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.

Black henbane can be either annual or biennial. We investigated which life cycle is found in four introduced western North American populations. Plants were grown in a greenhouse common garden until half were vernalized by exposure to natural winter temperatures, while the other half remained in the greenhouse above 20 C, with 16 h of light and 8 h of dark. In total the plants were monitored 313 d after germination. We measured whether plants bolted, the time it took for bolting to commence, and the size at bolting. All vernalized plants bolted after 117 d of active growth (within 26 d of the end of the vernalization treatment), whereas only 26% of the nonvernalized plants bolted after an average of 278 d of active growth. Vernalized plants bolted at a smaller size than the nonvernalized plants that bolted (28 vs. 41 leaves on average). In the nonvernalized plants, the relationship between time to bolting and size was strong, but not so with the vernalized plants. Our results indicate that introduced black henbane plants are biennial, and that vernalization is more critical to bolting and flowering than reaching a certain size. Nonetheless, the fact that nonvernalized plants were capable of bolting if grown long enough suggests that vernalization is not the only cue that can trigger reproduction in introduced populations.

Torilis japonica (Apiaceae) has a widespread distribution, extending from western Europe to eastern Asia. In Europe, it usually behaves as a spring-germinating biennial species. Ripe seeds of T. japonica have an underdeveloped embryo and can persist in the soil for several years. The aim of this research was to reveal the mechanisms regulating the seasonal emergence pattern of seedlings. Experiments in a natural environment were performed to study phenology of seedling emergence and embryo growth. Seasonal changes in the dormancy status of T. japonica seeds were examined by regularly exhuming buried seeds and incubating them in controlled conditions. The action of temperature and light in regulating dormancy, embryo growth and germination was studied in the laboratory. Results showed that seeds of T. japonica have non-deep, simple morphophysiological dormancy (MPD), whereby physiological dormancy is broken by moist chilling (5°C). Once MPD was broken, embryo growth and subsequent germination started in spring, when appropriate temperature and light conditions were present. Seeds buried at a depth where light could not reach them showed cyclic changes in their dormancy status; embryo growth in these seeds could not be initiated because of the lack of a light stimulus. As far as we know, this is the first extensive study of seasonal dormancy cycles in a spring-germinating species of the Apiaceae. T. japonica occurs in a temperate climate with cool winters and warm, moist summers. In this climate, spring is often the most favourable season for seedling establishment.