observed for plants treated with either 3% or 5% glyphosate. A treatment that included “bending and breaking” stems prior to treatment with 5% glyphosate provided control similar to 5% glyphosate alone. There was no there evidence that plants sprayed with only a mixture of the surfactant (Agridex), water, and a marking dye were affected beyond the short-term. These results indicate that 3% or 5% foliar applications with glyphosate were the most effective and consistent treatments for killing giant reed with a single late-season application. This result is especially important if the goal of the treatment program is to minimize the number of treatments, thus reducing labor costs and minimizing impacts on sensitive habitats by reducing the number of site visits.

Chemical, cultural, and biological methods have been developed to control leafy spurge in a variety of environments. Aphthona spp. biological control agents have established throughout the northern Great Plains and Rocky Mountain region and successfully controlled leafy spurge in many areas, but notable exceptions include areas with sandy soils. Leafy spurge control can be improved by combining methods such as chemical, biological, or cultural treatments, compared to a single method used alone. The effects of Aphthona spp., imazapic herbicide, and interseeded native grass species alone or in combination for leafy spurge control were evaluated at two locations in southeastern North Dakota for 5 yr. Both the Sheyenne National Grassland (SNG) and Walcott, ND, study locations had greater than 80% sand soil. Leafy spurge stem density, canopy cover, and yield were reduced for 1 to 2 yr in all treatments that included imazapic, with no difference in control between single and combination treatments. Aphthona spp. and interseeded native grasses alone or combined did not reduce leafy spurge density or cover. Aphthona spp. population remained low throughout the study at both locations. Forb yield increased during the study at the SNG but not the Walcott location. Conversely, warm-season grass yield increased at Walcott but not at the SNG. Leafy spurge stem density declined from 92 to 50 stems/m2 in 5 yr at the SNG site. The decline could not be attributed to specific treatments applied in this study and may be due to self-limitation or soil pathogens.

Aminopyralid efficacy on Canada thistle and changes in species composition in both Canada thistle–infested and native plant communities were evaluated in Theodore Roosevelt National Park, ND. Aminopyralid at 120 g ae/ha was applied in September 2004 to one-half of 30 native or 30 Canada thistle–infested sites. Canada thistle density 10 mo after treatment (MAT) averaged 2 stems/m2 compared to 31 stems/m2 in the control and 22 MAT averaged 16 stems/m2 compared to 42 stems/m2. Although Canada thistle was reduced in treated sites, species richness was similar to the untreated control. Species evenness increased in treated subplots compared to the control as foliar cover of low- and high-seral forbs and high-seral monocot species increased. Species richness was reduced in native plant communities treated with aminopyralid from approximately 12 to 9 species but evenness was not affected. Plant species diversity was similar in Canada thistle–infested treated and nontreated subplots but was reduced by aminopyralid in native subplots. Despite the decrease in Canada thistle and increased foliar cover of native perennial plants, composition and structure 22 MAT in the Canada thistle–infested plant community did not resemble native plant communities. Native plant communities were composed of many perennial monocots and high-seral forbs, which had increased but were in lower relative abundance in the treated Canada thistle–infested subplots.

The importance of dispersal in the establishment and proliferation of exotic populations make this life history stage critical in the prediction and management of biological invasions. We observed the dispersal of seeds by patches of smooth brome invading northern fescue prairies and applied an inverse power function model to explore its potential invasion patterns. Based on our observations of two northern fescue prairies in Riding Mountain National Park, Manitoba, Canada, patterns of potential invasion were contingent upon the dispersal of seeds as individual florets or aggregated within spikelets and panicles. For example, although the majority of dispersed seeds were intercepted within one meter, inside and outside the margins of invading patches, slopes of the log–log plots of seed number against their dispersal distance were steeper for seeds dispersed as spikelets than individual florets. Despite the observed aggregation of seeds along the margins of invading patches, the number of dispersed seeds was poorly correlated with that germinated from the seed bank. The shallow dispersal gradient of individual florets and spikelets, combined with the steeper gradient of panicles suggest that smooth brome is capable of simultaneously invading along dense fronts and by establishing isolated foci. Although low correlations between the number of dispersed seeds and their recruitment from the seed bank might suggest postdispersal transport of seeds, other mechanisms, including seed predation and pathogens, remain unexplored. Conservation and restoration of northern fescue prairies must include efforts to control the dispersal of smooth brome seeds and reduce opportunities for their establishment.

Invasion by cheatgrass and the associated high fire frequency can displace native plant communities from a perennial to an annual grass driven system. Our overall objective of this study was to determine the potential to favor desired native perennial bunchgrasses over annual grasses by altering plant available mineral nitrogen (N). In the first study, we grew cheatgrass and three native bunch grasses (native grasses were combined in equal proportions) in an addition series experimental design and applied one of three N treatments (0, 137, and 280 mg N/kg soil). Regression models were used to derive the effects of intra- and interspecific competition on individual plant yield of cheatgrass and the native bunch grasses (combined). In our second study, we compared the absolute growth rate of the four plant species grown in isolation in a randomized complete block design for 109 days under the same soil N treatments as the competition study. Predicted mean average weight of isolated individuals increased with increasing soil N concentrations for both cheatgrass and the three native perennials (P < 0.05). Biomass of cheatgrass and its competitive ability increased with increasing soil N concentrations (P < 0.0001) compared to the combined native bunchgrasses. However, the greatest resource partitioning occurred at the 137 mg N/kg soil N treatment compared to the 0 (control) and 280 mg N/kg soil treatments, suggesting there may be a level of N that minimizes competition. In the second study, the absolute growth of cheatgrass grown in isolation also increased with increasing N levels (P = 0.0297). Results and ecological implications of this study suggest that increasing soil N leads to greater competitive ability of cheatgrass, and that it may be possible to favor desired plant communities by modifying soil nutrient levels.

To eradicate a weed invasion, its extent must be delimited and each infestation must be extirpated. Measures for both of these criteria are utilized to assess the progress of current eradication programs targeting mikania vine and limnocharis in northern Australia. The known infested area for each species is less than 5 ha and has remained largely static for the last 3 or more years against a backdrop of refined and enhanced detection methods. This suggests that delimitation has been approached, if not achieved. Different methods of detection have their places, relative to the stage of the program and the spatial distribution of infestations. Although all known infestations of both species are effectively monitored and controlled, ongoing emergence from persistent seed banks limits progress towards the extirpation of infestations to a slow, but measurable, rate.

Invasion by annual grasses, such as cheatgrass, into the western U.S. sagebrush-steppe is a major concern of ecologists and resource managers. Maintaining or improving ecosystem health depends on our ability to protect or re-establish functioning, desired plant communities. In frequently disturbed ecosystems, nutrient status and the relative ability of species to acquire nutrients are important drivers of invasion, retrogression, and succession. Thus, these processes can potentially be modified to direct plant community dynamics toward a desired plant community. The overall objective of this review paper is to provide the ecological background of invasion by exotic plants and propose a concept to facilitate the use of soil nitrogen (N) management to achieve desired plant communities that resist invasion. Based on the literature, we propose a model that predicts the outcome of community dynamics based on N availability. The model predicts that at low N levels, native mid- and late-seral species are able to successfully out-compete early-seral and invasive annual species up to some optimal level. However, at some increased level of N, early-seral species and invasive annual grasses are able to grow and reproduce more successfully than native mid- and late-seral species. At the high end of N availability to plants, the community is most susceptible to invasion and ultimately, increased fire frequency. Soil N level can be managed by altering microbial communities, grazing, mowing, and using cover crops and bridge species during restoration. In these cases, management may be more sustainable since the underlying cause of invasion and succession is modified in the management process.

Garlic mustard is among the most important invasive weeds of North American eastern deciduous forests. Investigations of the mechanisms that enable its success as an invader require a simple method to propagate this weed in the laboratory and the greenhouse; we develop such a method in this study. Cold treatment (24-h dark cycle; maximum 6 C, minimum −1 C) for at least 100 d on a moist organic mix, followed by incubation at temperatures approximating spring (maximum 15 C, minimum 6 C), results in close to 100% germination. The information presented here will be valuable in studies requiring a steady supply of garlic mustard plants for experimentation and for the mass rearing of biological control agents.

Land managers commonly assume that nonindigenous plant species (NIS) are rapidly increasing in population size in all environments in which they occur. In fact, these plant species have differing levels of invasiveness depending on environment. A method was developed that quantifies invasiveness of a plant population based on annual changes in plant density and area occupied, within a series of permanently placed 1 m2 (10.76 ft2) monitoring plots. An invasiveness index (I) was calculated from the change in proportion of cells occupied and the proportions of cells that had growth rates > 1 and < 1; the possible value is restricted from −4 to +4. The method was tested on populations of yellow toadflax over 6 yr on a total of six populations within three distinct environments (Ridge, Valley, and Forest). Invasiveness values were different between environments. The Ridge populations had the highest mean level of invasiveness (I = 0.31), followed by the Valley (I = 0.26), and then the Forest (I = −0.90). Invasiveness also varied by year. The highest annual value of invasiveness was at the Ridge (I = 1.77) and the lowest was at the Forest (I = −1.90), both in 2005. Values of invasiveness were correlated (Pearson's correlation coefficient = 0.82) with the traditional calculations of population growth rate, but our method provides an enhanced measure of invasiveness because it includes information on both change in population area and density. This research shows that populations of yellow toadflax are not equally invasive in different environments or through time, although consistent patterns can be observed. The method presented and tested was implemented in approximately three person-days per year at less than $500 per year, and can be used to quantify the invasiveness of plant populations and thus allow land managers to prioritize the most invasive populations for management.

Cuban club-rush is an invasive aquatic weed that is spreading northward in the southeastern United States. It is reported for the first time from Mississippi and from significantly farther northward in Alabama than was previously known. Cuban club-rush dissemination and rapid population growth are attributed to two types of reproduction: corky floating achenes and asexual reproduction by fragmentation. An illustration of Cuban club-rush and photos of its habit and habitat are provided.