Little research has been done on this topic. There are many factors that affect the quality of collected water – roof composition, animals on the roof, water storage conditions, etc. It becomes a personal choice based on your level of comfort. Certainly, if you do decide to use it, we'd recommend you water at the base of the plant, right into the soil, as opposed to the leaves. Actually this advice also helps to prevent plant diseases too. If you are not comfortable using it on your edibles, feel free to water your other flowers and landscape plants with it.