The Role of Invasive Erodium Species in Restoration of Coastal Sage Scrub Communities and Techniques for Control
Three studies were conducted to investigate effects of invasive Erodium species on re-establishment of native species in the California coastal sage scrub (CSS) plant community, and how both chemical and non-chemical techniques might be used to reduce Erodium and increase success of restoration efforts. First, the effects of a range of Erodium densities were tested on the establishment of native shrubs and forbs in two experimental sites, one a seeding experiment and the other a weeding experiment. Analyses were done to detect a threshold value where Erodium density decreased the establishment of the native species, and showed that Erodium species inhibited the establishment of native shrubs at about 66 Erodium plants/m2. Erodium species often germinate at very high densities (thousands of plants/m2), indicating land managers will often need to actively control Erodium species densities in order to restore CSS vegetation after disturbance. Second, varying treatments of the non-chemical agricultural technique of solarization were tested without irrigation, as supplementing water may not be possible in some wildland situations. Laying sheets of clear plastic over tilled soil during the hot summer months reduced invasive annuals the most of all the treatments, even without the addition of water. Third, chemical control experiments tested a variety of herbicides with different modes of action and at varying rates at two sites. Some herbicides labeled as grass-specific also have activity on Erodium species but do not damage most CSS native forbs and shrubs, and were tested in a variety of concentrations. Chemicals tested in this study did not provide season-long control of Erodium species at rates allowed by the label, but one above-label rate proved effective. Multiple applications of herbicides with this mode of action (e.g., fluazifop) within approved rates should be tested to determine efficacy on Erodium. The broadleaf-selective chemical triclopyr provided the best control but will have to be used with caution in the CSS community as it also has activity on native shrubs and forbs. The broad spectrum herbicide glyphosate had good control at one site, but the second site had germination of a second cohort of Erodium after the first cohort was sprayed.