Understanding seed characteristics and seedling establishment patterns is essential for the development of effective management strategies for invasive annual species. Dittrichia graveolens (stinkwort) has increased its range rapidly within California since 1995, yet its biology is not well understood, which has led to poorly timed management. In this study, seed viability, germination, longevity, and dormancy, as well as seedling emergence characteristics of D. graveolens were evaluated in field, greenhouse, and laboratory experiments in Davis, CA, over a 2-yr period (fall 2010 to summer 2012). In the laboratory, seed germination of D. graveolens occurred at a wide range of constant temperatures (12 to 34 C). Cumulative germination was comparable to total seed viability (80 to 95%) at optimal germination temperatures, indicating that primary (innate) dormancy is likely absent. The base temperature for germination was identified using a thermal time model: 6.5 C and 4 C for 2010 and 2011 seed populations, respectively. In the field, seedlings emerged from fall through spring following precipitation events. A very low percentage of seedlings (2.5%) emerged in the second year after planting. Equivalent seedling emergence was observed over a wide range of light conditions (100, 50, 27, and 9% of available sunlight) in a greenhouse experiment, indicating that seed germination is not limited by high or low light. Results from these seed experiments improve our understanding of the reproductive biology of this rapidly expanding exotic annual and provide valuable information for developing effective timing and longevity of management programs.
Nomenclature: Stinkwort, Dittrichia graveolens (L.) Greuter. Synonyms: Inula graveolens L., Erigeron graveolens L
Management Implications: Dittrichia graveolens is an invasive annual broadleaf plant of Mediterranean origin. It has become well established in California and has spread effectively over the past two decades. Plants establish in disturbed areas such as roadsides, gravel mines, and heavily grazed rangeland; at wetland margins; and on riparian floodplains. Individual D. graveolens plants produce many thousands of small, pappus-bearing achenes that disperse along transportation corridors. Control of D. graveolens has been challenging in the early stages of its invasion due to the absence of information on basic biology and ecology, both scientific and anecdotal. Most importantly, misunderstanding of seedling emergence has led to improper timing of control efforts.
Dittrichia graveolens seeds disperse in late fall and, based on our findings, the majority of seeds germinate in the winter and spring following major rain events, with few seedlings emerging the second year after dispersal. Mature seeds have high viability (> 80%) and no primary dormancy. Together these factors indicate that seed longevity is likely to be on the order of 2 to 3 yr, suggesting that a short, intensive management program that successfully prevents seed production should greatly reduce or eliminate infestations. However, in disturbed areas and along transportation corridors the risk of recontamination is high; continued monitoring to identify new populations is advised in these situations. The base temperature for germination is moderate: 6.5 C and 4 C in the two populations examined, and seeds will germinate with temperatures up to 34 C as long as sufficient moisture is available. This temperature flexibility combined with the dependence on seasonal precipitation makes seed germination very opportunistic, such that emergence timing is year-specific. Preemergent control, therefore, requires careful planning with a high level of context specificity.