Hydrangea Problems: UK Diagnosis Guide

Hydrangeas have large leaves that lose water rapidly in warm, windy or sunny conditions. The name itself means 'water vessel'. Drought wilt is the most common reason a UK hydrangea suddenly droops — it is particularly rapid in pots, grow bags, dry shade under trees, and against warm walls. Pyracantha.co.uk and Horticulture Magazine both note that wilting followed by leaf browning is most often a simple water problem, and that resolving it promptly usually prevents long-term damage.

water automatically without checking the soil first — in wet soil, wilting has a different cause and more water makes it worse.

The most common reason a UK hydrangea produces lush leaves but no flowers is pruning at the wrong time. Mopheads (Hydrangea macrophylla) and lacecaps set their flower buds on the old wood they grew the previous summer — those buds sit on the stems through winter and open the following year. Pruning in autumn or spring removes them. The RHS pruning guide and Garden Ninja both explain that mopheads and lacecaps should be tidied in spring only by removing spent flowerheads and any dead or crossing wood, not cut back hard. H. paniculata and H. arborescens flower on this year's new growth and can be pruned in spring without losing flowers.

Stop it coming back:A mophead or lacecap that was pruned at the wrong time will not flower that summer. It should flower the following year if left unpruned. Late frost and very exposed sites can also nip flower buds even on correctly pruned plants.

hard prune all hydrangeas in autumn — this removes next season's flower buds on old-wood varieties and may be unnecessary on most types.

Powdery mildew on hydrangeas appears as a white or grey powdery coating on the upper surface of leaves and sometimes on shoots. Unlike most fungal diseases, it thrives in dry conditions with warm days and cool nights — according to Washington State University Extension, it is most prevalent when conditions are dry rather than wet. Drought-stressed hydrangeas are significantly more susceptible. The coating rubs off with a finger and can cause leaves to curl, brown and drop in severe cases.

assume mildew means the plant has been overwatered — powdery mildew thrives in dry conditions, so the fix is better watering, not less.

Botrytis cinerea (grey mould) causes brown pockmarks and patches on hydrangea flowers, buds and leaves, and in humid conditions produces a characteristic grey fluffy mould on the affected tissue. It thrives in cool, damp, still conditions — a particular risk for plants in sheltered spots, against walls, or in a crowded bed where air does not circulate. The University of Connecticut Extension notes that botrytis is encouraged by wounding, dead tissue, and poor airflow, and that infected flowers and leaves should be removed promptly.

compost botrytis-affected material — the fungus can persist in home compost and reinfect plants.

Hydrangea scale (Pulvinaria hydrangeae) is a specific scale insect noted by the RHS as producing white, waxy, flat egg masses on stems and the undersides of leaves, most visible in early summer. The insects are sap-suckers and can cause loss of vigour, leaf drop, and — through honeydew production — encourage black sooty mould. The RHS describes the characteristic flat, white waxy blobs as sometimes being confused with disease symptoms. Plants in sheltered, south-facing positions with poor airflow are most at risk.

spray systemic insecticides when flowers are open — this harms pollinators. Treat in late spring before flowering or after flowering has finished.

Chlorosis — yellowing between the leaf veins with the veins themselves staying green — in hydrangeas is most often caused by iron or manganese deficiency linked to alkaline soil conditions or the build-up of calcium from hard tap water. Hydrangeas, especially Hydrangea macrophylla, prefer slightly acidic conditions. In alkaline conditions, even if iron and manganese are present in the soil, the plant cannot absorb them. Chlorosis is particularly common in the south-east, East Anglia and the East Midlands where hard water is prevalent.

add ordinary garden compost containing builders' rubble or lime-rich materials near a hydrangea — this can further raise the pH.

In the UK, a late frost in April or May can blacken the soft new growth of hydrangeas — including the flower buds that have swollen and begun to develop on old-wood varieties like mopheads and lacecaps. The RHS notes that while most hydrangeas are hardy across much of the UK, the flower buds of mopheads are particularly vulnerable once they begin to swell. In exposed gardens and frost pockets, this can be the primary reason for poor or absent flowering in some years.

prune frost-damaged shoots while cold weather continues — wait until the risk has passed to avoid stimulating vulnerable new growth.