Youngia japonica (L.) DC. was recently discovered on the Azorean island of São Miguel. This species is a cosmopolitan herb that has been reported to be invasive in several regions of the world. It is unclear when and how this plant arrived in São Miguel, Azores. Nevertheless, we hypothesize that this is an unintentional and recent introduction. The populations are expanding in São Miguel, and action is needed to mitigate potential damage to native ecosystems.

Fireweed (Senecio madagascariensis Poir.) has invaded and colonized numerous habitats in the coastal areas of southeastern Australia and is a major weed in cultivated lands as well as in poorly grassed, neglected, and highly grazed pastures. To examine the seed germination ecology of two populations (Felton and Gatton) of S. madagascariensis, experiments were conducted in the laboratory and screen house. The germination of both populations increased as the alternating temperatures increased from the coolest temperatures (15/5 C) to warmer temperatures (25/15 C). However, the highest temperature regime (35/25 C) resulted in the lowest germination rates. The Gatton population exhibited greater tolerance to higher temperatures, resulting in significantly higher germination (2.4 times) than the Felton population at the highest alternating temperature of 35/25 C. Compared with the Felton population, the Gatton population demonstrated higher tolerance to salt and water stress. In comparison to alternating light and dark periods (12 h each) (97% to 98%), the germination of both populations of S. madagascariensis was significantly reduced under complete darkness (24 h) (33% to 39%). A screen house seed burial depth experiment revealed similar emergence of S. madagascariensis seedlings between the populations. The maximum emergence (60%) was observed for seeds placed at the soil surface, followed by a dramatic decline in seedling emergence with an increase in depth. No seedlings emerged from a burial depth of 4 cm. With the addition of wheat (Triticum aestivum L.) crop residue to the soil surface at rates comparable to 4,000 to 8,000 kg ha−1, seedling emergence of S. madagascariensis decreased significantly. Information acquired from this study could be utilized to manage and develop effective weed management strategies for controlling S. madagascariensis in different agroecological conditions.

Invasive thistles reduce the productivity of pastures and rangelands because their structural defenses make them unpalatable to grazing livestock. However, plants potentially alter their growth patterns, including their allocation of resources to defense, in response to the presence of competing vegetation. Understanding the effects of competition on the structural defense of problematic pasture weeds can inform management plans that reduce the economic harm caused by these pests. We grew musk thistle (Carduus nutans L., also commonly called nodding thistle) in a fully crossed factorial field experiment in a single abandoned pasture in 2017 with two experimental factors: the level of thistle infestation (1 thistle or 5, resulting in densities of 4 or 20 plants m−2) and the presence or absence of grazing (simulated by weekly trimming of competing vegetation). We assessed the effects of treatments on defense by counting prickles >3-mm long on leaves. Our analysis included leaf age and leaf size as covariates. Competition reduced the number of prickles present on leaves. Regression analysis showed that an increase from, for example, 50 g to 200 g of competing vegetation within 50 by 50 cm study plots reduced the expected average number of prickles on intermediate-aged leaves with average length 25.5 cm by 76.9 prickles per leaf, or 41%. This pattern was similar for leaves of all ages, although the oldest leaves generally had fewer prickles than younger leaves. We did not observe differences in defense structures between plants neighbored by conspecifics and those neighbored by other competitors. Carduus nutans has been previously managed using high densities of grazers, and this practice may be more likely to damage less-defended individuals such as those we observed in our treatments with competition. This finding suggests that maintaining competition in pastures may increase C. nutans vulnerability to grazing.

White-tailed deer (Odocoileus virginianus) (hereafter, ‘deer’) and invasive plants affect forest understories, but few studies have investigated their interaction. We investigated long-term (11-yr) effects of excluding deer and removing the invasive shrub, Amur honeysuckle [Lonicera maackii (Rupr.) Herder], on plants in southwest Ohio. Deer exclusion enhanced tree seedling richness and density, but reduced annual and bare ground cover. Vine density was reduced by Lonicera, especially where deer were excluded. Seedlings of several tree species, the invasive shrub, burning bush  [Euonymus alatus (Thunb.) Siebold], and the invasive vine, winter creeper [Euonymous fortunei (Turcz.) Hand.-Maz.], were indicator species of the deer exclosure by Lonicera removal treatment combination. The effect of deer on cover of native species depended on Lonicera treatment: where shrubs were removed, cover was higher where deer were excluded, but where shrubs were present, cover was higher where deer had access. We attribute these interactions to the reduced growth of, and shading by, Lonicera where deer had access and browsed this invasive shrub. Some of these effects were evident in the first 6 yr, but are now larger. Other effects were not evident in the earlier evaluation. These findings inform management of areas with high densities of deer and invasive shrubs palatable to deer. Control of only invasive shrubs will reduce native cover and not improve tree regeneration. Managing only deer will increase woody plants but reduce native cover. Management of both stressors is needed to promote tree regeneration and restoration of plant communities.

Three species of the Old World genus Dipsacus L. are considered invasive in the Americas, yet they may differ in how they spread and reproduce and in their genetic diversity. Differences in invasion method may suggest that different management techniques are needed for each species. We performed genetic analyses on 572 plants in 69 populations from the United States, Argentina, and Eurasia with the goals of analyzing taxonomy, diversity, mode of reproduction, population structure, and founder effect of each of these species’ invasions, as well as looking for evidence of recent or ongoing hybridization. We found Indian teasel [Dipsacus sativus (L.) Honck.] to be lowest in diversity and possibly reliant on self-pollination more than the other species, Fuller’s teasel (Dipsacus fullonum L.) and cutleaf teasel (Dipsacus laciniatus L.). We found no evidence of hybridization within the invasions and no support for D. sativus as a subspecies of D. fullonum. The closest genetic matches of D. fullonum from the United States to the native range were with Hungary and Spain, while the closest match for D. fullonum between Argentina and the native range was with Spain. Dipsacus laciniatus from the United States most closely matched with samples from Russia. Population structure information regarding these three weedy Dipsacus species can help us understand their invasive processes as well as give insight into their management and the development of a biological control program.

Medusahead [Taeniatherum caput-medusae (L.) Nevski] is an invasive winter annual grass of western North American grasslands and rangelands that negatively impacts forage production, wildlife habitat, and ecosystem processes. Growth regulator herbicides, such as aminopyralid, applied in spring reduced invasive annual grass seed viability in greenhouse and California annual grassland experiments. Beginning in fall 2017, we tested combinations of sequential fall (preemergence) and spring (postemergence) aminopyralid applications at low (103 g ae ha−1) and high (206 g ae ha−1) rates at two ecologically distinct sites in the Intermountain West. Preemergence and postemergence aminopyralid applications at low and high rates controlled T. caput-medusae by 76% to 100% the second summer after study initiation. At the Utah site (which is warmer, drier, and more degraded than the Idaho site), the high rate resulted in better control. The first summer, postemergence aminopyralid applications at low and high rates reduced seed viability 47% to 91% compared with nontreated seeds, with the greatest reductions seen in Utah, which was experiencing drought. Across study sites, reduced T. caput-medusae germination in one year was linked to improved control the following year. The Idaho site also had desirable perennial grasses, which we used to investigate non-target effects. In general, there was a correlation between high T. caput-medusae control and higher perennial grass cover, indicating that successful control can make desirable perennial grasses more vigorous in this system. The option of a spring aminopyralid application increases the management window for controlling invasive annual grasses by decreasing seed viability, thereby depleting short-lived seedbanks.

Hydrilla [Hydrilla verticillata (L. f.) Royle], an invasive aquatic weed, has had a rich introduction history into the United States, with multiple subspecies being introduced since the 1960s. The most recent occurred before 2016, when northern hydrilla (Hydrilla verticillata ssp. lithuanica) was discovered in the Connecticut River. By 2021, following a 3-yr survey from Agawam, MA, to the Long Island Sound by the Connecticut Agricultural Experiment Station Office of Aquatic Invasive Species, H. verticillata ssp. lithuanica was found in more than 113 km of the river, occupying 344 ha. Since this survey, there has been concern that H. verticillata ssp. lithuanica would spread to nearby waterbodies and have a significant negative impact. Here, we report the first documented spread and establishment of H. verticillata ssp. lithuanica from the Connecticut River to five waterbodies in Connecticut and one in Massachusetts. Of the eight sites where H. verticillata observations were made, 75% (n = 6) were confirmed to be H. verticillata ssp. lithuanica and 25% (n = 2) to be Hydrilla verticillata ssp. peregrina (wandering hydrilla). Except for one site, all six locations infested with H. verticillata ssp. lithuanica provide watercraft access through public or private boat ramps. The authors also postulate on the mechanisms facilitating the spread and establishment of this subspecies.