In places where multiple related taxa are invasive and known to hybridize, it is important to have correct identifications to enable an appropriate legal, ecological, and management understanding of each kind of invader. Invasive knotweeds in the genus Reynoutria Houtt. are noxious weeds in Europe, North America, Africa, and Oceania, where they disrupt native plant communities and negatively impact human activities. Two species (Japanese knotweed [Reynoutria japonica Houtt.; syn.: Polygonum cuspidatum Siebold & Zucc.] and giant knotweed [Reynoutria sachalinensis (F. Schmidt ex Maxim.) Nakai; syn.: Polygonum sachalinense F. Schmidt ex Maxim.]) and their hybrid (known as Bohemian knotweed [Reynoutria ×bohemica Chrtek & Chrtková; syn.: Polygonum ×bohemicum (J. Chrtek & Chrtková) Zika & Jacobson [cuspidatum × sachalinense]]) have similar invasive tendencies, although there are some noted differences among them in their reproduction potential, ecological tolerance, and effect on native communities. Prior studies demonstrated that not only one kind of interspecific hybrid exists, but in fact there are at least four kinds that differ in the sequence variants they possess from each parent. Thus, in addition to identifying plants as hybrids, it may become important to distinguish each kind of hybrid when considering control or treatment strategies. In the current study, we expand the available genetic information for invasive Reynoutria by providing expanded DNA sequence data for the low-copy nuclear gene LEAFY, which has become important for characterizing hybrids. Our methods recover the same LEAFY genotypes that were identified previously for the commonly sequenced second intron, and we also provide sequence data for the first intron and second exon of the gene.

Vallisneria × pseudorosulata S. Fujii & M. Maki is an invasive aquatic weed that has recently become a major issue within the U.S. Southeast. Vallisneria × pseudorosulata is a hybrid between two nonnative eelgrass species (Vallisneria spiralis L. and Vallisneria denseserrulata Makino) and has rapidly overtaken water bodies in Tennessee, Alabama, and Florida. This hybrid can reproduce rapidly through offshoot formation and floating propagules capable of drifting large distances before establishing. Vallisneria × pseudorosulata has been previously found in Japan and is thought to have been introduced in the United States by the aquarium trade or through dumping.

Urban areas are increasingly recognized as important centers of biodiversity. Nonetheless, invasive species can reduce this biodiversity, and cities can be hubs for alien plant invasions, highlighting the need to monitor urban biodiversity and problematic alien species. The goal of our study was to assess the distribution of wild chervil [Anthriscus sylvestris (L.) Hoffm.] and anise [Myrrhis odorata (L.) Scop.] in green spaces of Reykjavík, Iceland. This information is necessary to implement the city’s biodiversity strategy regarding invasive species. Both of these alien plants are spreading throughout Iceland, and Reykjavík’s high-latitude location (≥63°N) and remoteness make it an ideal case study to assess alien plant introductions and invasions in subarctic urban areas. We surveyed four green spaces (Laugarnes, Vatnsmýri, Elliðaárdalur, and Ægisiða) from May to October 2017 using AllTrailsPro and ArcGIS mobile applications. ANOVA and Bonferroni correction (post hoc test) were used to compare the distribution and patch sizes of A. sylvestris and M. odorata among the study sites. We found that A. sylvestris covered at least 10% (15.5 ha) of the total area surveyed (158 ha), while M. odorata only covered ≤1 ha. Both plants were abundant near buildings, pathways, riversides, and streams, and they are expanding their distribution in Reykjavík’s green spaces. While A. sylvestris is clearly more established and widespread with larger patches (>100 m2), the distribution of M. odorata is more localized, occurring mainly in smaller patches (<100 m2). We recommend long-term monitoring to further assess M. odorata’s invasive potential, as well as testing and adopting integrated weed management strategies via adaptive management to control the distribution of A. sylvestris and that of other problematic alien plants. These actions, which are applicable to other subarctic cities, will help foster more proactive management encouraging urban biodiversity.

Citizen science is becoming very useful in surveying and monitoring biodiversity. Within the European Union LIFE medCLIFFS project, a network of volunteers has been established for the detection and long-term monitoring of invasive plant species that threaten the endemic flora of Mediterranean cliffs in northeastern Spain. Through iNaturalist, volunteers record various data along a series of 1-km transects. Based on the ca. 700 observations collected by volunteers in 2023 (the first year of the project), a simple and visually attractive methodology for assessing the recorded populations has been developed. This method classifies populations into one of three population dynamics categories: (1) propagative behavior (i.e., populations with seedlings or young plants but lacking senescent or dead individuals); (2) senescent behavior (i.e., showing senescent/dead plants but lacking seedlings/juveniles); and (3) a mixed behavior (i.e., containing both). This methodology, whose outputs are easily interpretable as heat maps, allows the collection of large datasets on invasive plants by citizen scientists, with two main purposes: (1) knowing which species are most concerning based on simple, straightforward observations of their population dynamics; and (2) identifying which regions of the study area are more problematic and where management efforts should therefore be directed.

Reduced hack and squirt (RHS) is a herbicide-injection technique that controls woody species with a limited number of hacks. When used with aminopyralid or aminocyclopyrachlor, the technique controls multistemmed shrubs such as Brazilian peppertree (Schinus terebinthifolia Raddi) and Surinam cherry (Eugenia uniflora L.). However, additional data are needed to compare its effectiveness on other woody invasive species against standard individual-plant treatment techniques. We compared RHS with aminocyclopyrachlor or aminopyralid administered with 0.5 ml of undiluted concentrate (240 g L−1) against traditional basal bark and cut stump techniques using triclopyr at 96 and 120 g L−1, respectively, on six invasive trees in Florida, USA. For all species tested, the RHS technique with aminocyclopyrachlor was not different or was more effective than basal bark or cut stump treatment with triclopyr. The RHS technique with aminopyralid was effective on certain species but did not control as many species as RHS with aminocyclopyrachlor or cut stump treatment with triclopyr. The RHS technique required significantly less time than cut stump treatment but was not different from basal bark treatment. Across all species, when compared with basal bark and cut stump treatment with triclopyr, the RHS approach resulted in reductions of average total herbicide mix applied by 98% and 89%, respectively. For herbicide active ingredient, when compared with basal bark and cut stump treatment with triclopyr, RHS resulted in reductions of 95% and 86%, respectively. These results indicate that when compared to conventional individual plant treatment (IPT) approaches with triclopyr, the RHS approach yielded a high level of target efficacy on a broad spectrum of species, comparable or shorter application times, and substantial reductions in herbicide mix and active ingredient applied. Future research should seek continued refinement of this technique for woody plant control on other troublesome invaders.

Tree-afflicting pests, such as insects and pathogens, could change forests in ways promoting invasions by non-native plants. After tree death associated with the fungal pathogen oak wilt (Bretziella fagacearum) and its attempted containment (severing root connectivity and sanitation removal of infected trees), we examined change in cover of the non-native liana Oriental bittersweet (Celastrus orbiculatus Thunb.; hereafter Celastrus) at 28 sites in temperate black oak (Quercus velutina Lam.) forests, Ohio, USA. During our 5-yr study spanning 2020 to 2024, Celastrus cover increased significantly (P < 0.05) through time at oak wilt sites but not in untreated reference forest sites without evidence of oak wilt. Celastrus cover increased by an order of magnitude, up to an average of 32 times among oak wilt treatments up to 10 yr old. By 2024, Celastrus cover ranged from 6% to 22% on average in 5- to 10-yr-old oak wilt treatments, compared with 1% cover in reference forest. Results indicate that non-native plant invasion accelerated following disturbance associated with a fungal pathogen and its attempted containment and, more generally, suggest that tree-afflicting pests can promote invasive plants in forests. Co-management of tree-afflicting pests and non-native plants may become increasingly important to ensure forests recovering from tree mortality are dominated by native plants.

Invasive plants’ ability to extend their range depends upon their local environments and both positive and negative interactions with native species. Interactions between invasive and native plants may be indirectly linked to the soil fungal community, which may enhance or suppress invasion through mutualism or parasitism. Many invasive plants preferentially select fungal communities or change soil chemistry to gain a competitive advantage, and such changes can remain even after the invader is removed, known as legacy effects. Yellow toadflax (Linaria vulgaris Mill.) is an invasive forb that is aggressive in the western United States but is nonaggressive in the midwestern United States. We evaluated the relationship between soil abiotic properties, nitrogen (N) enrichment, arbuscular mycorrhizal fungal (AMF) community composition, and L. vulgaris invasion in aggressive (CO) and nonaggressive (IL) populations. We collected soil from uninvaded and invaded sites in Gothic, CO, and near Chicago, IL, and sequenced AMF community composition in each site. Using the same soil, we grew L. vulgaris and native species in pots for 120 d, with half of the pots receiving N fertilization, and harvested biomass. We also injected a 15N-labeled tracer in pots and analyzed plant tissue for 15N enrichment and net uptake rates (NUR). In CO soil, L. vulgaris rhizomes sprouted more in invaded soil, whereas in IL soil, L. vulgaris only sprouted in uninvaded soil. N fertilization had no impact on biomass, and NUR did not differ significantly between any treatments. AMF communities differed between the two sites but were not significantly influenced by invasion history. Our results suggest that L. vulgaris leaves legacy effects but that these effects are different between aggressive and nonaggressive populations. Legacy effects may facilitate reinvasion in CO, but we did not find conclusive evidence of legacy effects in IL, and differences between the sites could be shaped by endemic AMF communities.

Several Elaeagnus species (autumn olive [Elaeagnus umbellata Thunb.], Russian olive [Elaeagnus angustifolia L.], and thorny olive [Elaeagnus pungens Thunb.]) are invasive in North America. Elaeagnus pungens is prevalent throughout much of the southeastern United States, commonly overtaking wooded and natural areas, bottomlands, and roadsides. While many management methods, including several herbicide treatments, have been evaluated, the efficacy of these methods can vary based on the size and density of the target plants. Further, personal communication with land managers revealed a lack of information that incorporated application effort, duration, and associated cost into treatment efficacy and usefulness. We evaluated three herbicide application methods using the free acid formulation of triclopyr in an E. pungens–infested forest in South Carolina, USA, to determine the effectiveness of each application method. We estimated pretreatment E. pungens biomass and destructively harvested all live material posttreatment to obtain actual biomass values. Foliar herbicide application was ineffective, but both cut stump and basal bark application nearly eliminated E. pungens in the treatment plots. The basal bark application took slightly more time to complete than cut stump treatments but was described as less physically demanding by applicators. Based on treatment efficacy and time required, the basal bark application method seems most prudent for controlling E. pungens in these areas. These results will help land managers more effectively use their resources for invasive woody plant control.