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. 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.