Collecting, storing and using rainwater is a great way to maintain beds and landscape during those times when Mother Nature is not giving us enough water. A rainfall of one inch per hour on a 1,000 square foot surface will yield 10 gallons of water per minute, so it is possible to fill that barrel with the first rain event of the season!
Gardeners who are really into collecting rainwater will have several barrels. Some connected in series as well as barrels collecting water from the garage roof too. We have certainly progressed from using steel 55-gallon drums and makeshift collection hardware to more easily handled plastic barrels with nice collection lids with screen mesh and spigots to attach the garden hose. There are plans available to build your own rain barrels or organizations that sell them ready to go.
Like many other areas of our life, using rainwater used to be a simple matter. There are now some health concerns about using the collected rainwater, especially with garden vegetables. The concern revolves around the potential of spreading diseases carried by wildlife that have contaminated the water collected off our roofs. Birds, squirrels, and raccoons will frequent home rooftops. Bacteria and disease-causing organisms from animal waste are why there are "red flags."
So, how you are going to use the rainwater becomes important. Children (and adults for that matter) should not ever drink the rainwater, and use potable water from the house spigot to rinse off the dirt from vegetables before you take them indoors. However, you do have the all clear to use rainwater without any worry on any non-edible plants in the gardens, raised beds, other containers on the patio and the lawn.
If you are going to use the rain barrel water in the vegetable garden, please use it as you would any other source of "grey" water by watering the soil and NEVER water the foliage. The vegetable crops you grow should influence the choice. Leafy green crops are much more likely to become contaminated, and since we consume the entire plant, the risk is higher. Other crops where the fruits are well above ground are less risky. Staked or caged tomatoes (versus growing without support) and okra, which sets fruit high on the plant, are both safer than vine crops, for example. Rain barrel water should not be used close to harvest either. Let common sense guide you in your decision. Since potential contamination can occur even if you are not using rainwater, it is best to rinse any vegetable or food grown with drinkable water before you eat them.