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 (R. japonica Houtt., R. sachalinensis (F.Schmidt) Nakai) and their hybrid (known as R. × bohemica Chrtek & Chrtková) 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 there is not only one kind of interspecific hybrid, but in fact 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 knotweeds 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.

Ailanthus altissima (Mill.) Swingle readily exploits disturbances, grows quickly into dense monocultures, and suppresses native plant species. The vascular wilt pathogen, Verticillium nonalfalfae Inderb., native to the eastern U.S., has been proposed as a biocontrol agent for the invasive A. altissima. Studies consistently demonstrate the safety and efficacy of the bioherbicide, but they also note that the selective nature of the fungus does not preclude other invasive plants that commonly co-occur with A. altissima from occupying the site. We quantified the standing plant community and seedbank at several sites across Virginia five years after inoculation with V. nonalfalfae to understand which species are present or being naturally recruited. A. altissima remained dominant in untreated areas but was nearly eradicated from the treatment plots. Other nonnative species made up a large portion of the plant community and seedbank across all study areas, with no differences in their respective cover and count between treatments. While variability in plant community composition is high and site-specific context is important for establishing effective management strategies, planting native species and mitigating other invasives will be crucial to ensuring native species successfully establish in bioherbicide-treated areas.

Among the most widely distributed species globally, common reed (Phragmites australis (Cav.) Trin. ex Steud.) has generated extensive interest in invasive plant science and management because its introduced strains are highly invasive and often form monocultures that alter ecosystem properties. In desert wetlands in Las Vegas, Nevada, USA, where management goals included reducing hazardous P. australis fuels and increasing native plant diversity, we assessed variation in P. australis cover, the degree of native plant colonization, and soil seed banks after P. australis management treatments (cutting, glyphosate-imazapyr herbicide) and wildfires across gradients in soil properties. Based on change in P. australis cover during six measurement events over 24 months, 24 study sites formed three groups: 1) decreasing cover, where initially high P. australis cover (60-85%) decreased to < 5% following multiple cutting or herbicide treatments; 2) sustaining low cover, where wildfire or clearing was associated with initially low P. australis cover which remained low (< 30%) after multiple herbicide applications; and 3) sustaining high cover (45-100% initially and remaining 30-100%), including sites unmanaged or treated/burned only once. High soil salinity correlated with low post-management P. australis cover. No native plants were detected in the sustaining high P. australis cover group, despite natives occurring in the seed bank. Where management reduced P. australis cover, minimal native plant colonization did occur. Secondary invasion by other non-native plants was nearly absent. Our results suggest that if P. australis can be initially cleared, multiple herbicide applications can persistently keep cover low, especially on drier, saline soils. Slow native plant colonization suggests that a phased approach may be useful to initially reduce P. australis cover, keep it low via repeated treatments, and actively revegetate sites with native species tailored to the moisture-salinity gradient across P. australis-invaded habitats.

Effective regulation is essential for preventing the establishment of new invasive plants and managing the environmental, social, and economic impacts of those already established. Invasive plants are regulated by jurisdictions at a variety of local, regional, national, and international levels. Enhanced coordination of policy and regulations has been identified as a key strategy for addressing the impacts of invasive species (IPBES 2023; Pluess et al. 2012); however, coordination between jurisdictions, and even within jurisdictions, is not always considered. To review regulatory coordination in Australia, we compiled a comprehensive dataset of noxious weeds (defined as invasive plants and potentially invasive plants with controls specified in regulation) in each Australian jurisdiction (i.e., State or Territory). We found that jurisdictions on average shared c. 67% (SD = 15%) of noxious weed listings. Neighbouring jurisdictions were not more similar than separated jurisdictions in their noxious weed listings. There were significant differences in the biogeographic native ranges of noxious weeds between jurisdictions, with species native to temperate Asia being most frequently listed overall. The predominant likely entry pathway for noxious weeds in Australia was the ornamental trade. Listings were primarily dedicated to proactive control, prohibiting the cultivation of noxious weeds to avoid their naturalisation. There were 415 noxious weeds regulated in a harmonious manner across jurisdictions. However, there were 327 noxious weeds regulated by jurisdictions in a discordant manner, potentially leaving neighbouring jurisdictions vulnerable to invasion. We suggest jurisdictions reassess the regulation of these 327 discordant noxious weeds in Australia and utilise a national taxonomic standard to avoid problematic synonyms. Improved cohesion of policies could be achieved through wider adoption of existing regulatory systems and co-development of regulations between government and industry.

Herbicides that persist in the forest litter and soil following their use for managing invasive plant species may negatively affect restoration efforts as well as minimise re-invasion via their residual phytotoxic activity. This study determined the impact of an herbicide mixture comprising triclopyr, dicamba, picloram and aminopyralid (‘TDPA’) for the control of dense infestations of a woody invader, Pinus contorta, on the germination of re-invading P. contorta and three New Zealand native species (Chionochloa rubra, Fucospora cliffortioides, and Leptospermum scoparium) used in restoration. Given the essential role of ectomycorrhizal fungi in facilitating pine re-invasion, the impact of residual herbicides present in mineral soil on the ectomycorrhizal infection of P. contorta seedling roots was also examined. Germination trials were conducted using intact forest litter-soil cores collected at 27, 112 and 480 days (after herbicide spraying) from sprayed and adjacent unsprayed dense P. contorta infestations. At the same time, mineral soil was also collected for the ectomycorrhizal infection study. Post-spray herbicide residue bound in the litter significantly decreased germination rate, root and shoot growth and survival, and also caused malformation of P. contorta seedlings. Similar results were recorded for native species germination, however, overall viability of native seed was poor resulting in low germination rates. There was no difference in levels of ectomycorrhizal infection rates of P. contorta between treatments. Results indicate residual levels of TDPA herbicide in forest floor litter negatively affect P. contorta re-invasion, native recruitment and active restoration management. Ectomycorrhizal fungi, however, are unaffected by this herbicide mixture and therefore remain a risk to facilitating re-invasion as residual herbicide declines.

Crested floating heart [Nymphoides cristata (Roxb.) Kuntze] is an invasive aquatic plant in the southeastern United States. For clonal plants like N. cristata, clonal diversity may influence response to control tactics and/or evolutionary potential. However, little is known about the diversity of introduced N. cristata. In this study, we used genotyping-by-sequencing to quantify N. cristata diversity in the southeastern U.S. and determine how that diversity is distributed across the invaded range. Our results show that at least three distinct genetic lineages of N. cristata are present in the southeastern U.S. Geographic distribution of the lineages varied, with one widespread lineage identified across several states and others only found in a single waterbody. There is also evidence of extensive asexual reproduction, with invaded waterbodies often host to a single genetic lineage. The genetic diversity reported in this study likely results from multiple introductions of N. cristata to the southeastern U.S. and should be considered by managers when assessing control tactics such as screening for biocontrol agents or herbicide testing. The extent and distribution of genetic diversity should also be considered by researchers studying the potential for invasive spread of N. cristata within the U.S. or hybridization with native Nymphoides species.

Japanese hop (Humulus japonicus Sieb. & Zucc.) is an emerging invasive plant that has been observed to invade and spread throughout wetlands. As an annual vine, H. japonicus can smother native vegetation, forming dense stands and reducing biodiversity. At a restored floodplain forest in Joslin, Illinois, formerly used as an experimental site to test the effectiveness of different reforestation methods, H. japonicus has invaded stands of the previously dominant invasive, reed canarygrass (Phalaris arundinacea L.). We conducted an observational field study to examine the spatio-temporal dynamics of H. japonicus invasion relative to gradients in canopy cover and species composition. Ten transects, with half the transect extending into and half extending beyond H. japonicus patches, were established in October 2022. Seven quadrats per transect were surveyed for vegetation cover and canopy cover in October 2022, June 2023, and October 2023. Transects were evenly split between forested and open areas based on the reforestation treatments. H. japonicus cover significantly increased from October 2022 to October 2023, which has resulted in a slight decrease and replacement of P. arundinacea across the site. Shade reduced H. japonicus cover, indicating its preference for sunlit conditions. Species richness was higher in forested transects compared to open ones, most likely due to the absence of both P. arundinacea and H. japonicus in shaded transects. Along transects, quadrats that had been invaded by H. japonicus differed in species composition from quadrats that had not been invaded in both October 2022 and October 2023. H. japonicus cover was much lower in June than October, suggesting that temporal niche partitioning may allow P. arundinacea to persist, and indicating that monitoring for H. japonicus should occur late in the growing season. Both invasive species are shade intolerant, suggesting that planting fast-growing trees should be an effective long-term solution for controlling invasion.

Flowering rush (Butomus umbellatus L.) is an emergent perennial monocot that has invaded aquatic systems along the U.S. - Canadian border. Currently, there are two known cytotypes of flowering rush, diploid and triploid, within the invaded range. Although most studies have focused on the triploid cytotype, little information is known about diploid plants. Therefore, phenology and resource allocation were studied on the diploid cytotype of flowering rush in three study sites (Mentor Marsh, Ohio; Tonawanda Wildlife Management Area, New York; and Unity Island, New York) to understand seasonal resource allocation, environmental influences on growth, and to optimize management strategies. Samples were harvested once a month from May to November at each site from 2021 to 2023. Plant metrics were regressed to air temperature, water temperature, and water depth. Aboveground biomass peaked from July-September and comprised 50 to 70% of total biomass. Rhizome biomass peaked from September to November and comprised 40 to 50% of total biomass. Rhizome bulbil densities peaked from September to November at 3,000 to 16,000 rhizome bulbils m-2. Regression analysis resulted in strong negative relationships between rhizome starch content and air temperature (r2=0.52) and water temperature (r2=46). Other significant, though weak, relationships were found including a positive relationship between aboveground biomass and air temperature (r2=0.17), a negative relationship between rhizome bulbil biomass and air temperature (r2=0.18) and a positive relationship between leaf density and air temperature (r2=0.17). Rhizomes and rhizome bulbils combined stored up to 60% of total starch and present a unique challenge to management as these structures cannot be reached directly with herbicides. Therefore, management should target the aboveground tissue before peak production (July) to reduce internal starch storage and aim to limit regrowth over several years.

Invasive plant taxa are generally regulated at the species level, without considering infra- or inter-specific variation. However, cultivars or hybrids can be of a lower level of risk, e.g., due to sterility. We evaluate six general approaches to regulating cultivars and hybrids -1) “Globally guilty by association”; 2) “Nationally guilty by association”; 3) “Guilty until proven innocent”; 4) “Negotiated guilt”; 5) “Claimed to be innocent”; and 6) “Innocent until proven guilty”. We discuss these approaches in the context of South Africa (which has a typified “Negotiated guilt” approach). Following negotiations since 2001 between the South African horticultural industry/green industry and legislators, an unofficial consensus list of “presumed sterile” cultivars and hybrids was produced in 2014 containing 187 entities from 34 taxa. In 2020 this was reduced to 157 entities from 16 taxa. But the evidence supporting the original lists and the subsequent revisions were not published. To address this issue, we developed a generic pro-forma for reporting sterility based on observations and/or experiments on: flowering, fruiting, pollen, and seeds; the potential for vegetation propagation; and the potential for genetic changes (including hybridisation and reversion to fertility). We recommend that such information should be incorporated into risk analyses conducted specifically for infra- and inter- specific entities, and only if the risk of a harmful invasion is demonstrated to be acceptably low or can be easily mitigated should such entities be exempted from regulation. This will be time-consuming, but by setting out the evidence clearly, the approach is transparent and provides a clear route for stakeholders to seek exemptions for entities of importance. In summary, although we suspect the simplicity of the “Negotiated guilt” approach is desirable to many stakeholders; in general, and specifically for South Africa, we recommend a shift towards the “Guilty until proven innocent” approach.

Native aquatic macrophytes such as Vallisneria americana Michx. are often desirable in aquatic ecosystems due to the ecological benefits they provide but are threatened by competition from invasive taxa including nonnative Vallisneria taxa and hydrilla (Hydrilla verticillata L.f. Royle). Identifying potential selective herbicide management options can provide options to minimize impacts to native taxa in restoration and aquatic invasive plant management programs. Greenhouse mesocosm experiments were conducted in 2023 to investigate herbicide efficacy on two native eelgrass species (V. americana and V. neotropicalis Vict.), two nonnative taxa (V. australis S.W.L. Jacobs & Les and V. spiralis × V. denseserrulata Makino) and H. verticillata. Herbicide applications included endothall, diquat, florpyrauxifen-benzyl, fluridone, and flumioxazin and select combinations of these herbicides used in H. verticillata management. Endothall alone provided 90 to 100% aboveground biomass reduction at 3000 µg L-1 with at least 24 hours of continuous or intermittent exposure to all native and invasive species at six weeks after exposure, whereas florpyrauxifen-benzyl applied alone resulted in minimal aboveground biomass reduction. A 45 day of exposure of fluridone (10 µg L-1) resulted in 95% biomass reduction on V. americana and 7 to 48% other tested taxa. The combination of flumioxazin and florpyrauxifen-benzyl resulted in 90 to 100% aboveground biomass reduction and endothall combined with florpyrauxifen-benzyl resulted in 93 to 100% aboveground biomass reduction across taxa. Reductions in belowground biomass mirrored trends observed in aboveground biomass. No treatments selectively controlled invasive Vallisneria without injury to native Vallisneria, although efficacy was observed on hydrilla. These insights provide an understanding for differences between these Vallisneria for researchers moving forward with selectively targeting H. verticillata in the presence of native Vallisneria species and two new aquatic invasive plants. Future research should expand treatment scenarios, increase the study period, and identify potential integrated plant management strategies for field scenarios.