While it makes good sense for your environment -- and pocketbook -- collecting rainwater might also help your garden. The water in many municipalities has unnaturally high amounts of chemicals used to treat the water and keep it safe for you to drink. Your water may also have an especially high or low pH, which could give you trouble growing certain kinds of plants. For example, rhododendrons, azaleas, camellias, and gardenias do best in acidic soil. Watering with high-pH water can raise the soil pH to levels harmful to the shrubs.
It may not seem like much, but you can collect a considerable amount of water with a rain barrel. For every inch of rain on 500 square feet of roof, you collect about 300 gallons of water. What does this mean in reality? The United States Environmental Protection Agency estimates that a rain barrel will save most gardeners about 1,300 gallons of water during the hot summer months.
The simplest type of rain barrel is simply a water-tight container that you place at the bottom of your gutter system. Water from your roof will flow through the gutter and into the rain barrel. Any type of water-tight container will work, but one that has a fine mesh screen over the top will keep mosquitoes from laying eggs in your garden's new water supply.
You can make your own barrel or purchase one. Many manufactured rain barrels come with a spout at the bottom so you can attach a hose for watering. (Note, though: Rain barrels don't usually offer much water pressure so you probably won't be able to attach sprinklers to them.) Manufactured rain barrels may also offer an overflow system so that when they fill up, they direct the excess water away from your home's foundation.