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.

Following large-scale herbicide spraying and burning on Assateague Island, a barrier bar island located in Maryland and Virginia, the invasive common reed (Phragmites australis) was largely reduced from vast monocultures to less dense patches interspersed within maritime shrublands. To improve the control of these remnant/reemerging infestations and limit further nontarget damage, we tested three new treatments: mechanical cutting followed by dripping imazapyr onto stems, cutting followed 2 wk later by the foliar spraying of regrowth, and simple cutting with and without the removal of Phragmites litter. All herbicide treatments and cutting paired with litter removal significantly reduced Phragmites coverage (P ≤ 0.01) when compared with untreated controls. Native plant coverage was significantly greater after the cut-stem treatment than after traditional foliar spraying (P ≤ 0.01) because of the former's reduced herbicide use and more direct contact limited to Phragmites stems; native coverage was also greater after litter removal than when litter remained (P ≤ 0.001). Cutting followed by stem applications of herbicide is an effective means of treating scattered common reed stands in sensitive habitats, and litter removal after cutting can provide native vegetation with an advantage at recolonization.

Japanese knotweed is an invasive plant that occurs along waterways, highways, abandoned agricultural land, and other disturbed areas. It reduces plant diversity and can increase shoreline erosion. An experiment was conducted in Bible Hill and Antigonish, NS, Canada to evaluate early POST aminopyralid at 120 g ae ha−1 and POST imazapyr applications at 720 g ae ha−1 at maximum shoot height, flowering, senescence, maximum height+flowering, maximum height+senescence, flowering+senescence, and maximum height+flowering+senescence. Early POST aminopyralid only provided 10% to 15% control at 52 wk after treatment (WAT) whereas 83% to 100% control occurred following imazapyr applications at all application timings. Percent control at 2, 4, and 8 WAT tended to be higher where imazapyr followed aminopyralid. By 52 WAT, equivalent damage ratings and reductions in stem density occurred at both sites in all plots where imazapyr was applied. The use of aminopyralid or multiple imazapyr applications provided no additional benefit over a single imazapyr application. We conclude that early POST aminopyralid suppresses knotweed growth, which should facilitate late-season imazapyr applications, especially in large stands.

There is a limited understanding about the ecological mechanisms that enable certain plant species to become successful invaders of natural areas. This study was conducted to determine the soil and landscape characteristics that correlate with invasion of Chinese privet (CHP), and to develop a model to predict the probability of CHP invasion in Piedmont forests. A landscape ecosystem classification (LEC) system—based on the percentage of clay in the B horizon, depth to maximum clay (cm), exposure, terrain shape, and aspect (degrees)—was used to determine the soil moisture characteristics of invaded and uninvaded plots. Additional measurements included the cover classes of CHP and other species, litter depth (cm), slope (degrees), overstory basal area (m2 ha−1), and soil chemical properties. CHP invasion was negatively correlated with overstory basal area and slope and positively with litter depth and pH. A stepwise logistic regression model containing these four variables was highly sensitive, with an overall accuracy of 78%. Given the accuracy of this model, we propose that it can be used to calculate the probability of invasion in a given area, provided that some basic, readily obtainable site characteristics are known.

Invasive species management in natural landscapes is generally executed at the scale of independent jurisdictions, yet the ecological processes and biodiversity to be protected from invasion occur over large spatial scales and across multiple jurisdictions. Jurisdictional land boundaries can influence the flows and dynamics of ecological systems, as well as the social systems that exist in these complex landscapes. Land management entities in large, protected area–centered ecosystems may use different approaches to address cross-boundary management challenges. To understand these differing strategies and their effects on cooperative invasive plant management, we interviewed employees with federal, county, and state agencies, research organizations, nonprofits, and local stakeholder groups in two national parks and their surrounding lands in California, USA. Although all participants stressed the importance of working together, they did so along a continuum of strategies ranging from simple communication to coordination of independent efforts to active collaboration. Barriers to collaboration can be categorized as originating within or externally to the management unit, including limited resources, differing agency priorities, paperwork requirements, and lack of support by higher-level managers. Strategies to reduce barriers depend on where they originate.

Old World climbing fern (OWCF) is a highly invasive plant that is spreading rapidly and disrupting natural areas in Florida, with the potential to spread by wind-blown spores into Central and South America. Experiments were performed using gametophytes to determine the dose response of OWCF to metsulfuron methyl (metsulfuron). OWCF gametophytes were highly susceptible to metsulfuron with ≤ 1.4% survival at concentrations ≥ 27 mg ai L−1. Metsulfuron OWCF gametophyte 50 and 95% inhibition (I50 and I95) values were 6.1 and 26.4 mg ai L−1, respectively. Survival of treated gametophytes that developed into sporophytes was ≤ 0.014% at concentrations ≥ 27 mg ai L−1, but no sporophytes developed at concentrations ≥ 432 mg ai L−1. Metsulfuron OWCF sporophyte inhibition I50 and I95 values were 5.6 and 24.1 mg ai L−1, respectively. At 216 mg ai L−1 metsulfuron, half of the standard operational use rate, 1.1 × 10−8% of the treated gametophytes developed into sporophytes. The results indicate that the potential for tolerant, and potentially resistant, OWCF populations is higher using lower concentrations of metsulfuron as no gametophytes developed into sporophytes at the standard field treatment concentration of 432 mg ai L−1.

Japanese clematis, a herbaceous to woody vine native to Asia, has been widely used for landscaping in the southeastern United States and is now a naturalized and invasive in Florida. Herbicides that contain the active ingredients fluroxypyr, glyphosate, imazapic, metsulfuron, or triclopyr, applied alone or in combination on a spray-to-wet basis, were evaluated for their effectiveness to control the plant in a natural area of Gainesville, FL. All herbicide treatments provided some level of control 30, 60, and 90 d after application. The most effective herbicide treatments, imazapic 0.6 g ae/L, glyphosate as low as 3.6 g ae/L, triclopyr as low as 3.6 g ae/L, metsulfuron 0.07 g ai/L, and fluroxypyr as low as 1.8 g ae/L, resulted in 72% to 99% control 90 d after application, but were not significantly different from each other. Natural area managers throughout the range of Japanese clematis should be vigilant for this species and remove it whenever encountered. Our results indicate that it can be controlled by foliar application of several herbicides commonly used for management of pest plants in natural areas, but repeat applications will be necessary.

It is essential that we improve our ability to predict which nonnative species will become invasive in order to prevent their introduction and spread. Past attempts to foresee invasions have met with limited success, but increased computing power, increased availability of information about exotic species, and comprehensive evaluations of invasion potential are improving our ability to predict which species are likely to invade most successfully. We used data from Colorado and other states to develop an effective means of predicting the spread of invasive plant species among states. Qualitative criteria were used to develop a numerical threat index, which rates potential invaders based on distribution and abundance with respect to climate, biological characteristics, and preferred habitats of the species. Out of a compiled list of 388 species, we identified six invasive nonnative plants that are highly likely to invade Colorado, 10 with medium invasive potential and five with low potential. Species found to be likely to invade Colorado included garlic mustard, smooth distaff thistle, and Syrian beancaper.

Privet has escaped from cultivation and is invading natural areas throughout eastern North America. Understanding the pattern of invasion over time could help us develop more efficient management strategies. We studied the invasion history and spatial distribution pattern of privet by mapping age and spatial data for established patches in a 132-ha (326 ac) forested natural area in northeast Ohio. We determined the age of 331 geo-referenced patches by counting annual rings, and mapped them with corresponding land habitat. Age distribution and cumulative number of privet patches over about 40 yr showed three phases of invasion. The initial 19-yr lag phase was characterized as a dispersed spatial pattern (based on nearest neighbor analysis), with patches located mostly at edges of different habitats and open places. In a second phase of about 15 yr, an average of 19 patches were initiated yearly, in a pattern that trended towards clustered. The final phase began around 2007, as the rate of new patch establishment declined, possibly because of saturation of the suitable habitat. Establishment of new patches was not associated with specific habitats. Aggregation of patches with similar ages increased after 1998 and became significantly clustered. Mapping of clusters of old and young patches identified invasion hot spots and barriers. Results affirmed that the best time for invasive control is during the lag phase. By monitoring edge habitats associated with early establishment, managers might detect and control early invaders and delay the onset of the expansion phase.

Essential variables to consider for an efficient control strategy for invasive plants include dispersion pattern (i.e., satellite or invasion front) and patch expansion rate. These variables were demonstrated for buffelgrass [Pennisetum ciliare (L.) Link], a C4 perennial grass introduced from Africa, which has invaded broadly around the world. The study site was along a roadway in southern Arizona (USA). The P. ciliare plant distributions show the pattern of clumping associated with the satellite (nascent foci) colonization pattern (average nearest neighbor test, z-score −47.2, P<0.01). The distance between patches ranged from 0.743 to 12.8 km, with an average distance between patches of 5.6 km. Median patch expansion rate was 271% over the 3-yr monitoring period versus 136% found in other studies of established P. ciliare patches. Targeting P. ciliare satellite patches as a control strategy may exponentially reduce the areal doubling time, while targeting the largest patches may have less effect on the invasion speed.

Exotic conifers are rapidly spreading in many regions of New Zealand, as well as in many other countries, with detrimental impacts on both natural ecosystems and some productive sector environments. Herbicides, in particular the active ingredient triclopyr, are an important tool to manage invasive conifers, yet there is a paucity of information that quantifies the amount of herbicide required to kill trees of different sizes when applied as a basal bark treatment. Two sequential experiments were conducted to define the amount of triclopyr required to kill individual invasive lodgepole pine (Pinus contorta Douglas ex Loudon), trees of different sizes when applied in a methylated seed oil to bark (either the whole stem or base of the tree) and to determine which tree size variates (height, diameter at breast height [DBH], crown diameter [CD]) or derived attributes (crown area, crown volume index) best characterized this dose–response relationship. The outcomes of the dose–response research were compared with field operations where triclopyr was applied to the bark of trees from an aerial platform. Applying the herbicide to the whole stem, as opposed to the base of the tree only, significantly increased treatment efficacy. The tree size variates DBH, CD, crown area, and crown volume index all provided good fits to the tree mortality data, with >91% prediction accuracy. Of these variates, CD provided the most practical measure of tree size for ease of in-field calculation of dose by an operator. Herbicide rates used in field operations were seven to eight times higher than lethal doses calculated from experimental data. Our results highlight the potential for substantial reductions in herbicide rates for exotic conifer control, especially if dose–response data are combined with remotely sensed quantitative measurements of canopy area or volume using new precision technologies such as unmanned aerial vehicles.

Mesocosm studies were conducted in 2020 to evaluate the effects of carrier volume and application method on waterhyacinth [Eichhornia crassipes (Mart.) Solms] response to 2,4-D, glyphosate, and diquat. Carrier volumes of 935, 467, and 187 L ha−1 were applied using either a conventional stream, conventional cone, adjustable cone, or a drizzle-stream spray pattern. Reducing carrier volume from 935 L ha−1 reduced spray coverage up to 60%, depending on application method. However, reducing carrier volume did not diminish efficacy of any herbicide or application method. Alternatively, E. crassipes control from 2,4-D increased 10% to 26% when applied using 187 L ha−1 compared with 935 L ha−1. Likewise, E. crassipes biomass was reduced 91% when 2,4-D was applied using 935 L ha−1; however, treatment applied at 187 L ha−1 resulted in 99% biomass reduction. In general, 2,4-D resulted in roughly 10% greater control when conventional or adjustable cone applications were used compared with either stream applications. Eichhornia crassipes control at 7 d after treatment (DAT) from diquat increased with decreasing carrier volumes; however, treatment effects in diquat experiments were not detected at other evaluation intervals. Glyphosate efficacy was highly influenced by carrier volume, as E. crassipes control increased up to 61% when applied using 187 L ha−1 compared with 935 L ha−1. Moreover, E. crassipes biomass reduction increased from 55% in the 935 L ha−1 treatment to 97% in the 187 L ha−1 treatments. Glyphosate application methods consisting of conventional stream or conventional cone sprayers resulted in slightly increased E. crassipes control by 28 DAT; however, no differences among application methods were observed in E. crassipes biomass data. These data support further evaluations of alternative application techniques for E. crassipes control under field conditions and for other herbicides and aquatic plant species.

The presence of landscape disturbances increases the establishment of exotic plants inside natural areas. Here, we examine the effect of human disturbances to prairie landscapes on the distribution and abundance of Kentucky bluegrass and smooth brome, exotic grasses that threaten the integrity of prairie ecosystems throughout the northern Great Plains. Using plant inventory data from Riding Mountain National Park in Manitoba, Canada, we investigated how roads, trails, and the intensity of historic livestock grazing affect the distribution and abundance of exotic grasses. On the basis of our results, both Kentucky bluegrass and smooth brome were more abundant in areas closer to roads. Kentucky bluegrass was also more abundant in areas farther from trails and those historically grazed by cattle. Our research demonstrates that the effect of landscape disturbances on exotic grasses varies between species and suggests that patterns of invasion may reflect different introduction histories. Given our findings, conserving the integrity of northern fescue prairies should account for human disturbances of landscapes and their effects on the proliferation of exotic plants into areas of native prairie.

Arbuscular mycorrhizal fungi (AMF, Glomeromycota) are globally distributed symbionts of plant roots. Relationships with arbuscular mycorrhizae can provide crucial support for the establishment of any plant in an unfavorable environment. We hypothesized that invasions of neophytes are associated with changes in the colonization of native plants and early invaders (archeophytes) by AMF. We examined changes in AMF colonization in yarrow (Achillea millefolium L.) and wild carrot (Daucus carota L.) (native plants) and tansy (Tanacetum vulgare L.) and false oatgrass [Arrhenatherum elatius (L.) P. Beauv. ex J. Presl & C. Presl] (archeophytes) in response to the invasion of four neophytes from the Asteraceae family, namely great globethistle (Echinops sphaerocephalus L.), New York aster [Symphyotrichum novi-belgii (L.) G. L. Nesom agg.], annual fleabane [Erigeron annuus (L.) Pers.], and Canada goldenrod (Solidago canadensis L.). We found that the AMF colonization of the Asteraceae neophytes was high in the studied monodominant invasions, and the AMF colonization of the neophytes was higher than or equal to that of the studied native plants and archeophytes. Changes in plant dominance did not serve as predictors of the extent of AMF colonization of the native plants and archeophytes despite the invaded plots being associated with strong changes in the availability of primary and secondary mineral nutrients. The absence of a response of AMF colonization of native and archeophyte plant species to the invasion of neophytes suggests that AMF are passengers, rather than drivers, in the course of Asteraceae invasions in central European environments.

Local abiotic and biotic conditions can alter the strength of exotic species impacts. To better understand the effects of exotic species on invaded ecosystems and to prioritize management efforts, it is important that exotic species impacts are put in local environmental context. We studied how differences in plant community composition, photosynthetically active radiation (PAR), and available soil N associated with Russian olive presence are conditioned by local environmental variation within a western U.S. riparian ecosystem. In four sites along the South Fork of the Republican River in Colorado, we established 200 pairs of plots (underneath and apart from Russian olive) to measure the effects of invasion across the ecosystem. We used a series of a priori mixed models to identify environmental variables that altered the effects of Russian olive. For all response variables, models that included the interaction of environmental characteristics, such as presence/absence of an existing cottonwood canopy, with the presence/absence of Russian olive canopy were stronger candidate models than those that just included Russian olive canopy presence as a factor. Compared with reference plots outside of Russian olive canopy, plots underneath Russian olive had higher relative exotic cover (exotic/total cover), lower perennial C4 grass cover, and higher perennial forb cover. These effects were reduced, however, in the presence of a cottonwood canopy. As expected, Russian olive was associated with reduced PAR and increased N, but these effects were reduced under cottonwood canopy. Our results demonstrate that local abiotic and biotic environmental factors condition the effects of Russian olive within a heterogeneous riparian ecosystem and suggest that management efforts should be focused in open areas where Russian olive impacts are strongest.

Fennel is a major invasive plant in many lower elevation natural areas in coastal California. Three identical field experiments were conducted to evaluate glyphosate and triclopyr for control of fennel. Treatments included each herbicide applied alone and in various combinations. We also compared broadcast applications to spot spraying of individual fennel plants because spot spraying is a commonly used technique in natural area weed management. Most treatments controlled fennel well when evaluated 6 wk and 1 yr after treatment, with the exception of the lowest rate of glyphosate. Purple needlegrass, a native perennial grass, was present in two of the sites. In most, but not all, treatment and site combinations, it was not significantly harmed by the herbicides. The spot spray applications were less effective and used more herbicide per unit area than the broadcast spraying.

The biological control agent Aphthona nigriscutis Foudras (Chrysomelidae) established in Fremont County, WY since 1992 on leafy spurge was released into a mixed stand a native plant Euphorbia robusta Engelm. During host range testing, E. robusta was a likely host for A. nigriscutis under laboratory conditions. In 1999, A. nigriscutis was observed feeding on both E. esula and 31 of 36 E. robusta plants present on about 2 ha (5 ac) where the visually estimated E. esula canopy cover was 50%. By August 2001, E. esula cover had declined to less than 5% and E. robusta plants had increased to 450 plants with 26 (5.8%) showing feeding damage. In 2006 Euphorbia esula ground cover was 2% and of 598 E. robusta plants originally marked, 391 could be located and four of these had damage consistent with A. nigriscutis feeding. For the 8-yr period, E. esula ground cover was inversely correlated to E. robusta density and positively correlated to A. nigriscutis feeding damage on E. robusta. This study shows that while also acceptable to A. nigriscutis in the field, feeding on E. robusta declined with declining densities of the target weed while E. robusta population densities increased. It seems that some risk in this regard is acceptable in light of the damage from the target weed and the generally high level of selectivity provided by biological control agents.

We studied the response of naturalized kudzu plants to simulated herbivory at three locations: Delaware (DE), Pennsylvania (PA), and North Carolina (NC). At the DE and PA sites, plant mortality after the first yr was 14 and 50%, respectively, and was highest for plants that had a small starting root crown size. At both sites, 50 and 75% leaf and shoot clipping, and drilling one or two large holes from the root crown into roots, had no effect on aboveground biomass. In NC, all plants survived for 3 yr. Plants subject to 50% vine removal at this site showed significant decrease in aboveground biomass compared to the control, but 50% leaf cutting and root drilling had no effect. In the greenhouse, kudzu seedlings grown in 60 and 100% light compensated for 50% leaf removal, but 75% damage reduced aboveground biomass. Plants survived for 1 to 2 mo in 0% direct light, but only one of 53 plants survived to the end of the experiment. Results suggest that established kudzu plants are able to compensate for biomass removal, seedlings can survive for several weeks without light, and that effective biocontrol might require more than 2 to 3 yr of continuous damage.

Brachypodium sylvaticum, a shade-tolerant, forest dwelling, and aggressive invasive grass native to Eurasia, is a noxious weed in California, Oregon, and Washington. This species could cause ecosystem collapse by altering forest fire regimes. To examine interactions with fire, we divided two Willamette National Forest sites into eight units and randomly selected half for treatment with prescribed fire in spring 2011. We assessed the effect of B. sylvaticum on fire (severity and intensity) as well as the effect of fire on B. sylvaticum (cover, seedling emergence, and dispersal). We found that B. sylvaticum cover decreased fire severity but had no effect on intensity. Furthermore, fire severity influenced B. sylvaticum cover; in areas receiving low-severity fire, the grass increased from 21 ± 15.05 to 34 ± 15.81%, but in areas of high-severity fire, cover remained consistently around 0% (0 ± 0% cover in yr 1 to 0.2 5± 0.25% in yr 3). In the field, prescribed fire decreased seedling emergence by 32% compared to controls, but not in an associated greenhouse experiment. However, in the greenhouse, severely burned plots had zero emergence, compared to 0.29 ± 0.14 seedlings low-severity m−2 plot. Fire severity also influenced dispersal in the field; we monitored plots with < 0.5% cover B. sylvaticum initially; when these plots experienced low severity fire, they had greater B. sylvaticum cover (increasing 1,200%), suggesting increased dispersal with less severe fires. High-severity dispersal plots did not experience increased cover. High severity fires have the potential to control the grass, but low-severity fires will likely increase its cover.

The composition of nonnative floras is influenced by a region’s socioeconomic history, yet rarely are these factors studied alongside plant naturalization rates over time. Such information is especially critical for archipelagos, which often host large numbers of nonnative plants and would benefit from prevention of inter-island spread. We compiled the first record of occurrence and first record of naturalization for all naturalized plants in Hawai‘i alongside data on their origin, native climate types, taxonomy, and likely introduction pathway and compared rates of naturalization with socioeconomic trends. We found that the rate of total plant naturalizations has increased at a roughly constant rate during the past century without any sign of plateauing. However, this relatively steady increase is underlain by notable fluctuations in naturalization rates for different introduction pathways, with ornamentals increasing recently, while agriculture-related plants have decreased. Furthermore, this trend mirrors a shift from an agriculture-dominated economy to a tourism-based one associated with increases in both resident and tourist populations as well as general economic well-being. We further found that the average naturalized species spreads at a rate of 1.86 islands per decade, eventually occupying most major islands in the archipelago, and the rate of spread appears to be increasing since Hawai‘i’s economic shift. Our findings also emphasize the diversity of Hawai‘i’s nonnative flora, which originates from a variety of climates, continents, and taxonomic groups. We demonstrated that many nonnative species have native ranges that include temperate climates, which is important, because these climates typically co-occur with higher-elevation, remnant patches of native-dominated ecosystems in Hawai‘i. This study reveals trends that may help predict a species’ ability to naturalize and spread within and between islands, and we discuss management implications that may be extended to other regions.