I've heard both good and bad things about using rain water to water your gardens and flowers. Do you collect rain and use it? My parents did this and I remember the number of dead bugs on the top of the barrel.
We use these for watering all the flower beds too. We don't water the garden with it, but I would if there was enough water. I don't think there is a problem using the water. Our flowers thrive so I'm sure the water isn't hurting them.