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.

Nomenclature: Roughseed clammyweed, Polanisia dodecandra (L.) DC. PONGR; spiderflower, Cleome hassleriana Chod. CLEHA

Interpretive Summary: Gardens can be sources of nonnative, invasive, ornamental crops. Cleomes are popular ornamental garden plants that have escaped cultivation and naturalized in several countries including the United States. Native and nonnative cleome cultivars were evaluated in this study. The objective of this study was to compare establishment potential in gardens, roadsides, and prairie environments; effects of competition in prairies were also assessed. Garden settings are most suitable for reseeding, although germination is also possible in roadsides and prairies. 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. Thus, for flower-breeding programs to assess invasive potential, specific target environments—in which a potentially invasive species could invade—are more critical than testing in cultivated sites. Competition plays a significant role in decreasing population establishment at the seedling stage. Native North American species may be more likely to establish as weeds than nonnative species.