Because of economic and environmental constraints, alternatives to chemical management of Canada thistle (Cirsium arvense) are frequently sought, but adequate nonchemical suppression of this invasive species remains elusive. Combining biological control with other tactics may be an effective approach to suppress Canada thistle, but more information is needed about how environmental conditions affect interspecific interactions. We investigated effects of a biocontrol agent (Hadroplontus litura, a stem-mining weevil) and a potential plant competitor (common sunflower, Helianthus annuus, native annual) on Canada thistle under two soil nutrient regimes in outdoor microcosms. Larval mining damage was relatively light, and weevils negatively impacted only main shoot height and flower number. All measures of Canada thistle performance were reduced when plants were grown with common sunflower or in reduced nutrients, although effects of the latter on root biomass were not significant. Effects of common sunflower and soil nutrients on Canada thistle were generally additive, though a marginally insignificant interaction indicated a trend for greatest flower number with high nutrients and absence of common sunflower. Effects of weevils and common sunflower on Canada thistle were also additive rather than interactive. Although larval damage ratings were significantly greater on plants grown in high-nutrient soil, under our experimental conditions weevils and soil nutrients did not have a significant interactive effect on Canada thistle plants. Our results indicate that H. litura is a relatively weak biological control agent, but when combined with competitive desirable vegetation, some level of Canada thistle suppression may be possible, especially if soil nutrient levels are not highly enriched from agricultural runoff. Assessing the true ecological impacts of Canada thistle infestations may be an important direction for future research.

The common reed (Phragmites australis) is one of the most invasive vascular plants in northeastern North America. A competitive genotype from Eurasia has recently invaded road and agricultural ditches, which facilitate the dispersal of the plant over long distances. However, large tracts of roadsides—apparently propitious for the establishment of the plant—are not invaded by the grass. We hypothesized that the absence of the invader is associated with physical and biological characteristics of roadsides. To test this hypothesis, we collected field data and developed two statistical models to explain the presence or absence of the common reed along a highway of southern Quebec highly invaded by the plant but with contrasting patterns of common reed distribution. The models explained 23 to 30% of the total variance and correctly predicted the presence or absence of common reed 71% of the time. The models suggest that a dense woody plant cover over a drainage ditch limits the establishment and/or expansion of the common reed, probably by competition for light and space. Also, shaded ditches are not subjected to a frequent maintenance, and are therefore less disturbed, probably further reducing common reed invasion because the germination of their seeds is less likely without soil disturbance. This study yields insights on the potential of woody plants for controlling the expansion of invasive grasses, and could help to justify the preservation of dense shrubs and tree hedges along right-of-ways.

Preinvasion baseline data on entire communities are absent for most taxa in most places, and this limits our ability to connect long-term ecological changes to particular invasive species or invasion events. We obtained data on forest understory composition from 94 stands in the 1950s and again the 2000s. We recorded within-stand frequency of occurrence for garlic mustard, European buckthorn, and Bell's honeysuckle and identified changes in native plant species density in 20, 1-m2 quadrats in invaded and noninvaded stands. All three invasive species were absent from all study sites 50 yr ago, yet at least one was present in 77.7% of the stands by the 2000s. All three species were present in 14.9% of the stands. Garlic mustard and European buckthorn were found at 47.9% of resurveyed sites, and Bell's honeysuckle was found in 40.4% of resurveyed sites. Native understory plant species density declined an average of 23.1% during the past 50 yr. Declines were not significantly different in stands with or without invasive plants. The absence of a measurable effect by invasive plant presence or frequency could be due to invasive plants being too few to have a measurable effect at the plot scale, species density being an insensitive response variable, time lags between invasions and effects, or regional factors like development pressure and fire suppression driving density declines in both invasives and native species.

Kentucky bluegrass (Poa pratensis L.) invades northern Great Plains rangelands. On the Sheyenne National Grassland in southeastern North Dakota, three research sites, each with a different level of Kentucky bluegrass invasion, were chosen to evaluate effectiveness of burning and burning–herbicide combinations to control Kentucky bluegrass. Initial Kentucky bluegrass invasion levels were 37%, 77%, and 91% for LOW, MODERATE, and HIGH invaded sites, respectively. Within each invaded site, four replicated strips (20 by 60 m) were established, with half of each strip burned in late October 2005 and the other half burned in early May 2006. Herbicide treatments of (1) no herbicide, (2) 2.24 kg ha−1 of glyphosate, and (3) 0.43 kg ha−1 of imazapic were randomly assigned to 10 by 20 m subplots within each burn. Control plots were established at the same time. Relative basal cover of native grass, native forb, and Kentucky bluegrass was estimated annually using 50 10-point frames within each subplot. On the HIGH site in 2006, fall-burned plots with a spring glyphosate application had three times the native grass cover and only one fourth of the Kentucky bluegrass cover compared with controls. Similar results with the same treatment occurred at the MODERATE site. Native grasses became the most abundant plant community on these plots in the MODERATE and HIGH sites within 1 yr. Treatment differences were transitory, and the LOW site differed from the MODERATE and HIGH sites. In 2007, on fall-burned plots with spring glyphosate application, the amount of Kentucky bluegrass was 14% and 30%, and native grass species were 52% and 42% on the MODERATE and HIGH sites, respectively, which was similar to the initial values on the LOW site. These data emphasize the importance of initial invasion level in developing restoration strategies and provide evidence burning and herbicide combinations can be valuable management tools even on heavily invaded grasslands.

The extent and nature of spread of exotic plant species to and within Alaska by shipment of hay and straw was studied. The amounts of hay and straw imported into Alaska and the amounts and types of seed in imported and locally produced hay and straw was determined We purchased alfalfa hay, wheat straw, ryegrass straw, and timothy hay produced in Washington and Oregon (WA–OR) and locally produced straw and hay. The hay and straw were shaken over screens, and the remaining fines were mixed with sterile potting soil and incubated in the greenhouse. Forty-nine plant species were identified from hay and straw, nine of which are ranked as invasive in Alaska, including downy brome, foxtail barley, hare barley, narrowleaf hawksbeard, and quackgrass—a prohibited weed in Alaska. The number of seeds ranged from 0 to 6,205 seeds kg−1, with an average of 585 seeds kg−1, and the number of species ranged from 0 to 12, with an average of 4.2 species per bale. Crop seed comprised a large proportion of the germinated seeds in ryegrass straw, wheat straw, and timothy/brome hay (98, 78, and 62%, respectively), but was less prevalent (ranging from 0 to 38%) in the other three hay and straw crop types. Hay and straw from Alaska contained more total seeds and species than hay and straw from WA-OR, but the difference was not significant when only weed seed was used in the analysis. Alaska-grown timothy/brome hay contained significantly more total seed than alfalfa hay and wheat straw from WA–OR and Alaska-grown barley straw. The grower or distributor of the hay and straw also influenced the number of seeds and species among some crop types. Results of this study document that large numbers of alien plant species are transported by movements of hay and straw into and within Alaska.

The objective of this article is to extend the reported period in which flood-distributed knotweed propagules may be successfully managed using only manual labor. During a second round of early detection and rapid response (EDRR) management along the Green River in Guilford, VT, we collected and measured all Japanese knotweed propagules that had been distributed by flooding approximately 21 mo earlier, in August 2011, at a single site. Our data suggest that knotweed s.l. prioritizes the growth of new stems over new rhizomes at the start of a growing season. Because the limiting factor for successful removal of new knotweed s.l. plants by hand is the size of the rhizome system, our findings support extending the time frame for EDRR management of flood-distributed knotweed s.l. into the second spring after its initial dispersal. Additionally, in November 2013, surveys of our work sites found no new knotweed s.l. plants in locations accessible to work crews. In addition to validating our EDRR management techniques, this implies that knotweed s.l. fragment viability does not extend past the second spring following its dispersal.

Waterhyacinth is a free-floating aquatic weed that is considered a nuisance worldwide. Excessive growth of waterhyacinth limits recreational use of water bodies as well as interferes with many ecological processes. Accurate estimates of biomass are useful to assess the effectiveness of control methods to manage this aquatic weed. While large water bodies require significant labor inputs with respect to ground-truth surveys, available technology like remote sensing could be capable of providing temporal and spatial information from a target area at a much reduced cost. Studies were conducted at Lakes Columbus and Aberdeen (Mississippi) during the growing seasons of 2005 and 2006 over established populations of waterhyacinth. The objective was to estimate biomass based on nondestructive methods using the normalized difference vegetation index (NDVI) derived from Landsat 5 TM simulated data. Biomass was collected monthly using a 0.10m2 quadrat at 25 randomly-located locations at each site. Morphometric plant parameters were also collected to enhance the use of NDVI for biomass estimation. Reflectance measurements using a hyperspectral sensor were taken every month at each site during biomass collection. These spectral signatures were then transformed into a Landsat 5 TM simulated data set using MatLab® software. A positive linear relationship (r2 = 0.28) was found between measured biomass of waterhyacinth and NDVI values from the simulated dataset. While this relationship appears weak, the addition of morphological parameters such as leaf area index (LAI) and leaf length enhanced the relationship yielding an r2 = 0.66. Empirically, NDVI saturates at high LAI, which may limit its use to estimate the biomass in very dense vegetation. Further studies using NDVI calculated from narrower spectral bands than those contained in Landsat 5 TM are recommended.

One of the greatest threats to biodiversity is the invasion of ecological communities by alien plants. Management strategies for alien plants, however, rarely focus on specific biodiversity-conservation outcomes, and recovery actions rarely address the threat on a landscape scale. There are many reasons for these failures, including (1) limited knowledge of the native species at risk, (2) a disconnect between policy and management, (3) a disconnect between the fields of weed science and biodiversity conservation, (4) a dearth of data from management actions (for both threat abatement and recovery), and (5) the broadly held assumption that control of alien plants will by itself lead to a positive biodiversity response is often incorrect. Thus, alien plant management strategies with a conservation aim need to include planning processes and assessment measures to ensure that the aim is achieved. Here, I outline an example of the planning steps needed to ensure that the management of widespread alien plant species achieves the greatest possible conservation outcome, using the alien plant program for bitou bush (Chrysanthemoides monilifena) as a case study. In addition, I present an overview of the challenges faced and solutions developed during the transition from planning to management, along with some initial data to demonstrate the success and value of this investment in both planning and implementation during the past decade. Interim results from a series of sites have highlighted the success of the program and led to the program being acknowledged as an important ecological restoration project in Australasia. This success is highly dependent on extensive stakeholder involvement (across land tenures), dedicated coordination, and leadership. The planning process is now being adopted for other alien plants in Australia and can be modified to abate the threat from other alien organisms or other threats to biodiversity because the principles of each step are fundamentally similar.

The purpose of this study was to determine the effect of whole-plant composting on the viability of seeds and other propagules of the invasive plant species waterhyacinth, waterlettuce, hydrilla, and giant reed while producing a valuable compost product. Invasive species were subjected to preliminary germination and growth tests and oven mortality tests to evaluate whether species distribution was via seeds, vegetative propagules, or both, as well as whether the composting process had the potential, through the high temperatures obtained, to kill seeds and other propagules. Germination and growth tests determined the means by which invasive species spread. Oven tests determined the temperatures at which unscarified and scarified seeds and propagules were rendered inviable. Achieving temperatures of at least 57.2 C was necessary within constructed compost piles to effectively kill the plants without the danger of redistribution. In the field, the study successfully developed a large-scale composting operation using invasive plant species as the primary feedstock. Analysis of field-scale composting showed final materials were within satisfactory to ideal levels for samples analyzed by the U.S. Compost Council's Seal of Testing Assurance Program and were, therefore, a valuable compost product.

Black swallowwort [Vincetoxicum nigrum (L.) Moench] and pale swallowwort [Vincetoxicum rossicum (Kleopow) Barb.] are perennial vines of European origin that invade natural areas and perennial cropping systems in the northeastern United States and southeastern Canada. Both species reproduce via wind-dispersed seeds in the form of achenes with comas, but little is known about the extent of dispersal of these seeds. We studied the relationship of seed release height (0.75 m, 2 m), wind speed at the time of release, seed mass, and settling rate on distance traveled. Vincetoxicum nigrum and V. rossicum seeds traveled up to 72.1 and 79.6 m, respectively. Seeds of both species released from 2 m traveled greater distances than seeds released from 0.75 m, which fell within 20 m of the release point. Release height was the most important factor influencing long-distance dispersal events. Wind speed also strongly interacted with release height for long-distance dispersal of V. nigrum. Vincetoxicum nigrum seed mass was greater and settling rates faster than for V. rossicum. Increasing seed mass generally increased settling rate, which in turn decreased distance traveled, except in V. rossicum, for which longer distance–dispersing seeds had a faster settling rate. Our findings suggest that management efforts focus on reducing the presence of these two vines, especially if there is potential for them to climb up taller vegetation such as trees. Seeds released from these greater heights are more likely to travel far from source populations and initiate new populations. Preventing seed production in small, nonclimbing patches will also help suppress the expansion of these two vines, as seeds can still disperse up to 20 m away from parent plants.

Downy brome (Bromus tectorum L.) is a highly invasive winter annual grass that can fill open niches in native plant communities. Prescribed burning is often used to control B. tectorum and can be combined with herbicide treatments to extend the duration of control and promote the native plant community. Several herbicides have been evaluated in conjunction with burning for B. tectorum control, although the herbicide indaziflam has not. In September 2017, two B. tectorum–infested sites were burned in Colorado foothill shrublands. In March 2018, indaziflam was applied alone or in combination with glyphosate, rimsulfuron, or imazapic. These treatments were compared with imazapic plus glyphosate as a standard. All treatments were made within burned and non-burned areas in a crossed-nested design. Bromus tectorum cover and the desirable plant community responses were evaluated 1 and 2 yr after treatment (YAT). In non-burned areas, all indaziflam treatments reduced B. tectorum cover compared with the control. In contrast, reductions from the imazapic treatments did not persist after the first year. Most post-burn treatments further decreased B. tectorum cover compared with the non-burned treatments. The most effective treatments (indaziflam 44 and 73 g ai ha−1 + imazapic 123 g ae ha−1) provided similar levels of control (<1% B. tectorum cover at 2 YAT), with or without burning. Desirable plant cover, richness, and diversity were not negatively impacted by burning or herbicide treatments. Plant diversity and species richness increased at Site 2 when burning was followed by indaziflam treatments. This study indicates that B. tectorum control using indaziflam can be enhanced when applied after burning, and the combinations with imazapic or rimsulfuron provide a wider application window compared with the combination with glyphosate.

Prevention, early detection, rapid response, and prioritization are essential components of effective and cost-efficient invasive plant management. However, successfully implementing these strategies requires the ability to accurately predict the temporal and spatial dynamics of newly/recently detected nonnative species. Why some nonnative species become invasive and the source of variation in lag time between arrival and the onset of invasive expansion are poorly understood. One tool to fill these knowledge gaps is the “invasion curve,” which tracks nonnative species abundance (i.e., area invaded) over time after arrival in a new area. Since invasive species curves rely primarily on records from herbarium collections, we propose that these collections can be used as a springboard to develop a standardized approach to building invasion curves. This would allow researchers to compare the trajectories of nonnative species, improving risk assessment and our ability to recognize potential invasive species and factors contributing to both invasibility and invasiveness. While there have been admirable efforts to produce invasion curves, several barriers exist to their reliable production and standardization. In this paper, we explore the challenges related to the efficient production of these curves for plants using herbarium data and suggest ways in which progress could occur. It is our hope that this will better position herbaria and researchers to aid natural resource managers to prioritize needs, make effective management decisions, and develop targeted prevention and monitoring programs by taking advantage of lag times to implement timely responses.

Garlic mustard, a biennial forb native to Europe, has invaded native ecosystems in forested regions in the United States. In anticipation of a biological control program being implemented in the United States for this plant, a garlic mustard monitoring program was initiated. The objective of this study was to characterize garlic mustard populations and the associated plant communities and their response to environmental conditions in Minnesota hardwood forest ecosystems. Additionally, we developed a baseline for long-term studies to determine future benefits and impacts of biological control agents on plant communities infested with garlic mustard, should they be released. To monitor garlic mustard populations, we used a nationally standardized protocol in which data were collected on garlic mustard population density and cover, garlic mustard plant heights and silique production, insect damage to garlic mustard, cover of the associated plant community, and litter cover. We also collected data on available photosynthetically active radiation in the understory. The results underscore the variability in garlic mustard population dynamics. At only 6 of 12 sites did garlic mustard densities follow the predicted two-point cycles due to their biennial life cycle, with the first- or second-year life stage dominating in any given year. Available light did not differ strongly among sites, but shading by adult plants is implicated in keeping the populations of first-year plants low. Sites with greater garlic mustard cover had lower native species richness and cover than sites with lower garlic mustard cover. Absent biological control agents, garlic mustard is currently experiencing very little herbivory in Minnesota with an average of 2% of leaf area removed by herbivores. Our work shows the importance of pre-release monitoring at multiple sites over multiple years to adequately characterize populations. Without control, garlic mustard will likely continue to have negative impacts on northern forests.

Previous research showed growth regulator herbicides, such as picloram and aminopyralid, have a sterilizing effect on Japanese brome (Bromus japonicus Thunb.) that can reduce this invasive annual grass's seed production nearly 100%. This suggests growth regulators might be used to control invasive annual grasses by depleting their short-lived seed banks. The goal of this study was to extend the previous Japanese brome research to downy brome (Bromus tectorum L.), the most damaging invasive annual grass of U.S. grasslands. In a greenhouse, we found picloram did not greatly influence downy brome seed production, while point estimates suggest aminopyralid reduced seed production 55 to 80%. If not for a highly abnormal retillering response that we somewhat doubt would occur in the field, point estimates suggest aminopyralid would have reduced downy brome seed production approximately 90% when applied at the heading stage and approximately 98% when applied at three earlier growth stages. Our greenhouse study should encourage field studies designed to further explore the potential for using growth regulators to control downy brome and other invasive annual grasses.

Dioscorea bulbifera is a serious invader of various ecosystems in Florida, where plants generated by its two morphotypes climb aggressively and smother supporting vegetation. There is a dearth of published research on its invasive biological attributes including vine growth and biomass production by plants generated from bulbils. Herein, we assessed these parameters in common garden studies by planting bulbils from four biomass categories (PBBCs I–IV) of both morphotypes. Vine lengths, longevity-based growth rates (VLGR), biomass, and quantities of leaves and daughter bulbils in both morphotypes showed positive correlation with the biomass of planted parental bulbils. This indicated similarity between corresponding attributes in two morphotypes. Total vine length showed strong positive correlation with VLGR, biomass, and quantities of leaves and bulbils. Overall vine longevity among plants from PBBCs I–IV did not significantly differ whereas the total vine lengths, VLGRs, number of branches, and quantities of leaves and bulbils increased with the biomass of the parental bulbils. Plants recruited by smaller bulbils allocated more biomass to leaves and tubers compared to stems and bulbils, whereas the plants recruited by larger bulbils allocated more biomass to leaves and bulbils compared to tubers and stems. Higher proportion of biomass allocation to leaves and bulbils presumably ensures immediate faster growth, longer vines, and a greater number of daughter bulbils for future recruitment of new plants. Vine length (associated with faster growth rate, capable of blanketing supporting structures and producing large quantities of bulbils) has been noted as the primary invasive biological attribute that facilitates D. bulbifera's status as a noxious exotic weed in Florida. Control measure that can reduce vine length should reduce or eliminate the invasive behavior of D. bulbifera in Florida.

Hybridization between two introduced plant species can influence the invasion capabilities of the exotic taxa, but the role of hybridization will likely differ in different invasions, even of the same species. Until now, studies concerning the ploidy of Japanese knotweed, giant knotweed, and their hybrids have been conducted in Europe or native ranges in Asia. Here, we assess the role of hybridization and ploidy in a U.S. invasion. We use flow cytometry to characterize DNA content in (1) established families in a common garden, (2) seedlings grown from common garden parents, and (3) wild populations. We also measured fertility in the garden and the field and vegetative growth traits in the garden. Although the majority of our parental and hybrid samples had ploidy levels previously documented in Europe (4X and 8X for parental species; 6X for hybrids), we found a wider range of knotweed cytotypes established in our garden (4X, 6X, 7X, 8X, 9X, and 10X) and additionally detected 5X, 11X, 12X, and possibly 14X ploidy levels in progeny from garden seed parents. The unexpected cytotypes were not confined to the greenhouse or common garden, in that all < 11X ploidy levels were also found in field populations in Massachusetts. In several cases, these data contradicted our expectations on the basis of morphological and molecular analysis, suggesting both significant introgression and the introduction of multiple cytotypes from Asia. With one exception (14X), we found all cytotypes were capable of strong vegetative growth, seed set, and the production of viable pollen. Without barriers to sexual reproduction, introgression is expected to progress, creating a progressively more diverse swarm of invasive genotypes.

The continued threat of an invasive, annual brome (Bromus) species in the western United States has created the need for integrated approaches to postfire restoration. Additionally, the high germination rate, high seed production, and seed bank carryover of annual bromes points to the need to assay soil seed banks as part of monitoring programs. We sampled the soil seed bank to help assess the effectiveness of treatments utilizing the herbicide Plateau® (imazapic) and a perennial native seed mix to control annual Bromus species and enhance perennial native plant establishment following a wildfire in Zion National Park, Utah. This study is one of few that have monitored the effects of imazapic and native seeding on a soil seed bank community and the only one that we know of that has done so in a pinyon–juniper woodland. The study made use of untreated, replicated controls, which is not common for seed bank studies. One year posttreatment, Bromus was significantly reduced in plots sprayed with herbicide. By the second year posttreatment, the effects of imazapic were less evident and convergence with the controls was evident. Emergence of seeded species was low for the duration of the study. Dry conditions and possible interactions with imazapic probably contributed to the lack of emergence of seeded native species. The perennial grass sand dropseed outperformed the other species included in the seed mix. We also examined how the treatments affected the soil seed bank community as a whole. We found evidence that the herbicide was reducing several native annual forbs and one nonnative annual forb. However, overall effects on the community were not significant. The results of our study were similar to what others have found in that imazapic is effective in providing a short-term reduction in Bromus density, although it can impact emergence of nontarget species.

Lesser celandine (Ranunculus ficaria) is an ephemeral perennial introduced to North America from Europe for ornamental use. This species is becoming widespread in shady, moist woodlands and lawns in the northern United States and southern Canada. Despite its invasive attributes, lesser celandine continues to be marketed by the nursery sector. A hardy and easy plant to grow, lesser celandine has a dense compact growth form that makes it ideal for border plantings and showy flowers that make it a desirable garden species for early spring color. The species easily escapes cultivation, colonizing primarily moist woodland habitats. Once established, lesser celandine creates a monoculture and is purported to displace native ephemeral species. Several factors make control of lesser celandine difficult, including a large root structure, which facilitates clonal reproduction and spread. The ephemerality of lesser celandine results in a relatively brief window in which to apply control measures. Land managers need to be aware of this common garden species as a potentially invasive weed and need to be able to differentiate it from the similar native marsh marigold. Effective management strategies are needed to protect native woodland understory species and biodiversity in natural and seminatural areas of affected regions.

Ecological restoration of shrub–steppe communities in the western United States is often hampered by invasion of exotic annual grasses during the process. An important question is how to create restored communities that can better resist reinvasion by these weeds. One hypothesis is that communities comprised of species that are functionally similar to the invader will best resist invasion, while an alternative hypothesis is that structurally more complex and diverse communities will result in more effective competitive exclusion. In this field experiment, we examined the effects of restored community structure on the invasion success of three annual grass weeds (downy brome, jointed goatgrass, and cereal rye). We created replicated community plots that varied in species composition, structural complexity and density, then seeded in annual grass weeds and measured their biomass and seed production the following year, and their cover after 1 and 3 yr. Annual grass weeds were not strongly suppressed by any of the restored communities, indicating that it was difficult for native species to completely capture available resources and exclude annual grass weeds in the first years after planting. Perennial grass monocultures, particularly of the early seral grass bottlebrush squirreltail, were the most highly invaded communities, while structurally complex and diverse mixtures of shrubs (big sagebrush, rubber rabbitbrush), perennial grasses (bluebunch wheatgrass and bottlebrush squirreltail) and forbs (Lewis flax, Utah sweetvetch, hairy golden aster, gooseberryleaf globemallow) were more resistant to invasion. These results suggest that restoration of sagebrush steppe communities resistant to annual grass invasion benefits from higher species diversity; significant reduction of weed propagule pressure prior to restoration may be required.

Native forbs are important for plant community function and diversity, and provide food and cover for wildlife. Field studies have been conducted throughout the western United States to determine the impact of herbicides applied to control invasive weeds on native forbs. However, locating adequate populations of native forbs for evaluation of tolerance to herbicides is often difficult. The susceptibility of nine native prairie forbs to aminopyralid applied at 30 to 120 g ha−1 was evaluated in a greenhouse study and results compared to the same or similar species in field trials. Forb susceptibility to aminopyralid varied by species. Of the forbs evaluated, azure aster, purple coneflower, and closed bottle gentian were the most tolerant to aminopyralid while prairie coneflower, great blue lobelia, harebell, and white prairie clover were the most susceptible and likely would be killed in the field. Blanket flower and showy goldenrod were moderately tolerant to aminopyralid even when applied at 120 g ha−1. The susceptibility of greenhouse-grown forbs to aminopyralid was comparable to results for the same or similar species in the field. Results from greenhouse trials could be used to predict native forb tolerance in the field.