Manuka oil shows promise as natural herbicide

The current issue of the journal Weed Science reports on the possibilities of manuka oil as a natural herbicide. Distilled from the manuka tree, this essential oil showed good results in field tests as both a preemergent and postemergent herbicide. Field tests were conducted in Stoneville, Mississippi, against crabgrass, velvetleaf, pigweed, and other species of broadleaf and grass weeds.

Natural alternatives to synthetic herbicides are often essential oils, used after weeds emerge to “burn down” the undesired plants. To be an effective herbicide, these oils often require multiple applications in high amounts. The cost of the oil and the cost of making numerous applications drive up the overall expense.

Manuka oil contains natural beta-triketones, which target the same plant enzyme as some commercial synthetic herbicides. With this component, small amounts of manuka oil can be combined with a commercial organic herbicide of lemongrass oil to achieve greater results. In this study, this combination made the lemongrass oil more potent in postemergent applications, causing as much as a 94 percent reduction in dry weight of remaining crabgrass collected.

However, it is the potential of manuka oil as a preemergent treatment that makes it an attractive option for developing a new natural herbicide. Large crabgrass growth was reduced 50 percent to 90 percent in the current study, depending on the dose of manuka oil used. No other essential oil currently in commercial use for weed control has shown such strong activity.

This study found that manuka oil and its main active ingredient, leptospermone, were stable in soil for up to 7 days, with half-lives of 18 and 15 days, respectively.