Two commonly used organic weed killers are vinegar (active ingredient acetic acid) and corn gluten meal (CGM). Before giving details about using them to kill weeds, I want to advise caution for home gardeners who mistakenly think products labeled “organic” or “natural” are totally harmless to humans. That is not the case. Organic weed killers may trigger a bad reaction in someone with chemical sensitivity. Home gardeners need to be informed and cautious when using every pesticide. Know how each works and what the effects should be. Also evaluate individual products as to cost effectiveness. Organic herbicides are composed of chemicals, as are all things animal, vegetable and mineral in the natural world. Handle all pesticides safely. Take equal care when mixing and/or applying organic and synthetic horticultural remedies. Avoid skin contact. Do not inhale or ingest them.
Certain manufactured herbicides are formulated to be ‘selective’, meaning they are deadly to one plant group but not others. For example, broadleaf herbicides when sprayed on a lawn, cause broadleaf weeds like dandelions to die but turfgrass is not harmed. There are also herbicides formulated to kill grassy weeds in gardens without hurting broadleaf shrubs and herbaceous plants growing there. Many organic/natural weed killers are ‘generalized’ or ‘non-specific’ herbicides like Roundup. They kill almost every type plant. Any weed killer in this category (whether synthetically manufactured or derived from a natural source) may eliminate desirable plants as well as weeds/unwanted plants unless carefully applied.
Both vinegar (active ingredient acetic acid) and corn gluten are generalized organic weed killers. When vinegar is sprayed on plants, the acetic acid in it kills by drawing water out of leaves. Treated leaves wilt, then shrivel and the plant dies. No need to drench the plant or the soil around it. Vinegar works best in warm, sunny conditions, not so well where it is shady or when temperatures are cool. Vinegar from the grocery store contains 5% acetic acid and is not expensive. Specially made horticultural vinegar is more highly concentrated (20% acetic acid or stronger) and more costly. Some users add a splash of dishwashing soap to the vinegar spray. Soap helps acetic acid stick despite foliage that has a slick skin or a hairy surface. Otherwise liquids would bead up and roll off that type of leaf. Roots are usually not affected. Repeat applications may be needed to kill the plant. However, follow-up doses won’t break your budget because vinegar doesn’t cost a lot. Corn gluten meal (CGM) was discovered and patented by an Iowa State University horticulturist as a natural weed and feed product. It is a by-product of making corn starch and corn syrup. It is sold in the form of a yellow corn meal or light brown granules. It contains about 10% nitrogen fertilizer. CGM prevents sprouting seeds from developing normal roots. Treated seedlings die due to lack of moisture when soil is dry. Timing the application when weed seeds are sprouting and the weather is warm and sunny is critical to successful weed control with CGM. It is said to be effective against crabgrass, creeping bentgrass, smart weed, dandelions, purslane, lambs quarter, foxtail, barnyard grass and Bermuda grass. Not all scientists recommend CGM. On the internet, a Washington State University Extension horticulturist points out CGM drawbacks: no effect on existing weeds– just on seedlings, it inhibits survival of all seedlings– those of desirable plants as well as of weeds and the necessary repeat applications make corn gluten meal more expensive than other equally environmentally-friendly weed treatments.
Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region. UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (931-484-6743) has answers for horticulture questions, free publications and details on how to become a Master Gardener. Send email comments or yard & garden inquiries to Master Gardener Rae, email@example.com.