Planting California hedgerows: Native forb mix success at different densities

Background/Question/Methods

In California’s Central Valley, restorationists and agriculturists are focusing increased attention on planting native species in hedgerows along once bare or weed-infested field margins. In conjunction with pollinator services research, this study examined how three native forb seed mixes succeeded in hedgerows. At sites owned by Yolo County growers, three seed mixes were planted in winter 2008 at three different densities (1x, 2x and 4x) in nine 8m² blocks. The species mixes, along with the seeding densities, were based on attractiveness to pollinators, flowering time, and recommendations from local experts. We specifically examined germination success, amount of cover, flowering rates, and cost-effectiveness, all important considerations for restoration and ecosystem services.

Results/Conclusions

During the growing season in 2008, the seeding rate, type of mix, and site location were all significant factors for most planted species. A strict majority (five out of nine) of species had a higher germination rate and cover in mix 3, which contained all nine species, but the other four species often did best in mix 2 which contained only five species. Surprisingly, many species had just as high or higher rates of germination and cover at a seeding rate of 2x as they did at the highest seeding rate of 4x. Three species, Eschscholzia californica, Grindelia camporum, and Phacelia californica, were the most cost-effective species in terms of density and cover throughout the growing season. However, those species were not always the most prolific bloomers, an important consideration when providing pollinator services. These first-year results demonstrate the importance of seed mix and the “right” density for both restoration and ecosystem services projects. Practitioners also must consider their specific project objectives, cost-effectiveness vs. flowering rate vs. germination success, when determining which mix to plant at which rate. Second-year results plus new data from four new sites planted in winter 2009 will provide additional insight into these trends.