Sickly looking rhododendrons

Snapping away dead flower heads of rhododendrons is not strictly necessary, but if you have a sickly plant, relieving it from the burden of making seeds is a sensible plan

- ANNE SWITHINBANK

Q A few years ago, I planted several rhododendrons in a neutral soil, but they are beginning to look sickly and are not flowering well. What can I do? Julie Everett, Padstow, Cornwall

A In the wild, rhododendrons grow on acidic soils in wooded areas of Himalayan regions, China and Japan. Soil pH is a number describing the acidity or alkalinity of the soil, with pH7 being neutral. Numbers below this indicate acidic soil, while above are soils described as alkaline or limy.

Rhododendrons enjoy a soil pH of 4.5-6 and just one rise in the pH number represents a tenfold increase in alkalinity. The calcium content of alkaline soils means iron becomes less available to plants and this hampers the ability of rhododendrons to use sunlight for photosynthesis, with leaves turning yellow as a result. Before investing in plants, test soil using a soil pH kit in several parts of the garden.