Deciduous shrub. New growth more or less hairless. Leaves broadly ovate, mostly shiny above, to 20 cm long, margins with large teeth. Flowers of wild species in more or less flattened clusters but those of garden variants generally more or less spherical (moptops or hortensias), although garden variants of the flattened cluster type also available (lacecaps); summer. Sepals 4 or 5, small and inconspicuous except in sterile flowers when there may be 3-5, large, showy and petal-like, arranged around the flower or, when all the flowers are sterile, in a ball. Petals 4 or 5, white, blue or pink to purplish, often deepening in colour with age. Ovary inferior, becoming half-inferior in fruit. Fruit capsule woody, with 3 diverging styles.
Japan
[H. hortensis Siebold, H. opuloides (Lam.) Anon.]
The exact origin of the common garden form with a ball of sterile flowers is not known with certainty. It has been suggested that it is a natural variant derived from H. macrophylla var. normalis E.H.Wilson which has a lacecap flower; it has also been suggested that it is the result of horticultural breeding and selection.
Garden cultivars of this species are of 2 kinds: the Mopheads (Hortensias) and Lacecaps. Mopheads have all flowers sterile, forming a more or less spherical cluster. Lacecaps have flowers that form a more or less flat-topped cluster, the large sterile flowers being arranged around the outside and the small fertile flowers clustered in the centre.The distinction between mopheads and lacecaps is not always clear: some cultivars are variable and may intergrade, e.g. 'Libelle'.There are over 500 cultivars worldwide, derived largely from H. macrophylla and H. serrata; the list presented here is a selection of cultivars in Australia and the number would double if it included cultivars in specialist collections.
There is an uncanny similarity between these flower forms and those of some viburnums although in Hydrangea the sepals are free, not fused as in Viburnum, and the leaves of the 2 genera are quite different.
Flower colour is affected primarily by pH (which influences the availability of aluminium, the main determinant of flower colour). In alkaline soils without aluminium the flower colour is pink, in strongly acid soils flowers are blue. The addition of aluminium sulphate can change flower colour from pink to blue.
MOPHEADS (HORTENSIAS) Flowers in this group are mostly large and sterile in a more or less spherical cluster. Mopheads have been raised mostly by Dutch, French, German and Swiss breeders beginning in the 19th century, but many have been introduced in the 20th century. Flower heads variable in size and flower density. Sterile flowers are single or double and the petal-like sepals have margins that are rounded, wavy, or toothed. Shrubs may vary in size according to growing conditions. Flower size, colour and even sepal structure may vary with soil conditions and pruning regimes. Flower colour ranges through white and cream to pinks, crimson, purple, violet and blue. Distinguishing cultivars is often difficult as differences are often small; names have sometimes been guessed at or invented anew. The following list is an attempt to come to terms with the existing names but a complete revision of all the available cultivars is needed.
LACECAPS Flower heads more or less flat, the centre of small fertile flowers surrounded by an outer ring of large, sterile flowers. These are best known under their English names.
A range of new releases introduced from Japan includes the compact bicolors: 'Mariko', blue and white; 'Nobuko', purple and white; 'Taiko', violet blue and white; 'Machiko', pink and white; 'Sumiko', white and pink; and 'Reiko', a pink and white lacecap.
Source: (2002). Hydrangeaceae. In: . Horticultural Flora of South-eastern Australia. Volume 3. Flowering plants. Dicotyledons. Part 2. The identification of garden and cultivated plants. University of New South Wales Press. (as Hydrangea macrophylla subsp. macrophylla)
