Looking for a no-fuss, least-toxic herbicide? Have you seen advertisements for herbicidal soaps and wondered if they work? As with most products, herbicidal soap is useful but not a silver bullet. It is similar to dish soap except it is highly refined to penetrate the protective coating of plants, causing the plant to dehydrate and die. Herbicidal soap does not contain any additional herbicidal ingredients. It is a least-toxic weed killer that is appropriate for some weeds and ineffective for others.
Herbicidal soaps can very effectively kill plant parts with which they come in contact. However, they will not affect underground portions of the plant. This means they can be quite effective for annual weeds, but they are ineffective for perennial weeds with extensive root systems that reproduce from vegetative buds on the roots, perennials like field bindweed, mouseear chickweed, quackgrass, creeping bellflower, Canada thistle, or tall grasses. It will kill exposed portions of these perennial weeds within hours, but does nothing to affect the extensive root system, and new plants will generate from the roots in a few days. The same is true for perennials such as dandelions; the tops will die, but the crown area of the root system will simply produce another plant. (However, with several treatments, dandelion plants will be unable to generate more regrowth and will die. Herbicidal soap kills only the portions of a plant with which it comes in contact, not the roots.
You may wish to try herbicidal soaps for annual or biennial weeds in flower beds, vegetable gardens, in sidewalk cracks, driveways, pathways, or in mulched areas. Some weeds for which you will find it effective are common chickweed, plantain, crabgrass (which is uncommon in Montana and often confused with the perennial quackgrass), first-year burdock plants (rosettes), and many other annuals and biennials. The soaps are less effective on grasses and older weeds. You may wish to try herbicidal soap, and see which weeds it will work for in your yard. Don't expect much on perennials or on larger, mature plants and grasses, but you will probably be pleasantly surprised at the efficacy on annuals and biennials.
Although herbicidal soap is of very low toxicity, it will kill both weeds and desirable plants, so it must be applied very carefully and allowed to touch only the individual plants you wish to kill. Formulations are also available for use on moss on buildings and in lawns.
References:
Flint, M.L. 1990. Pests of the Garden and Small Farm. University of California.
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997