Phytotoxic effects of Salvia rosmarinus essential oil on Acacia saligna seedling growth

Using essential oils (EOs) to suppress unwanted species at the seedling stage is a promising way to decrease the use of synthetic herbicides and thus protect native plant diversity. We verified the effects of rosemary (Salvia rosmarinus) EOs at different concentrations on the growth and mortality of seedlings of the invasive alien species Acacia saligna derived from seeds collected at four wild populations in Sardinia and Sicily (Italy), by monitoring their survival, dry weight, shoot and root length during their early development. Six different spray solutions were applied to A. saligna seedlings: one with only distilled water; one with a solution of distilled water, ethanol and Tween 20; three with a rosemary EO concentration of 3.9, 7.8 and 15.6 mL/L; and one with the commercial herbicide DICOTEX© RTU. The seedling survival and growth decreased with the increase of EO concentrations; the patterns were similar for all populations. The highest rosemary EO concentration tested (15.6 mL/L) strongly inhibited shoot and root length, dry weight and survival of A. saligna seedlings and its effect was significantly different compared to the other EO and control treatments, except for the control with commercial herbicide, that was the more effective compared to all other treatments. Our results suggest that the rosemary EO concentration at 15.6 mL/L is a valid tool for the biological control of A. saligna. This treatment is promising because it could be applied primarily to reduce the seedlings’ emergence/development and establishment while ensuring the conservation of native plant diversity and the preservation of the Mediterranean habitats.