Weeds have a greater impact on crop yields than any other pests. Over the past several decades, farmers have continually turned to synthetic herbicides because they are the most effective deterrent against weeds. However, demand for organic food is rising, and public sentiment toward synthetic herbicides is increasingly negative. There is a need — and a market — for new, natural weed management tools.

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.

Agrochemical companies are looking for “greener” options to meet market and government demands. This study found that manuka oil and its main active ingredient, leptospermone, were stable in soil for up to seven days, with half-lives of 18 and 15 days, respectively. This longer-lasting, but natural, effectiveness opens more possibilities for this oil in both organic and conventional farming.

Full text of the article, “Manuka Oil, A Natural Herbicide with Preemergence Activity,” Weed Science, Vol. 59, No. 4, October-December 2011, is available at http://www.wssajournals.org/.