One of the best homemade formulas for killing weeds, according to the Garden Counselor Lawn Care website, involves vinegar. White vinegar sprayed directly onto the plant (and into its roots) should kill a weed in a few days. Otherwise, mix 1 gallon of vinegar with 1 oz. of dish soap. The soap helps the vinegar stick to the plant, and the acetic acid in the vinegar kills the plant. Combine the soap and vinegar in a spray bottle, shake to mix and spray. Garden Counselor recommends using this formula on a sunny day to help the vinegar's acetic acid draw more moisture out of the weeds' leaves and roots. The only downside to a vinegar-based homemade formula for killing weeds is that it may increase the acidity of your soil, and it may not work on plants with waxy leaves. The formula also may not kill the roots immediately, so it may take a few applications before the weed dies. Another vinegar-based formula involves 1 qt. of vinegar and 4 oz. of lemon juice concentrate. By drenching the weed in this formula, you can kill the root, but again, this is an acidic formula and may damage the root systems of the plants nearby.
Rock salt is another great way to kill weeds. Sprinkling salt directly over the plant you want to kill can be very effective. Boil 4 qt. of vinegar, and adding 1 cup of salt and 1 tbsp. of dish detergent. After this formula cools, pour the mixture into a spray bottle and spray it directly on the weed. This homemade formula for killing weeds will not damage your soil as much as a vinegar-dishsoap formula, and it may help kill the weed faster.
A completely organic and efficient way to kill a weed is to pour boiling water directly onto the plant. According to the Vegetable Gardening Gnomes website, this homemade formula for killing weeds involves no preparation except boiling water. The heat from the water boils the weed to death, and has no adverse affect on the soil (unless an earthworm is nearby).
The Small Farm Permaculture and Sustainable Living website recommends a bleach solution of 50-percent bleach and water to kill weeds. This homemade formula for killing weeds can be dangerous if it gets on the skin or in the eyes, so use proper precautions. Just like a vinegar-based formula can make soil more acidic, a bleach formula can make soil more alkaline. Depending on the type of soil you have, consider using a homemade formula that will not damage your soil's pH level.