Hydrangea macrophylla

''Hydrangea macrophylla'' is a species of flowering plant in the family Hydrangeaceae, native and endemic to Japan. It is a deciduous shrub growing to tall by or more broad with large heads of pink or blue flowers in summer and autumn. It is currently treated as monotypic, with no subspecies or varieties. Common names include bigleaf hydrangea, and, for particular cultivar groups, the names lacecap hydrangea, mophead hydrangea, and hortensia. It is widely cultivated in many parts of the world in many climates. It is not to be confused with '' H. aspera'' 'Macrophylla'.

Distribution and habitat

''Hydrangea macrophylla'' is endemic to Japan, where it occurs in coastal habitats from Honshu southwards. Natural wild plants, formerly sometimes distinguished as ''H. macrophylla'' var. ''normalis'' E.H.Wilson, have "lacecap" structure flowerheads, with a few large sterile showy bract-like flowers that act as targets for pollinators, surrounding a central cluster of small fertile flowers. A closely related hydrangea from the mountains of the interior of Japan, '' Hydrangea serrata'', was treated as a variety of ''H. macrophylla'' by some authors in the past, but is currently treated as a separate species.

The species is naturalised in China, Korea, Siberia, New Zealand and the Americas, and has become an invasive species in the Azores and Madeira archipelagos.

Description

The term ''macrophylla'' means 'large- or long-leaved'. The opposite leaves can grow to in length. They are simple, membranous, orbicular to elliptic and acuminate. They are generally serrated.

The natural inflorescence of wild ''Hydrangea macrophylla'' is a corymb, with all flowers placed in a plane. Two distinct types of flowers are found; numerous central, small, fertile pentamerous ones, and a few peripheral, large, tetramerous ones; the latter are usually sterile, and whitish to pale blue or pinkish. The small flowers have five small greenish sepals and five small petals. Flowering begins in early summer and lasts until early winter. The fruit is a subglobose capsule.

In cultivation as an ornamental plant, numerous variants have been developed as cultivars; in most of these (over 500 cultivars), the small central flowers replaced by large, sterile or mostly sterile tetramerous flowers, the inflorescence forming a hemisphere or a whole sphere rather than in a flat plane. These cultivar groups are known as "mophead" or "hortensia" hydrangeas. These large flowers have colours ranging from pale pink to red, fuchsia, purple, to blue. A much smaller cultivar group (over 20 cultivars), known as "lacecap" hydrangeas, retain the natural form of flat flowerheads with small flowers surrounded by a halo of large sterile flowers, but varying from the wild plants in more intense colours. Some cultivars (possibly many, and particularly those selected for greater cold tolerance) derive from hybrids between ''Hydrangea macrophylla'' and '' Hydrangea serrata'', the hill hydrangea of the mountains of Japan.

Colour and soil acidity

The flowers of ''Hydrangea macrophylla'' cultivars can be blue, red, pink, light purple, or dark purple. The colour is affected by soil pH. An acidic soil (pH below 7) will usually produce a flower colour closer to blue, whereas an alkaline soil (pH above 7) will produce flowers of a pink, or even a red colour. This is caused by a colour change of the flower pigments in the presence of aluminium ions which can be taken up into hyperaccumulating plants. Scientists do not understand why this happens, whether it is due to predation or to attract pollinators.

Chemistry

Phyllodulcin, hydrangenol, and their 8-O-glucosides, and thunberginols A and F can be found in ''H. macrophylla''. Thunberginol B, the dihydroisocoumarins thunberginol C, D and E, the dihydroisocoumarin glycosides thunberginol G 3'-O-glucoside and (−)- hydrangenol 4'-O-glucoside and four kaempferol and quercetin oligoglycosides can be found in ''Hydrangeae Dulcis Folium'', the processed leaves of ''H. macrophylla'' var. ''thunbergii''. The leaves also contain the stilbenoid hydrangeic acid.

The various colours, such as red, mauve, purple, violet and blue, in ''H. macrophylla'' are developed from one simple anthocyanin, delphinidin 3-glucoside ( myrtillin), which forms complexes with metal ions called metalloanthocyanins.

Lunularic acid, lunularin, 3,4′-dihydroxystilbene and a glycoside of lunularic acid have been found in the roots of ''H. macrophylla''.

Hydrangine is another name for the coumarin umbelliferone, and may be responsible for the possible toxicity of the plant.

Uses

Amacha is a Japanese beverage made from fermented leaves of ''Hydrangea macrophylla'' var. ''thunbergii''.

''Hydrangeae Dulcis Folium'' is a drug made from the fermented and dried leaves of ''H. macrophylla var. thunbergii'' with possible antiallergic and antimicrobial properties. It also has a hepatoprotective activity by suppression of D-galactosamine-induced liver injury in vitro and in vivo.

''Hydrangea macrophylla'' is included in the Tasmanian Fire Service's list of low flammability plants, indicating that it is suitable for growing within a building protection zone.

Leaf extracts of ''Hydrangea macrophylla'' are being investigated as a possible source of new chemical compounds with antimalarial activity. Hydrangeic acid from the leaves is being investigated as a possible anti-diabetic drug as it significantly lowered blood glucose, triglyceride, and free fatty acid levels in laboratory animals.

Cultivars

The two main groups of ''H. macrophylla'' cultivars are called "lacecap" and "mophead".

Some popular hydrangea cultivars (those marked have gained the Royal Horticultural Society's Award of Garden Merit) include:

*'All Summer Beauty'; cold-hardy, floriferous