Crab grass sprouts in patches in late summer and grows until the first frost of fall. The grass takes its name from the flat, dense mat of grass and the three-pronged seed heads that resemble a crab with raised claws. Crabgrass will not sprout in dense grass, due to a lack of sunlight. Once crabgrass sprouts, you can control it using a homemade weed killer.
Boiling water will literally cook weeds to the ground when you pour it onto them. Simply pour 3 gallons of water into a stock pot and bring it to a rolling boil. Then carry the water out to the mat of crab grass and pour it onto the grass. This method works best to eliminate grass growing in sidewalk cracks, since it will also boil desirable plants that grow near the crab grass if you accidentally pour the water onto them.
The active ingredient in vinegar that kills crab grass is acetic acid. Some organic weed killers are manufactured with high concentrations of acetic acid. There is some debate over how effective vinegar is, since ordinary kitchen vinegar has a lower strength of acetic acid. But mixing vinegar with other household products, such as salt, may increase its effectiveness. In small amounts, salt will not harm the soil. However, salt should be used very carefully in weed killers because over time it can build up until nothing will grow in the soil.
Soap is frequently applied to crabgrass because it improves the plant's absorption of other weed killers. Only a small amount of soap is needed--typically less than a tablespoon. Soap breaks down the protective coating on crabgrass, making it more susceptible to the active ingredient of the other weed killers. Soap is also made from oil, which can help to kill crabgrass if applied in a high enough concentration.
The active ingredient in rubbing alcohol is isopropyl alcohol, a drying agent. When applied to a weed's surface it pulls out their water, leaving them dry and brown.