The oxalic acid in rhubarb leaves is a safe control agent for aphids, particularly those on roses. Cut 450g (1lb) rhubarb leaves, place in an old saucepan with 1.1 litres (2pt) water and boil for half an hour, topping up as necessary. When cool, add ltsp soap flakes dissolved in 275ml (l/2pt) warm water. This acts as the wetting agent when added to the strained rhubarb liquid. Stir the mixture thoroughly and use undiluted as a spray.