Hydrangeas, sturdy stalwarts of many a neglected garden, are actually quite specific in their cultural requirements and need some care if they are to give of their best
Providing suitable growing conditions
Hydrangeas are popular garden plants, be they traditional mophead cultivars (hortensias) (right), dainty lacecaps, selections of H. serrata (below) or other species. All thrive in moist, well-drained, fertile soil in partial shade. Hot, dry conditions encourage problems such as powdery mildew and red spider mite, while wet, humid environments promote botrytis on foliage, as well as root and stem rots.
Shallow chalky soils or light, acid sandy soils may cause yellowing leaves. To alleviate this, apply Epsom salts (magnesium sulphate) as a foliar spray at a rate of 200g per 11-litres (7oz per gallon).
Flowering problems
A common problem for gardeners is blue-flowered plants turning pink. The trace element aluminium is responsible for the blue colour and while this
is available to plants in acid soils, it is not in neutral or alkaline areas. Some cultivars will change from pink to blue using a proprietary hydrangea-bluing preparation, but only if you have no chalk or lime in your soil.
If you have slightly alkaline soil, you can boost the pink colour by applying ground limestone or chalk at 75-100g per sq m (2oz per sq yd) during winter. White-flowered cultivars remain white whatever you do.
The main cause of non-flowering, especially in mophead hydrangeas, is pruning too hard. In early spring simply cut off old flowerheads to the first pair of fat buds.
Pests
Hydrangeas are prone to attack by several pests, most commonly red spider mite, capsid bug, aphids and vine weevil. Hydrangea scale and glasshouse red spider mite may also be an occasional problem.
Fiona Dennis