Lessons learned in the use of herbicides and activated charcoal for restoration of retired agricultural lands in the western San Joaquin Valley (California)

Restoration of former agricultural lands to native plant communities in the San Joaquin Valley has been very problematic, in part because of immediate encroachment of weeds upon cessation of cropping. Tapping technology from the turfgrass seed industry, we began investigating the use of activated charcoal ‘safeners’ to ameliorate the effects of pre-emergent herbicides on seeded species. During the 2004-05 growing season, four native species (Grindelia camporum, Phacelia ciliata, Atriplex polycarpa, Suaeda moquinii) were seeded into a mustard (Sisymbrium irio) dominated site. Two charcoal treatments were applied over the drill rows in conjunction with seeding. The site was treated with glyphosate prior to drilled seeding, and five pre-emergent herbicides were applied post-seeding. Three herbicides (oxyfluorfen, clomazone, and sulfentrazone) significantly reduced the average cover of weeds while increasing native species establishment. During the 2005-06 growing season, a second similar experiment was conducted on a site dominated by aggressive winter-annual grasses (primarily Hordeum and Bromus spp.). Native species establishment was poor across all treatments. In contrast to the previous year’s trial, no glyphosate was applied prior to seeding. Much more abundant weed seed was present on the soil surface, promoting greatly increased competition within the charcoal-safened drill row. Heavy precipitation that occurred late in the growing season (i.e., after herbicide dissipation in the soil) also facilitated rapid weed growth. Results from both studies will be examined, with specific recommendations provided for herbicide selection and rates, treatment sequencing, and charcoal banding technique.