There are several different recipes for homemade organic weed killer. Most work by either temporarily raising or lowering the Ph of the weeds, sending the weeds into a tailspin. The nice thing about the homemade organic weed killer recommended here is that within just a few minutes the weed killer will combine with the oxygen in the atmosphere and become absolutely harmless, leaving behind no residue that could harm humans or pets. Keep in mind that the organic weed killer described in this article does not kill the roots of weeds. This means that, while this weed killer may very well kill a young weed with a weak root system, more than one application will be necessary to kill older, well-established weeds.
Mix 1 quart white vinegar with 4 ounces lemon juice and 2 ounces dish soap. Pour the mixture into a spray bottle. The dish soap will act as a surfactant, allowing the vinegar and the lemon juice to penetrate the leaves of the weeds more easily.
Carefully spray the vinegar mixture onto the leaves of the weeds you wish to kill. Spray on a warm day that is not windy. Use a small sheet of cardboard if necessary to protect other plants while you are spraying the weeds--any plant that the spray touches will die.
Wait 24 hours. Inspect the sprayed weeds. The leaves of the sprayed weeds should be brown or turning brown. If the weeds' leaves are not already turning brown, carefully spray again, making certain to protect any plants that you do not wish to kill.
Inspect the sprayed weeds in 14 days. Respray any that have regrown, being careful not to get the spray on plants you wish to keep. Inspect again in another 14 days, and spray a third time if weeds reappear. Except for crabgrass and certain other weeds with very deep rhizomes (roots that grow new plants), all weeds should be dead after a third spraying.
Spread salt (table salt or rock salt) around the base of crabgrass and other hardy weeds, and spray once more with the vinegar/lemon/dish-soap mixture. Keep the salt confined to the roots of the weeds, as the salt can kill other plants.
Once all weeds are dead, water the area thoroughly to dissipate the salt.