Germination of Native Species: Efforts to Guide Revegetation in a Mexican Petunia-Invaded Floodplain in Florida

Active revegetation with native species has been suggested as a way to not only restore native plant communities, but also prevent reinvasion of previously controlled invasive species (Ammondt et al. 2013). When sowing native species’ seeds for this purpose, understanding what seasonal conditions promote germination identifies a potentially optimal sowing period for native plant establishment (e.g., Kettenring and Galatowitsch 2011, Oliveira et al. 2012, Farley et al. 2013). Similarly, knowing what seasonal conditions preclude invasive species germination identifies a sowing period in which invasive species germination is potentially lower. Ideally, this combination of information suggests sowing at a time that promotes germination and establishment of native species, even when invasive species reinvasion likelihood is high. Despite the interest in sowing native species seeds for both revegetation and invasive species suppression, in many scenarios, germination requirements for both native species and invasive species are unknown.

Effective seeding approaches are especially critical in urban watersheds, where many factors challenge restoration efforts, including invasive species propagule pressure. Floodplain forests in urban watersheds of Florida, for example, receive stormwater runoff and subsequently experience dispersal of propagules of the invasive Mexican petunia (Ruellia simplex), a commonly planted ornamental landscape plant in the upstream watershed (Hupp et al. 2009, Wunderlin and Hansen 2014). A recent study demonstrated that chemical control can initially reduce Mexican petunia invasions (Reinhardt Adams et al. 2014), but Mexican petunia reinvasion following control is significant (Reinhardt Adams et al., University of Florida, unpub. data). Limited native species recolonization followed these effective control treatments, suggesting that active revegetation is needed to restore the native plant community. Therefore, information on both native species and Mexican petunia germination requirements are needed to inform effective revegetation practices, specifically by identifying conditions corresponding to a time of year that promotes native species germination and limits Mexican petunia germination. Specific requirements to promote optimal germination for native species of southeastern U.S. flood-plains are under-researched. Mexican petunia germination requirements have been examined by Wilson et al. (2004), who found that seeds will germinate (21–100%) across wide ranges of moisture levels, light regimes (with or without), and temperatures (15 to 33°C), and suggested that germination across a broad range of conditions may contribute to its invasiveness. We note that because Mexican petunia has many traits in common with other problematic invasive species (i.e., long flowering time, rapid growth, rhizomatous spread, explosive seed dispersal, high germination rates), and because urban wetlands are particularly prone to plant invasions, revegetation approaches that promote native species establishment in this scenario may offer promise to the restoration of similar plant invasions.

To better guide timing of native species seed sowing efforts, we determined germination behavior of selected native species and Mexican petunia by conducting a germination study in incubators under different temperature regimes representing Florida seasons (fall, winter, spring, summer). We hypothesized that Mexican petunia seeds would germinate quickly under most temperature regimes, while native species seeds would germinate more slowly and under certain temperature regimes. Native species chosen met selection criteria for effective revegetation in floodplain forests (e.g., readily available, competitive under current site conditions, characteristic vegetation of ecosystem, and ability to withstand a wide range of water depths; Smith et al., University of Florida, unpub. data) were bushy bluestem (Andropogon glomeratus), common rush (Juncus effusus), redtop panicgrass (Coleataenia longifolia sp. longifolia), and pinebarren goldenrod (Solidago fistulosa). Native species seeds from natural Florida populations were obtained from Ernst Conservation Seeds (Meadville, PA) (bushy bluestem, common rush, redtop panicgrass) or The Natives, Inc. (Davenport, FL) (pinebarren goldenrod). Mexican petunia seeds were hand collected from the Lake Jesup Conservation Area in October 2012. All seeds were stored in a plastic bag in a refrigerator at 10°C. Seeds with visible pathogen or insect damage were removed and discarded. On March 1, 2013, seeds for each species were divided into four replications of 50, and each replication was placed in single 10.9 × 10.9 cm transparent polystyrene germination box (Hoffman Manufacturing, Inc., Albany, OR) containing...