Soil Matters: propagation success of Viola pedunculata and implications for conservation of an endangered butterfly

This study examines the propagation methods and survival of Viola pedunculata, a wild violet that is the only host plant for larvae of a federally listed endangered butterfly species in the San Francisco Bay Area in California. The primary goal was to determine how soil composition affects seedling survival and long-term growth, ensuring that these plants can thrive and continue providing a vital food source for Speyeria Scudder butterfly populations. Previous research by students in our lab showed high rates of success germinating Viola pedunculata using stratification following a drying period and that additional treatments were not required for germination. However, that work did not test methods for growing the germinated seeds. Here we germinated the field collected seeds and planted the germinating seeds in four soil treatments under controlled laboratory and greenhouse conditions. The four soil treatments were: autoclaved native soil, non-autoclaved native soil, a mixture approximating “UC Mix 2”, and a mixture approximating ”UC Mix 3”. These treatments vary in soil moisture and acidity providing a gradient to examine how soil composition and sterilization impact propagation success. Preliminary findings indicate that non-autoclaved native soil provides the most favorable conditions for germination and seedling emergence, yielding 32 Viola seedlings of 43 seeds planted (77%) —likely due to beneficial microbial interactions absent in sterilized media. In total, 79 seedlings emerged in greenhouse conditions and 92 in the laboratory. These results contribute to understanding optimal propagation practices for Viola pedunculata to help support future restoration efforts by outplanting propagated plants, or planting seeds from propagated plants, to butterfly habitats to help maintain and increase populations of threatened butterflies.