LAST month, a campaign by the Royal Horticultural Society urged UK gardeners to be more environmentally friendly. One piece of advice was to get a water butt and use rainwater for watering instead of the mains. You might think there is plenty of water to go around in a rainy country like the UK, but it is a waste of energy to give plants tap water, which has been intensively treated so it is fit to drink. Rainwater is perfectly good enough for most plants and may even be better, as tap water contains dissolved magnesium and calcium, which accumulate around the roots, raising the soil's pH and lowering the availability of other nutrients. The only exceptions are tiny seedlings, which benefit from clean tap water as they are vulnerable to infections.
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