Providing there has been enough rainfall to sufficiently water plants, then rainwater is enough to water plants. Rainwater has benefits for plants, but its presence will depend on location, climate, time of year, and whether you have been collecting it. During the summer months in many US hardiness zones, there may not be enough rain water to irrigate plants. This is where it will need to be supplemented with other water sources. This can come from mains water, while you can also water plants with bath water or water plants with distilled water.