Ornamental grasses may become invasive weeds depending on their ability to naturalize and outcompete other species. Miscanthus sacchariflorus (Maxim) Franch. (Amur silvergrass) is a tall, self-incompatible, nonnative grass that has become naturalized in eastern North America, forming monospecific stands and raising concerns about its potential invasiveness. To understand the extent of clonal and sexual reproduction in feral populations, we examined their population genetic structure, seed production, and ploidy. We surveyed 21 populations in Iowa and Minnesota using eight polymorphic microsatellite markers. Only 34 multilocus genotypes (MLGs) were detected from a total of 390 samples. All of the study populations had more than one MLG, thereby allowing cross-pollination with near neighbors, but most were dominated by one or a few MLGs. Low genetic divergence suggests that all populations may have originated from similar cultivars. Cluster analysis showed that the six populations from Minnesota were extremely similar to each other, whereas the 15 populations from Iowa were somewhat more diverse. Seed production was quantified for 20 populations and ploidy for 11 populations. Average seed production was very low (< 0.30 seeds per panicle), although most populations did produce seeds. Because the populations were diploid (2x), they also may have the potential to hybridize with ornamental varieties of Miscanthus sinensis (Chinese silvergrass; eulaliagrass), a diploid close relative. Clonal growth, self-incompatibility, and spatial isolation of compatible clones may contribute to pollen-limited seed set in these populations. Low seed set may affect the rate of spread of feral M. sacchariflorus, which appears to disperse vegetatively as well as by seed. Although this species is not widely viewed as invasive, it is worth monitoring as a species that may become more widespread in the future.

Establishment of nonindigenous (NI) aquatic plants in the nearshore regions of freshwater ecosystems has resulted in environmental degradation, recreation concerns, economic impacts, and substantial management challenges. To reduce these undesirable effects, NI aquatic plants are often targeted for removal or control by management agencies, but the efficacy of implementation is often not documented or sustained. In this study, we developed a management plan to completely remove all NI plants from Emerald Bay, Lake Tahoe, CA, using only physical control techniques. Management plan priorities were based on previous research and lessons learned, including the need for (1) integrated weed management using multiple physical control techniques, (2) a large initial treatment investment, (3) ongoing early detection and rapid response, (4) detailed ecological monitoring, and (5) a long-term commitment to annual maintenance removal. Application of this management plan resulted in complete removal of all NI aquatic plants from Emerald Bay and substantial cost savings each year after the initial large investment. Annual maintenance removal and monitoring will need to continue as long as NI aquatic plants continue to enter Emerald Bay on boats and currents from other areas of Lake Tahoe.

Introductions of biocontrol beetles (tamarisk beetles) are causing dieback of exotic tamarisk in riparian zones across the western United States, yet factors that determine plant communities that follow tamarisk dieback are poorly understood. Tamarisk-dominated soils are generally higher in nutrients, organic matter, and salts than nearby soils, and these soil attributes might influence the trajectory of community change. To assess physical and chemical drivers of plant colonization after beetle-induced tamarisk dieback, we conducted separate germination and growth experiments using soil and litter collected beneath defoliated tamarisk trees. Focal species were two common native (red threeawn, sand dropseed) and two common invasive exotic plants (Russian knapweed, downy brome), planted alone and in combination. Nutrient, salinity, wood chip, and litter manipulations examined how tamarisk litter affects the growth of other species in a context of riparian zone management. Tamarisk litter, tamarisk litter leachate, and fertilization with inorganic nutrients increased growth in all species, but the effect was larger on the exotic plants. Salinity of 4 dS m−1 benefitted Russian knapweed, which also showed the largest positive responses to added nutrients. Litter and wood chips generally delayed and decreased germination; however, a thinner layer of wood chips increased growth slightly. Time to germination was lengthened by most treatments for natives, was not affected in exotic Russian knapweed, and was sometimes decreased in downy brome. Because natives showed only small positive responses to litter and fertilization and large negative responses to competition, Russian knapweed and downy brome are likely to perform better than these two native species following tamarisk dieback.

Tropical soda apple is an aggressive prickly perennial shrub growing up to 2 m high. It invades open to semishaded areas, including pastures, forests, riparian zones, roadsides, recreational areas, and horticultural and cropping areas. In Australia during August 2010, the weed was identified on the New South Wales Mid North Coast. It is believed that tropical soda apple has been present in this area for a number of years and both systematic and ad hoc surveys have found the weed in other satellite locations. The discovery of tropical soda apple at several cattle handling facilities indicated that cattle are a significant vector for the weed. The aim of this project was to use the National Livestock Identification System (NLIS) data to trace cattle movements from affected properties throughout New South Wales and into other Australian states. This has proved advantageous, as there are few other nonecological mechanisms to systematically trace significant weed movement. We have been able to conduct a pathway analysis of where this weed is likely to occur across New South Wales through the use of NLIS. Importantly, we can use this information to pinpoint surveillance activities for local managers, thus ensuring better use of resources. We have also been able to create a stochastic model for incursions at these sites using information gleaned from the NLIS data.

Multiple weed control strategies employed in combination can often aid the successful management of perennial weed species. This review article provides examples of integrated control programs that could aid in the management of several invasive perennial weed species that are problematic in the Pacific Northwest and elsewhere in North America. The development of an integrated management control program for wild chervil, a relatively recent invader to the Pacific Northwest of the United States and adjacent Canada, provides an example for this process. Through use of mechanical (mowing and tillage), cultural (establishment of competitive vegetation), and chemical (specific herbicides) strategies, control of this short-lived perennial species was greatly improved as compared to foliar herbicide applications alone. Such integrated strategies have been shown to enhance control of many perennial weed species, while potentially reducing the amount of herbicide applied, lessening the possibility of injury to adjacent desirable vegetation and increasing the stability of the ecological community at the site.

An experiment was initiated to study the effects of rubber benthic barriers vs. aggressive cutting on the invasive aquatic emergent plant, yellow flag iris. Treatments were compared against a control at two locations within British Columbia, Canada (Vaseux Lake and Dutch Lake). Yellow flag iris response was significantly different between the two sites, but biologically the results were identical: the benthic barrier killed yellow flag iris rhizomes within 70 d of treatment. Over the extent of the research, at Vaseux Lake the effect of aggressive cutting was no different from the control, while aggressive cutting was statistically no different than the benthic barrier at Dutch Lake. Vegetation regrowth approximately 200 d after the benthic barriers were removed was not detected at either location. These results indicate that rubber benthic barriers may be an effective treatment for yellow flag iris and maybe suitable for other, similar species.

Managing invasive winter annual grasses on noncrop and rangeland remains a constant challenge throughout many regions of the United States. Currently, there are limited management options for controlling winter annual grasses that work consistently, provide multiple years of control, and do not injure desirable plant communities. Imazapic has been one of the most widely used herbicides for downy brome control on rangeland; however, control with imazapic has been inconsistent beyond the application year and perennial grass injury is not uncommon. Indaziflam, a new herbicide mode of action for rangeland weed management, has shown promise in providing long-term downy brome control. A greenhouse study was conducted to compare pre-emergence activity of imazapic and indaziflam on six invasive winter annual grasses: downy brome, cereal or feral rye, jointed goatgrass, Japanese brome, medusahead, and ventenata. For both herbicides, seven rates were used to develop dose-response curves for each species. Log-logistic regression was conducted to determine the herbicide dose required to reduce biomass by 50% (GR50 values). Indaziflam was significantly more active across all species compared to imazapic, with the exception of jointed goatgrass. Comparing all species, the GR50 values for imazapic were on average 12 times higher than indaziflam. Japanese brome was the most sensitive to both herbicides, whereas jointed goatgrass and feral rye were the most difficult winter annual grasses to control with indaziflam and imazapic, respectively. This research provides evidence of a potential new mode of action for land managers to control the major invasive winter annual grasses.

Invasive species with distributions that encompass much of the North American environment often demand a range of management approaches, for several key reasons. Firstly, the North American environment includes a large number of highly variable habitats in terms of climatic, edaphic, and landscape features. Secondly, these regional habitat differences are accentuated by jurisdictions within Canada and the United States, whereby approaches and available resources differ at local, regional, and national scales. Another important consideration is whether an invasive species or complex also possesses genetic variation. All three of these factors render the knotweed complex in North America a highly variable target for management. In this paper we review existing knowledge of the variable nature of knotweed species (Fallopia japonica (Houtt.) Ronse Decr., Fallopia sachalinensis (F. Schmidt ex Maxim) Ronse Decr., and Fallopia × bohemica, (Chrtek and Chrtková) J. P. Bailey in North America, and evaluate how herbicidal, mechanical and biological control measures must account for this genetic variation, as well as accounting for regional differences and the potential northward expansion of knotweed under climate change. The imminent release of the psyllid, Aphalara itadori Shinji as a biological control agent in North America must also navigate regional and genetic differences. Prior European experience dealing with the three knotweed species should prove useful, but additional research is needed to meet the emerging challenge posed by F. × bohemica in North America, including the possibility of glyphosate resistance. Managers also face challenges associated with posttreatment restoration measures. Furthermore, disparities in resources available to address knotweed management across the continent need to be addressed to contain the rapid spread of this highly persistent and adaptable species. Linking practitioners dealing with knotweed “on the ground” with academic research is a crucial step in the process of marshalling all available resources to reduce the rapidly spreading populations of knotweed.

More than 100 years ago, Japanese knotweed was introduced to North America. Given its vigorous rhizome system and capability to grow from rhizome and stem fragments, it persists and spreads locally, forming monotypic stands. The Japanese knotweed clone originally introduced was a male sterile female clone; thus, early in the invasion, reproduction from seed was not an issue. The implication was that long-distance dispersal was relatively rare. However, recently, widespread hybridization between Japanese knotweed and Sakhalin (giant) knotweed has been reported, with the hybrid species, Bohemian knotweed, forming the majority of knotweed plants in many areas and possessing higher variability than the parent species. The hybrids produce large numbers of wind-dispersed viable seeds that germinate at rates approaching 100% in some populations. As temperatures increase, knotweed is predicted to expand its range farther north and to higher elevations. With the ability to regenerate from vegetative fragments and disperse via seeds, invasive knotweed species are on the move. An arsenal of chemical weapons, the ability to shade out competitors, and the ability to adapt rapidly through epigenetic change makes knotweed a formidable invader. We observed that knotweed species clearly possess 8 of the 12 ideal weed characteristics, with Bohemian knotweed likely exhibiting still more because of prolific seed production. More research is needed to answer pressing questions. How does hybridization affect knotweed epigenetics? Under what conditions might seed production become more frequent? What kind of niche expansion is possible with the increased variability? Given the considerable challenges posed by knotweed species that promise to become even greater with the proliferation and spread of Bohemian ecotypes, only a thoroughly researched, well-informed approach to knotweed management across North America can be successful.