The potential of culinary vegetable oils as herbicides in organic farming: The effect of oil type and repeated applications on plant growth

Abstract

This study tested the herbicidal effects of raw and processed culinary oils (rapeseed oil, sunflower oil, olive oil, flax/linseed oils) on nine plant species (poppy, white clover, alyssum, lupin, buckwheat, mustard, oats, perennial ryegrass, tall fescue). Oils were also tested on weeds that naturally emerged from farm soil. Plants were coated with oil using a pressurized hand pump, then maintained in a glasshouse or outdoor plant facility until harvested approximately 5 weeks after oil application. All the oils tested caused a decrease in plant dry matter compared with water-sprayed control treatments, although there was some inconsistency in herbicidal effects among plant species, trials and growing conditions. Spraying plants with rapeseed oil on multiple occasions tended not to be more phytotoxic than only a single spraying. Raw or organic oils were not consistently more phytotoxic than processed versions of the same oil type. There was some evidence that negative effects of oils on plant growth were more apparent under warmer conditions. The results suggest that culinary oils could reduce the biomass of weeds in an environmentally friendly way that is permissible to organic growers. However, the herbicidal activity of these oils appears low, and the quantities required to obtain substantial weed control may not be economically viable in all instances.