The Basics of Alternative Herbicides
This is an excerpt from the Ornamental Crops IPM E-Newsletter Summer 2007.
The full issue (with biocontrol photos) and past issues are available online at http://nysipm.cornell.edu/nursery_ghouse/newsletters/default.asp
There are a variety of herbicide products, based on naturally occurring compounds that are of interest as alternatives to synthetic herbicides. These products include corn gluten meal, vinegar, and various plant oils as the active ingredients. There are several soaps used as herbicides, which, while not naturally occurring, are often considered alternatives, as well. Although these herbicides are considered least-toxic alternatives, they require proper handling and can be skin, eye, or lung irritants. As with all herbicides, applicators must follow the application requirements on the label, including use of personal protective equipment.
What are they?: Corn gluten meal is a by-product of the corn milling industry. Acetic acid is the basic ingredient in vinegar. (However, the acetic acid levels in household vinegar are not high enough to use it as an herbicide.) Eugenol is an oil extracted from plants, especially from clove, nutmeg, cinnamon, and bay leaf. Pelargonic acid is found in oil of pelargonium, among other plant sources. Soaps used as herbicides are potassium salts of fatty acids, which, while not naturally occurring, are considered less toxic.
Corn gluten meal contains compounds that inhibit seed germination, so in certain settings it has some potential as a pre-emergence herbicide. However, part of its activity in weed control, particularly for turf, is as a nitrogen source, encouraging rapid grass growth to outcompete weeds. It has no activity on established weeds and will interfere with seed germination of desirable plants as well as weeds, so should be applied after desired plants have true leaves and are 2-3 inches tall.
Acetic acid and eugenol interfere with the action of the cell membrane, allowing cell contents to leak out and causing the death of the cell. Pelargonic acid and soaps remove the waxy cuticle of plant tissue, which allows the cells beneath to dry out and die. In both cases, herbicide application results in a burning of the leaves, which can be seen almost immediately. There is no movement of the active ingredients throughout the plant, so good coverage of the foliage is essential for weed control. Also, mature perennials and other plants that can regenerate from the root system will not be killed because the herbicide is not translocated to the root system. These herbicides are non-selective , so they will damage the foliage of any plant contacted. They are most useful for control of young, actively growing, annual weeds in a situation where desirable plants are either not yet present or can be protected from the herbicide.
There are many different products available with these chemistries and the concentration of the active ingredient can vary dramatically. Users should check both the amount of active ingredient in the concentrate or ready-to-use formulation and the appropriate dilution ratio for the situation, as described on the label. Cost per application can be high for some products, especially if reapplication is necessary for complete control. It should be noted that not all of the products described are labeled for certified organic food crop production. Users should check the OMRI list or other lists of products accepted under the National Organic Production (NOP) regulations. There is no equivalent list of products labeled for organic production of ornamentals.
Betsy Lamb, NYS IPM eml38@cornell.edu
The information in this article is taken from 2 factsheets in the Alternative Pest Management Series, by Leslie Weston and Elizabeth Lamb. The complete fact sheets are available at the Landscape Horticulture Program Work Team website under Resources
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