Hydrangea

Hydrangea is a genus of more than 70 species of flowering plants native to Asia and the Americas. Hydrangea is also used as the common name for the genus; some are also often called hortensia. The genus was first described from Virginia in North America, but by far the greatest species diversity is in eastern Asia, notably China, Korea, and Japan. Most are shrubs tall, but some are small trees, and others lianas reaching up to by climbing up trees. They can be either deciduous or evergreen, though the widely cultivated temperate species are all deciduous.
The flowers of many hydrangeas act as natural pH indicators, producing blue flowers when the soil is acidic and pink ones when the soil is alkaline.

Etymology

Hydrangea is derived from Greek and means 'water vessel', in reference to the shape of its seed capsules. The earlier name, Hortensia, is a Latinised version of the French given name Hortense. Philibert Commerson had attempted to call the flower Lepautia or Peautia in honor of French astronomer and mathematician Nicole-Reine Lepaute, but the common name for the flower instead became Hortensia. This led people to believe that Lepaute's name was Hortense, but the Larousse remarks that this is erroneous, and that the name probably came from hortus, garden.

Life cycle

Hydrangea flowers are produced from early spring to late autumn; they grow in flowerheads most often at the ends of the stems. Typically the flowerheads contain two types of flowers: small non-showy fertile flowers in the center or interior of the flowerhead, and large, sterile showy flowers with large colorful sepals. These showy flowers are often extended in a ring, or to the exterior of the small flowers. Plants in wild populations typically have few to none of the showy flowers, while cultivated hydrangeas have been bred and selected to have more of the larger type flowers.
There are two flower arrangements in hydrangeas with corymb style inflorescences, which includes the commonly grown "bigleaf hydrangea"—Hydrangea macrophylla. Mophead flowers are large round flowerheads resembling pom-poms or, as the name implies, the head of a mop. In contrast, lacecap flowers bear round, flat flowerheads with a center core of subdued, small flowers surrounded by outer rings of larger flowers having showy sepals or tepals. The flowers of some rhododendrons and viburnums can appear, at first glance, similar to those of some hydrangeas.

Colors and soil acidity

Hydrangea flower color can change based on the pH in soil. As the graph depicts, soil with a pH of 5.5 or lower will produce blue flowers, a pH of 6.5 or higher will produce pink hydrangeas, and soil in between 5.5 and 6.5 will have purple hydrangeas. White hydrangeas cannot be color-manipulated by soil pH because they do not produce pigment for color. In other words, while the hue of the inflorescence is variable dependent upon cultural factors, the color saturation is genetically predetermined. In most species, the flowers are white. In some, however,, they can be blue, red, or purple, with color saturation levels ranging from the palest of pinks, lavenders & powder blues, to deep, rich purples, reds, and royal blues. In these species, floral color change occurs due to the availability of aluminium ions, a variable which itself depends upon the soil pH. For H. macrophylla and H. serrata cultivars, the flower color can be determined by the relative acidity of the soil: an acidic soil, will have available aluminium ions and typically produce flowers that are blue to purple, whereas an alkaline soil will tie up aluminium ions and result in pink or red flowers. This is caused by a color change of the flower pigments in the presence of aluminium ions which can be taken up into hyperaccumulating plants.

Species

98 species are accepted.

Hydrangea acuminata
Hydrangea albostellata
Hydrangea alternifolia
Hydrangea × amagiana
Hydrangea amamiohsimensis
Hydrangea ampla
Hydrangea anomala – Himalaya, southwest China
Hydrangea arborescens – eastern North America
Hydrangea arguta
Hydrangea aspera – China, Himalaya
Hydrangea asterolasia
Hydrangea barbara
Hydrangea bifida
Hydrangea breedlovei
Hydrangea bretschneideri – China
Hydrangea caerulea
Hydrangea carroniae
Hydrangea chungii – China
Hydrangea cinerea – eastern United States
Hydrangea coenobialis – China
Hydrangea corylifolia
Hydrangea crassa
Hydrangea daimingshanensis
Hydrangea davidii – China
Hydrangea densifolia
Hydrangea diplostemona
Hydrangea fauriei
Hydrangea febrifuga – central & southern China to Malesia and New Guinea
Hydrangea glaucescens – China, Myanmar and Vietnam
Hydrangea gracilis – China
Hydrangea heteromalla – Himalaya, west and north China
Hydrangea hirsuta
Hydrangea hirta – Japan
Hydrangea hwangii
Hydrangea hydrangeoides – Ulleungdo, Japan, Kurils
Hydrangea hypoglauca – China
Hydrangea integrifolia – China
Hydrangea involucrata – Japan, Taiwan
Hydrangea jelskii – Andes
Hydrangea kawagoeana
Hydrangea kwangsiensis – China
Hydrangea kwangtungensis – China
Hydrangea lalashanensis
Hydrangea lingii – China
Hydrangea linkweiensis – China
Hydrangea liukiuensis
Hydrangea lobbii
Hydrangea longifolia – China
Hydrangea longipes – western China
Hydrangea luteovenosa
Hydrangea macrocarpa – China
Hydrangea macrophylla – southeast Japan, southern China
Hydrangea mangshanensis – China
Hydrangea marunoi
Hydrangea mathewsii
Hydrangea megalocarpa
Hydrangea minamitanii
Hydrangea × mizushimarum
Hydrangea moellendorffii
Hydrangea mollissima
Hydrangea nahaensis
Hydrangea nebulicola
Hydrangea obtusifolia
Hydrangea ofeliae
Hydrangea otontepecensis
Hydrangea paniculata – eastern China, Japan, Korea, Sakhalin
Hydrangea peruviana – Costa Rica and Panama, Andes
Hydrangea petiolaris – Japan, Korea, Sakhalin
Hydrangea pingtungensis
Hydrangea platyarguta
Hydrangea pottingeri – Arunachal Pradesh, Myanmar, southeastern China, and Taiwan
Hydrangea preslii
Hydrangea quercifolia – southeast United States
Hydrangea radiata – southeast United States
Hydrangea robusta – China, Himalaya
Hydrangea sargentiana – western China
Hydrangea scandens – southern Japan south to the Philippines
Hydrangea seemannii - northern Mexico, to Nayarit
Hydrangea serrata – Japan, Korea
Hydrangea serratifolia – Chile, western Argentina
Hydrangea sikokiana
Hydrangea sousae
Hydrangea steyermarkii
Hydrangea strigosa – China
Hydrangea stylosa – China
Hydrangea taiwaniana
Hydrangea tapalapensis
Hydrangea tarapotensis – Andes
Hydrangea tomentella
Hydrangea × versicolor
Hydrangea viburnoides
Hydrangea wallichii
Hydrangea xanthoneura – China
Hydrangea xinfeniae
Hydrangea yaoshanensis
Hydrangea yayeyamensis
Hydrangea × ytiensis
Hydrangea yunnanensis
Hydrangea zhewanensis – China
Fossil record

Hydrangea alaskana is a fossil species recovered from Paleogene strata at Jaw Mountain Alaska. †Hydrangea knowltoni has been described from leaves and flowers recovered from the Miocene Langhian Latah Formation of the inland Pacific Northwest United states. The related Miocene species †Hydrangea bendirei is known to from the Mascall Formation in Oregon, and †Hydrangea reticulata is documented from the Weaverville Formation in California.
Four fossil seeds of †Hydrangea polonica have been extracted from borehole samples of the Middle Miocene fresh water deposits in Nowy Sacz Basin, West Carpathians, Poland.

Cultivation and uses

Hydrangeas are popular ornamental plants, grown for their large flowerheads, with Hydrangea macrophylla being by far the most widely grown. It has over 600 named cultivars, many selected to have only large sterile flowers in the flowerheads. Hydrangea macrophylla, also known as bigleaf hydrangea, can be broken up into two main categories; mophead hydrangea and lacecap hydrangea. Some are best pruned on an annual basis when the new leaf buds begin to appear. If not pruned regularly, the bush will become very "leggy", growing upwards until the weight of the stems is greater than their strength, at which point the stems will sag down to the ground and possibly break. Other species only flower on "old wood". Thus, new wood resulting from pruning will not produce flowers until the following season.
The following cultivars and species have gained the Royal Horticultural Society's Award of Garden Merit under the synonym Schizophragma:

S. hydrangeoides var. concolor 'Moonlight'
S. hydrangeoides var. hydrangeoides 'Roseum'
S. integrifolium

Hydrangea root and rhizome are indicated for the treatment of conditions of the urinary tract in the Physicians' Desk Reference for Herbal Medicine and may have diuretic properties. Hydrangeas are moderately toxic if eaten, with all parts of the plant containing cyanogenic glycosides. Hydrangea paniculata is reportedly sometimes smoked as an intoxicant, despite the danger of illness and/or death due to the cyanide.
The flowers on a hydrangea shrub can change from blue to pink or from pink to blue from one season to the next depending on the acidity level of the soil. Adding organic materials such as coffee grounds and citrus peel will increase acidity and turn hydrangea flowers blue.
A popular pink hydrangea called Vanilla Strawberry has been named "Top Plant" by the American Nursery and Landscape Association.
A hybrid "Runaway Bride Snow White", from Japan, won Plant of the Year at the 2018 RHS Chelsea Flower Show.