Concurrent weed growth suppression with essential oils and species-specific response to fractionated coconut oil

Plant-derived oils are phytotoxic and thus are potential non-synthetic herbicides. The present study investigated the effectiveness of three essential oils (red thyme, clove bud, cinnamon bark) and a vegetable oil, fractionated coconut oil (FCO), used alone or in 2-way essential/FCO mixes for controlling some troublesome weeds at seedling stage. The tested weeds were mugwort (Artemisia vulgaris L.), crown daisy (Glebionis coronaria (L.) Spach), milk thistle (Sylibum marianum (L.) Gaertn.), dense-flowered mullein (Verbascum densiflorum Bertol.), goosegrass (Eleusine indica L. (Gaertn.)), entireleaf morningglory (Ipomoea hederacea (L.) Jacq.), small-flower morningglory (Jacquemontia tamnifolia (L.) Griseb), large crabgrass (Digitaria sanguinalis (L.) Scop.), hemp sesbania (Sesbania exaltata (Raf.) Rydb. ex A.W. Hill). Greenhouse trials included a negative control (non-treated) and a positive control (glyphosate) and were conducted in Sassari (IT) and Auburn (USA). In both locations, all the essential oils used severely injured weeds by 5 days after treatment (DAT), with least or no recovery by plants, with no harvestable plant biomass 20 DAT. FCO provoked more diverse species-specific responses, reducing biomass of crown daisy, large crabgrass, entireleaf morningglory and hemp sesbania compared to the non-treated control while it stimulated the growth of mugwort, dense-flowered mullein, goosegrass, and small-flower morningglory. Milk thistle was the only plant not influenced by FCO compared to the non-treated control. The essential oils mixed with FCO confirmed high phytotoxic effects with a significant improved action when red thyme oil + FCO was applied to goosegrass. Our research confirms the potential herbicidal effect of some phytotoxic essential oils and the potential beneficial growth effect of FCO to some species.