A group of popular deciduous shrubs and climbers, with large distinctive flower heads. Most forms are late flowering. Thrives best in coastal and milder areas of the UK.
Botanical Name: Hydrangea
Foliage: Deciduous (except for H. seemanii). Large broadly ovate green leaves.
Flowers: Large showy flower clusters, in shades of white, pink or blue. Flower size and form depends on variety. Flower colour is affected by the soil pH.
Flowering Period: Mid to late summer.
Soil: Moist but well-drained soil (clay, sand or loam). Acid or neutral pH. Soil pH level affects flower colour.
Conditions: Sun or partial shade. Can be grown in a north, south, east or west-facing aspect, in a sheltered or exposed location.
Habit: Bushy or climbing.
Type: Medium sized shrub.
Origin: Asia and the Americas.
Hardiness: Reasonably hardy in most regions of the UK.
Toxicity: Do NOT ingest, may cause mild stomach upset.
They prefer a rich moist soil, in a sunny sheltered position. Plant outside in late autumn or early spring. Container grown plants can be planted out at any reasonable time of year.
Most varieties like acid or neutral soils but there are a few that are suitable for alkaline or chalky soils, such as H. aspera.
Hydrangeas are one of the few flowering shrubs that perform well in shade. Light shade provides the best flowering conditions. Although they can take full sun, this can damage the flowers. Climbers do very well on shady walls.
Ideal for herbaceous and mixed borders, where late flowering forms provide much needed late colour in early autumn. The flower heads can also be left on over-winter to provide architectural structure.
The large flowered forms are excellent as cut flowers and for drying.
Flower colour is influenced by soil conditions. On alkaline soils the blooms tend to be pink. On acid soils they will be mainly blue. On neutral soil the colour will be somewhere in between or a mix of the two. Proprietary "blueing agents" can be added to alkaline soils to achieve blue flowers but these will need to be applied regularly. It should also be noted that white forms often turn reddish in full sun.
Hydrangeas like plenty of moisture, so water thoroughly during dry weather and never let the soil dry out. In dry spells the leaves will start to wilt, indicating it is time to get the hose out. They are also hungry plants that need a rich soil and regularly feeding. Mulch with organic matter in spring and apply a general slow release fertilizer suitable for shrubs.
Hydrangeas are reasonably hardy (down to about -15°C) and are rarely killed outright by hard frosts. Any frost damaged growth should be cut back in late spring.
Pruning depends on variety and flowering season. If not pruned regularly the plants can become drawn and straggly. It is best to leave pruning of late flowering forms until the spring, so that the old growth provides some frost protection. On many varieties it is possible to cut the whole shrub down to ground level, if necessary.
Susceptible to attack by aphids, capsid bug and hydrangea scale.
Generally disease free. Alkaline soils can cause chlorosis (yellowing of the leaves). Large leaf forms are the most susceptible. If this happens, treat by adding chelated iron to the soil and apply a good mulch of organic matter to improve soil acidity.
Softwood cuttings can be taken in summer. For further details see Hydrangea Cuttings.
Take cuttings of climbers in early summer.
Hydrangeas are mainly classified by their flower type. For example, mophead hortensias, have large rounded heads, in shades of pink, red and blue, such as H. macrophylla. Lacecaps have flattish elegance blooms with fewer florets, arranged mainly round the edge of much smaller buds.
Hydrangea arborescens (Tree Hydrangea) has large, rounded flower heads, from summer into autumn. Height to 10ft (3m). There are a wide variety of cultivars available, mainly white and light pink blooms.
Hydrangea arborescens 'Annabelle' has very large flower heads, up to 10in (25cm) across. Height to 8ft (2.5m). Available at Thompson and Morgan.
Hydrangea arborescens 'Grandiflora' is bushy deciduous shrub, with broad ovate leaves, and white flowers in a looser habit. Height to 8ft (2.5m).
Hydrangea aspera has white, pink,
mauve or blue lacecap flowers surrounded by
white ray florets.
Suitable for growing on chalk. Height to 13ft (4m).
There are a good number of cultivars available.
Hydrangea macrophylla is a very large group with two distinct forms: lacecaps with flat heads and hortensias with round or mophead blooms. Average height and spread to 6ft (1.8m). Macrophylla cultivars are some the most popular forms of hydrangea sold in flower shops and garden centres.
Hydrangea paniculata forms a large bush with large pyramid shaped white flowers. The white blooms often fade to pink. Height from 8 to 12ft (2.5-4m), spread 5 to 8ft (1.5-2.5m). There are a good number of cultivars available.
Hydrangea petiolaris (H. anomala subsp. petiolaris) is a popular self climbing form with white lacecap flowers. It can tolerate deep shade and is well suited north-facing walls. It can also be grown horizontally, such as over a low wall or down a bank.
Hydrangea quercifolia (oak-leaved hydrangea) is a spreading form with large oak shaped leaves and white conical flowers. Height to 5ft (1.5m), spread to 8ft (2.5m). Good autumn colour with deep reds and purples.
Hydrangea sargentiana forms a large upright medium-sized deciduous shrub, with peeling bark and large, ovate furry leaves. The large flat head flowers appear in purple or blue, surrounded by white ray florets. Height to 10ft (3m). Good autumn colour.
Hydrangea serrata is a compact
form with large flat, lacecap flowers, in pink, blue
or white. Height to 3ft (1m).