Is it okay to use rain water from the garage roof for my rhododendrons?
Robert Preece, by email
Stefan says: Although rain is a natural source of water, when we collect it for garden uses, it is runoff, usually from roofs, and flows through pipes into a collection vessel. If the water runs from a tiled or slate roof, there will be no problems. If it flows, as it does in your case, over a garage roof of bitumen and limestone chippings, difficulties could arise.
The limestone chippings could well result in some lime content developing in the water, and this could have a detrimental effect on lime-hating rhododendrons and azaleas. Bitumen could present a more general difficulty because when fresh it certainly contains substances that are toxic to plants, and my advice would be not to use water from a new bituminised roof until the bitumen has had a chance to weather – I’d guess about a year would be long enough.