While collecting rainwater for your garden is a great idea, a surprisingly small number of gardeners use rain barrels. 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.
Source: Better Homes & Gardens