Invasive species have the potential to reduce the abundance and diversity of native plants; this is particularly true in sensitive coastal dune habitats. Gypsophila paniculata is an aggressive invasive plant in sand dune communities of northwest Michigan. Restoration efforts in these communities are frustrated by regrowth of G. paniculata in treated areas, thought to reflect both resprouting from taproots and recruitment from seeds. We are assessing efficacy of treatment methods currently used to remove G. paniculata, including timing of herbicide treatment relative to the phenology of seed maturation. G. paniculata has high potential for seed dispersal, but the phenology of seed maturation is poorly known. We therefore conducted a study of populations in Michigan to document the phenology of seed maturation. Seeds were collected from untreated plants weekly during July and August 2016, stored, and tested to determine the proportion of seeds from each collection date that germinated. To assess the relationship between timing of glyphosate treatment and efficacy in preventing seed maturation, seeds from plants treated with glyphosate at different times during a 3-week period in July 2016 were collected at the end of the growing season and tested for germinability. Statistical comparisons were performed using nonparametric time-to-event analysis.
The results show that percent germination increased markedly from 20% to 90% in seeds collected between July 22 and July 28 and increased to 99% in the Aug 23 collection. Seeds from plants sprayed over a three-week period in July 2016 showed up to 20% germination, with the lowest percent (0%) in seeds from plants treated in early July and the highest in seeds from plants treated in late July. In 2016, herbicide treatment would have been most effective if spraying had been completed by mid July. Improving our ability to predict the timing of seed maturation (e.g., by using growing degree-days) would aid in limiting the number of germinable seeds deposited in treated areas each year and optimize use of limited resources for restoration.