Organic farmers are keen to have access to new weed management techniques and share knowledge with each other. Researchers around the world are working to address this challenge, including those supported by Canada’s Organic Science Cluster program. One tool that is missing from the organic weed management toolbox is a cost-effective herbicide that is organically acceptable. Can this gap be filled by the essential oil of manuka (Leptospermum scoparium), a shrub native to Australia and New Zealand?
Naturally derived herbicides can be an acceptable option under Canadian organic standards, with approval on a case-by-case basis. Products such as acetic acid and pine oil have been tested previously as contact herbicides but have not been sufficiently cost-effective to warrant commercialization for organic agricultural production. However, manuka oil had been identified as having potential as an organically acceptable herbicide. Canadian researchers began to further explore manuka oil as an organic herbicide under Organic Science Cluster 2 (2013-2018).
Activity Leader Rene Van Acker and his research team (University of Guelph, Ontario) conducted research in 2015 to “evaluate manuka oil on selected broadleaf and grass weeds for its effect on weed control and determine its pre- and post- weed emergence activity and its synergy/additive effects with other new and currently approved organic weed control products.”
Essential oils permitted in organic production (lemongrass, clove, cinnamon) tend to be fast-acting but not long-lasting. Manuka oil is attracting worldwide attention because it works systemically to harm the plant and remains active in the soil longer than other essential oil products. Rather than “burning down” weeds, manuka oil enters the plant through its roots and interferes with photosynthesis and the creation of chlorophyll, resulting in “bleached” tissues before plant death.
In field studies at Simcoe and Ridgetown, ON, mixtures of manuka oil and other essential oils (cinnamon-clove or citrus) sprayed once reduced weed incidence in corn and tomato plots by 63-97%. An application of manuka oil added to vinegar reduced weeds by 97%. Mixtures were significantly more effective than essential oils applied alone.
While this trial did not find manuka oil had weed control effects when applied prior to weed emergence, other studies have. A previous American study in a lab setting detected bleaching effects of manuka oil on later emerging weeds. These researchers determined that the active ingredient in manuka oil persisted in soil for up to 7 days after application.
Rachel Riddle, who oversaw research in this study conducted at Simcoe, weighs the pros and cons: activity longevity increases weed control effectiveness and decreases the demand for multiple applications, but it may also harm crops planted during that time, if they are sensitive to manuka oil.
Similar to many natural herbicides, manuka oil has non-selective effects on weeds, affecting broadleaf and grass weeds of Canada, including pigweed, crabgrass, cleaver, ryegrass and sterile oat.
For organic wild blueberry and vegetable farmer Wayne Edgar, of North of Nuttby Farm near Tatamagouche, Nova Scotia, grass weed species in particular demand a lot of labour to control sufficiently. Currently, Edgar uses methods such as hand weeding, burning, and organic pH modification used to battle weeds on his 8 acres of wild blueberry. He is working to expand blueberry production area. Weed management will need to improve so he can keep up with an increase in area.
Edgar says he would be interested in such a product as a manuka oil mixture. “I would expect that a thorough scientific review of manuka products would guarantee that there would be no detrimental effects on the ecology and environment of a farm.”
Natural chemical weed controls like essential oils might be less ecologically harmful than synthetic chemicals because microorganisms recognize and quickly degrade them in the environment. While the relatively long activity time of manuka oil makes it effective and attractive as a herbicide, would there be concerns relating to this mode of action? Does manuka oil pose additional environmental risk due to its longer residency in soil? Does its longevity increase its risk of harming non-target organisms? All pesticides must be reviewed by the Pesticide Management Regulatory Agency; however, further review is also needed to be permitted under organic standards.
The organic standards also require farmers to adopt preventative practices (such as using clean seed) and cultural practices (such as crop rotation, planting timing, seeding rate) before adopting physical (cultivation) and then chemical controls.
Weed control effects of manuka oil treatments on pepper plants in 2015 research study in Ontario:
1. Manuka Oil & Vinegar; 2. Only manuka oil; 3. Non-weeded control (Photo by O’Sullivan et al., 2016)
However, progress toward commercialization of manuka oil herbicides has stalled. Riddle sees cost as the major reason for a stall in further Canadian research. “Since manuka oil [is] marketed for pharmaceuticals, the cost of using this in agriculture makes no sense. If you could… significantly reduce the cost, then maybe there would be consideration for organic certification and registration.”
Riddle believes that research into manuka oil should continue if growers show interest in it. “To continue to look at manuka oil, there should be a discussion with companies that produce the product to see if there is interest in making this product available in large amounts for a lower price.”
Due to the enhanced weed control effects observed with mixtures of essential oils, Riddle adds, “Perhaps [the] next research objectives would be to look at products that are currently available and how they might work in combination with each other.”
Research on organic herbicides also leads to other questions. Close attention is being paid to the environmental performance of organic agriculture and the tradeoffs that exist among practices. For example, what are the tradeoffs among mechanical tillage, synthetic mulches, natural mulches, and a herbicide such as manuka oil in terms of efficacy, crop productivity, and cost (e.g. net carbon emissions, and financial)?
If its costs can be reduced sufficiently, would you as a producer be interested in incorporating this product into your organic weed management plan? If you chose to apply manuka oil, how would you integrate it into your current practices so that your weed management plan supports organic ecological principles?
For more information, visit OSC2 Activity C.37 at : www.dal.ca/oacc/oscii
(OSC3) Originally published in Organic Science Canada magazine, Issue #3, Spring 2021
