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.

Nomenclature: Black henbane, Hyoscyamus niger L. HSYNI.

Management Implications: Knowledge of the life cycle of an introduced plant is a fundamental component of its successful management. Black henbane (Hyoscyamus niger) is a state-listed noxious weed that is toxic to both livestock and humans. There is surprisingly little information on the introduced populations in North America. We explored whether introduced populations follow an annual life cycle, biennial life cycle, or both. We first collected seeds from several naturalized populations. The seeds were germinated, and the resulting plants were grown in a common environment in Fort Collins, CO. We then subjected half of the plants to a winter cold treatment (vernalization), while the other half remained in the greenhouse. All plants sampled appeared to be biennials, with cold being required for timely flowering. This has important implications for the potential of black henbane to spread: it is likely limited to areas that experience at least 10 wk of cold (3 to 11 C) winter temperatures. Combining this information with this species being a poor competitor, requiring open space to thrive, we can infer that it will perform best in fairly open western North American environments with a cold winter. Additionally, given that it is biennial, where active management is necessary in the western United States, monitoring and managing populations over multiple years will likely be key to effective control. Finally, it is critical to guard against the introduction of annual henbane plants to North America. Currently, USDA-APHIS requires a permit to import any part of the black henbane plant or plant products into the United States. Adhering to the current set of national regulations will help limit the range of naturalized black henbane populations.