Cultivar and Site-Specific Variation Affect Establishment Potential of the Cleomes Roughseed Clammyweed (Polanisia dodecandra) and Spiderflower (Cleome hassleriana)

Establishment potential is one of the primary components of invasive species risk assessment. Models that predict establishment of potentially invasive ornamental crops often ignore differences among cultivars and the variability in plant response to site-specific factors. The objective of this study was to determine the extent to which differences among cultivars and habitat characteristics affect establishment of 5 cultivars of ornamental cleome. Experiments were conducted to compare germination, survival, and growth of cultivars in cultivated (gardens) and noncultivated (roadsides and prairies) environments and, in prairies, the effects of competition (low, intermediate, and high). In the first experiment, germination, survival, and growth were recorded in gardens, prairies, and roadsides in four locations in Minnesota. In the second experiment, the effects of competition with resident species were studied in five seed lots from three cultivars in three prairie sites. Additionally, a quantitative description of germination and transplantable seedling quality, when grown under greenhouse production standards, was obtained and compared with results from the cultivated and noncultivated outdoor environments. Germination in greenhouse conditions was significantly greater (78%) than in garden, prairies, or roadsides (< 46%). Mortality was greater in noncultivated than in cultivated environments (3% wk−1 and 1.4% wk−1, respectively). Survival was affected by competition, which reduced population establishment. Cultivar differences were most pronounced at seedling emergence, whereas habitat characteristics were more influential at later stages of the life cycle. Germination and plant height were similar among noncultivated environments. Variability in seedling emergence, survival, and growth in response to cultivar, habitat, and competition are important determinants of establishment potential. Among the cultivars studied, the native cleome, roughseed clammyweed, has a greater establishment potential than the nonnative cleome, spiderflower.