Why let rainwater flow off your roof and into a storm drain? You can easily collect it using a rain barrel. And it’s certainly easy enough to do: just install a rain barrel or reservoir (use a plastic garbage can, a recycled barrel, etc.) on the downspout of your home’s gutter. Make sure it’s raised above the ground at least a foot (30 cm) so water will easily flow out by the force of gravity. Next, add a spigot (available in any hardware store) near the base and you’ll have free rainwater the next time you need to water your plants. You can use it to fill your watering can or attach it to a soaker hose for even easier watering. And plants prefer rainwater, which is naturally soft, to the hard water that comes from most municipal systems and wells.
If you feel unable to cobble together your own rain barrel system, commercial models are widely available. All the parts are included: you only have to install the unit according to the instructions! But check with your municipality before buying: more and more of them are now subsidizing the purchase of rain barrels.