Old World climbing fern (OWCF) is a highly invasive fern that disrupts natural communities in central and southern Florida. OCWF produces copious, wind-blown spores that have propelled its rapid invasion of Florida's natural areas over the last few decades. Current management of OWCF is limited to herbicides and natural resource managers in Florida have questioned if herbicides inhibit spore germination. This study compared spore germination rates of OWCF exposed to six herbicides and a surfactant from 1 to 24 h under laboratory conditions. Spores of OWCF were highly susceptible to metsulfuron, but exhibited tolerance to imazapyr, glyphosate, fluroxypyr, asulam, and triclopyr. Spore germination rates were 0.4% for spores exposed to 0.1 g ai L⁻¹ of metsulfuron, but 0% for rates greater than or equal to 0.2 g ai L⁻¹ at 30 d after treatment (DAT). Reduction in spore germination was observed with all other concentrations of herbicides tested, ranging from 10.4% with triclopyr (40 g ai L⁻¹) to 42.6% with asulam (4.2 g ai L⁻¹) compared to 47.9% germination for untreated checks 30 DAT. Spores were highly sensitive to metsulfuron with herbicide concentrations required for 50 and 95% inhibition of spore germination (I₅₀ and I₉₅) measuring 0.014 and 0.063 g ai L⁻¹, respectively; spores were greater than 1,000-fold more sensitive to metsulfuron compared to I₉₅ concentrations of any other herbicide tested. These results indicate that metsulfuron exhibits potential to control OWCF spore germination but spores are tolerant to the five other herbicides tested.

Nomenclature: Asulam; fluroxypyr; glyphosate; imazapyr; metsulfuron; triclopyr; Old World climbing fern, Lygodium microphyllum (Thunb.) Swartz.

Interpretive Summary: Old World climbing fern is a highly invasive plant in natural areas of Florida and spreads by wind-blown spores. Herbicide control of OWCF can be achieved but long-term management is required for new growth from germinated spores blown in by wind currents. No previous research has examined the effects of any herbicide on the spores of this species. In this study, metsulfuron inhibited OWCF spore germination to less than 0.5% at rates below 0.1 g ai L⁻¹ at 30 d after treatment compared to 47.9% germination for untreated checks. No other herbicide tested on Old World climbing fern spores inhibited spore germination by less than 11%. With the exception of metsulfuron, there was a gradual decrease in percentage of spore germination for imazapyr, glyphosate, fluroxypyr, triclopyr and asulam, but the concentrations of these herbicides required for 95% inhibition of spore germination were well above the legal label limit. Our results suggest that the standard aerial application rate of 0.084 kg ai 76 L⁻¹ metsulfuron used in Florida for control of OWCF may inhibit greater than 95% of the spores if it contacts the spores or translocates into the sporangia, whereas the standard ground aerial application rate of 0.084 kg ai 379 L⁻¹ is unlikely to inhibit 95% of the OWCF spores.