The herbicide Roundup, manufactured by the Scotts Miracle Grow Company, comes in several formulations. Roundup is only effective when it comes in contact with plant leaves and stems; concentrated forms of the herbicide must be diluted with water.

The ratio of Roundup to water depends on the specific Roundup formula you have. Mix 5 tablespoons, which is 2.5 fluid ounces, of Roundup Super Concentrate per gallon of water. Mix 12 tablespoons, which is 6 fluid ounces, of Round Up Concentrate Plus per gallon of water.

You may use half of the specified amount of Roundup per gallon if you are only spraying young weed seedlings, the manufacturer advises. A gallon of prepared Roundup is enough to spray about 300 square feet of weeds.

Spray prepared Roundup on unwanted plants' leaves and you should see them begin to turn yellow after a few hours. It takes up to two weeks for the plants to completely die.