Vinegar, salt and dish soap are a lethal combination for weeds, but nontoxic for humans and pets. Vinegar and salt are also harmful to plants; plants in the vicinity of weeds being treated may be lost as well. Salt in particular is difficult to manage once it accumulates in soil. It's best to confine the application of this formula to patios and paths.
The formula for this common nontoxic weed killer is one quart of white vinegar, 1/4 pound salt (any kind), and 2 teaspoons of a liquid dish detergent that doesn't contain bleach. No water is included. Mix together and pour into a spray bottle. The solution should be applied topically, and preferably away from desirable plants. Salt especially is detrimental to soil quality and plants; it may reduce water uptake, inhibit seed germination and slow plant growth. Salt accumulation in soil can't be fixed with chemicals, conditioners or fertilizers.
Vinegar is an effective weed killer, proven in research conducted by the U.S. Department of Agriculture. The vinegar was hand-sprayed on the weeds. Common lamb's-quarters, giant foxtail, velvetleaf, smooth pigweed and Canada thistle succumbed to vinegar at 5 and 10 percent acetic acid concentrations, applied in the first two weeks of their appearance. Higher concentrations of vinegar were needed to kill mature weeds; the kill rate was 85 to 100 percent. Perennial weeds were only temporarily deterred; they grew new roots.
Stronger Vinegar Sold As Herbicide
Vinegar is an acid, but doesn't persist in soil, so it's unlikely to accumulate in amounts that affect the pH balance of soil. USDA only tested vinegar on weeds around corn crops, which weren't harmed. Vinegar can burn plant tissue, so care needs to be taken when applying it on weeds near flowers. Household vinegar is usually 5 percent concentration. Stronger vinegars are sold as herbicides; it's illegal to use vinegar to kill weeds unless it's labeled for that purpose.
How the Formula Works
Salt and vinegar are both desiccants; they draw moisture from weeds and other plants. Combining them increases the power. Adding liquid soap detergent increases the absorption of the spray. The soap breaks down the waxy surface of the plant, lowering its defenses against salt and the acetic acid in vinegar. Water sticks to the leaves instead of washing the solution from them. Soap can kill some weeds by itself; oil in soap injures plants.