[Cynthia, Prescott, AZ]

Rain water is great for most watering, but don't mix fertilizer in it. Use tap water when you fertilize, unless you buy special fertilizer for pure water. Most fertilizer manufactures expect some hardness to water and make the fertilizer acidic to compensate. In pure water, this fertilizer could go very acidic, so I use at least some portion of tap water when using anything other that MSU (Michigan State University) fertilizer. Since you are in the UK, you may have very much better tap water than us in the US, so the problems with fertilizer may be a little different there, but I will bet that your tap water is very good for the plants. You might want to check with your city for a water quality report to find out what the hardness is.