Hydrangea

Cultivation notes

Hydrangeas thrive in a moist, but well-drained soil, in a cool, semi-shady part of the garden. Try to avoid exposed east-facing sites, where cold winds may damage young spring growth, and also avoid dry, sunny spots.

Work plenty of organic matter into the soil prior to planting, and periodically over time. This could be well-rotted leafmould, garden compost, composted bark, or farmyard manure.

On lighter, sandier soils, feed in early spring with a flowering shrub fertiliser. This should be enough for the season, but should not be necessary on richer soils. Too much feeding can encourage excessive soft, leafy growth, with plants less likely to develop flower buds and more at risk from frost in colder winters.

Pruning and training

Climbing hydrangeas

Hydrangea anomala subsp. petiolaris, and H. seemanii are pruned after flowering, simply by removing spent flower heads and trimming any wayward shoots back to healthy buds.

Shrubby hydrangeas

See the advice page on pruning shrubby hydrangeas (such as H. macrophylla, H. paniculata and H. quercifolia). Most of these are pruned in early spring.

Flower colour

One of the most interesting quirks of some hydrangeas is the ability of the flowers to change colour. This is not true of all hydrangeas, only the mophead cultivars (H. macrophylla).

This colour change is due to the soil pH which affects aluminium availability. Those with blue or pink flowers tend to be blue in acid soil conditions (high available aluminium levels), mauve in acid to neutral soil conditions, and pink in alkaline conditions. To get the best flower colour, choose cultivars that give the best colours for the pH in question.

White flowers, and also green-flowered cultivars, remain white or green regardless of soil pH.

Try to use rainwater to water hydrangeas, since mains hard water can affect the flower colour, turning blue flowers mauve or pink.

Cultivars with blue flowers can be kept blue by growing the plants in acidic soil (pH 4.5-5), or by using hydrangea blueing compounds according to the manufacturer’s instructions. These compounds contain aluminium sulphate and are available from most garden centres. If the soil is very alkaline, or if there is any obvious chalk in the soil, this treatment will not work, but can be very effective for container-grown plants.

If you wish to enhance red or pink flowers, apply a dressing of ground limestone or chalk at a rate of 75-100g per sq m (2-3oz sq yd) in winter.

Propagation

Hydrangeas are easy to grow from softwood, semi-ripe or hardwood cuttings, but the climbing types are best layered.

Species hydrangeas can also be propagated from seed in spring.

Cultivar selection

A popular shrub, hydrangeas come in a number of habits and colours, from mophead, lacecap and paniculata types, to oak-leaved and climbing forms. A few are mentioned below but for a fuller list, see the RHS Plant Selector.

• Hydrangea macrophylla – cultivars of these mophead types are popular for mass planting or informal hedges, especially in milder coastal regions, as they have good tolerance of salt-laden sea winds. H. ‘Europa’ AGM to 2.5m (8ft) has pink or pale blue flowers, depending on soil conditions
• H. quercifolia ‘Little Honey’ – one of the oak-leafed hydrangeas with golden leaves, and white flowers, followed by red autumn leaf tints. It is a compact, mound forming variety reaching a height of 1-1.5m (39in–5ft)
• H. paniculata Pink Diamond 'Interhydia’ AGM – white flowers up to 30cm (1ft) long that fade to deep pink. It can be kept to a height of 1.5m (5ft) by regular pruning

Links

AGM Plants
RHS Plant Finder

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Hydrangea arborescens 'Annabelle'

Hydrangea paniculata 'Floribunda'

Hydrangea serrata 'Graciosa'

Hydrangea serrata 'Miyamayae-murasaki'

Problems

Hydrangeas are usually robust shrubs. Non-flowering can be caused by frost damage to flowering wood, over-zealous pruning, pruning at the wrong time, or over-feeding with fertiliser too high in nitrogen.

Hydrangeas may occasionally be affected by pests such as scale insects, hydrangea scale, capsid bugs, aphids and vine weevil.