Successful pollinator seed mixes include low grass density and high forb richness across a range of total seeding densities

Pollinator habitat restorations and enhancements are an important approach to mitigate pollinator declines due to habitat loss and other stressors. Foraging resources can be provided by wildflower plantings grown from seed mixes composed of pollinator-friendly plant species. Despite clear evidence on the benefits of such plantings for bees and other pollinators, there is limited knowledge on how variation in multiple seed mix design parameters affects floral resources over time. Such information is crucial to improve the effectiveness of different habitat applications and to potentially lower seed costs, which remain an impediment to adoption. We conducted an incomplete factorial field experiment to test the impacts of density, diversity, and forb:grass ratio of seed mixes on floral resource availability and phenological coverage of the resulting plantings. We quantified floral area and phenological coverage over the whole blooming season for three consecutive years in replicated plots of wildflower mixes in two regions of the U.S.A, Northern California's Central Valley and Midwestern Tallgrass Prairie. When different seed densities were fully replicated (e.g. within high forb:grass seed ratios in Minnesota, or forb-only seedings in California), seeding density had no further impact on floral area and floral phenological coverage.