Borax is a chemical compound with the formula Na2B4O7, also known as sodium borate or sodium pyroborate. Among its other uses, borax can act as a herbicide to kill certain undesirable plants in your garden.
Borax contains boron, which in trace quantities is an essential nutrient for plants. When levels in the soil climb too high, however, boron becomes toxic and can kill plants instead. Symptoms include yellow and brown spots near the edge of leaves, wilting stems and plant death.
The amount of borax to apply depends on the area and the kinds of weeds you're trying to control. The University of Minnesota Extension and the Iowa State University Extension both recommend mixing 10 oz. of borax in 4 oz. of water then diluting this solution in 2 1/2 gallons of water. The resulting mixture is enough to treat 1,000 square feet.
It's extremely important not to overapply borax. Not only can the borax kill other desirable plants like turf grass, but the excess boron may accumulate and linger in the soil for a long time, creating a "hot spot" where other plants cannot grow.