Family: Hydrangeaceae

There are around 20 species in this genus of lush, showy, evergreen or deciduous shrubs from temperate Asia and North America. They are grown for their spectacular, domed or flat flower clusters which consist of minute flowers surrounded by coloured bracts which in the orig­inal species were red, pink, white or blue. They mostly flower in midsummer. Many hybrids are now available and these come in rich reds, soft and deep pinks, and purplish blues. They are generally less vigorous than the original species. Hydrangeas range in height from shrubs of 1 m (3 ft) tall to climbers which reach 18 m (60 ft). Most have large, oval leaves with serrated edges. Some are prolific bloomers. Both sterile and fertile florets are sometimes found on the one flower head, while flower heads of many of the cultivated forms are made up of mostly sterile florets. Hydrangeas come with double, semi-double, or serrated bracts. They can be grown indoors or outdoors.

Species

H. anomala subsp. petiolaris is the climbing hydrangea, zone 5, a deciduous plant reaching at least 15 m (50 ft) and clinging to flat surfaces by means of aerial roots. Its flat heads of white flowers appear in summer.

H. arborescens, zone 3, a rounded shrub, to 2.5 m (8 ft), has domed white flowers. Cultivar 'Grandiflora' has clustered, sterile flowers.

H. aspera, zone 7, from southern and eastern Asia, is a much-branched, deciduous shrub, to 2 m (6 ft), with white, sterile flowers. Subspecies sargentiana is a beautiful, upright, deciduous shrub which grows to a height and spread of 3 m (10 ft). It has velvety, oblong, tapered leaves and late-flowering, blue to purple, usually fer­tile flowers.

H. heteromalla, zone 6, a branch­ing shrub, to 2 m (6 ft) or more, has both fertile and sterile white flowers.

H. macrophylla, common hydrangea, from Japan, zone 5, is a deciduous shrub of neat, rounded habit, up to 2 m (6 ft) in height. It has flat heads of blue or pink flowers in mid to late summer. However, it is the many cultivars that are grown by garden­ers. There are two groups of cultivars: the Hortensias or mophead hydrangeas, with large ball-shaped flower heads, consisting of sterile flowers, and the Lacecaps, with flat heads of fertile flowers, surrounded by large sterile flow­ers. There are many to choose from in each group. Flower colours may be blue, pink, red or white, or combinations.

H. paniculata, zone 3, from China and japan, is a deciduous shrub or small tree, to 7 m (20 ft), with yellowish white, conical flowers. Cultivar 'Grandiflora', with large, showy, white flowers, is most commonly grown.

H. quercifolia, zone 5, from North America, is a bushy, deciduous shrub, to 2 m (6 ft), and has white flower cones, to 25 cm (10 in) long, and lobed leaves colouring attrac­tively in autumn.

Cultivation

Hydrangeas need a rich, well-drained, but moisture-retentive soil, with added leaf mould, cow manure, peat moss or com­post. They can be grown in full sun or partial shade but need protection from cold winds. They can grow for many years, provided they are pruned regularly and generally cared for. To
produce blue or purple bracts from pinkish vari­eties, aluminium sulphate or other commercial preparations may be added monthly to acidify the soil. An annual addition of lime to the soil will tend to favor the production of pink or red tones. Weather conditions may affect flower colour. Most newer hybrids are more colour stable than the older types. Propagate from hardwood cuttings, 25 cm (10 in) long, planted singly in sandy soil in a sheltered position, in late autumn. They will be well rooted by the autumn of the fol­lowing year. Some species can also be propa­gated by layering or by separating suckers. They can also be struck from tip cuttings during the growing season.

Climate

There are species suited to various cli­matic zones.