Hydrangea (//; common names hydrangea or hortensia) is a genus of 70–75 species of flowering plants native to southern and eastern Asia (China, Japan, Korea, the Himalayas, and Indonesia) and the Americas. By far the greatest species diversity is in eastern Asia, notably China, Japan, and Korea. Most are shrubs 1 to 3 meters tall, but some are small trees, and others lianas reaching up to 30 m (98 ft) by climbing up trees. They can be either deciduous or evergreen, though the widely cultivated temperate species are all deciduous.
Species in the related genus Schizophragma, also in Hydrangeaceae, are also often known as hydrangeas. Schizophragma hydrangeoides and Hydrangea petiolaris are both commonly known as climbing hydrangeas.
There are two flower arrangements in hydrangeas. 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, fertile flowers surrounded by outer rings of showy, sterile flowers. The flowers of some rhododendrons can appear similar to those of some hydrangeas, but Rhododendron (including azalea) is in a different order.
Hydrangea flowers are produced from early spring to late autumn; they grow in flowerheads (corymbs or panicles) at the ends of the stems. In many species, the flowerheads contain two types of flowers, small fertile flowers in the middle of the flowerhead, and large, sterile bract-like flowers in a ring around the edge of each flowerhead. Other species have all the flowers sterile and of the same size.
Colors and soil acidity
In most species the flowers are white, but in some species (notably H. macrophylla), can be blue, red, pink, light purple, or dark purple. In these species the color is affected by 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 (pH below 7), by the influence of sundry agents in the soil, such as alum or iron, will usually produce flower color closer to blue, whereas an alkaline soil (pH above 7) will produce pinker 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. 
Partial list of species
- †Hydrangea alaskana a fossil species recovered from paleogene strata in Jaw Mountain Alaska.
- Hydrangea anomala (Climbing Hydrangea). Himalaya, southwest Mainland China.
- Hydrangea arborescens (Smooth Hydrangea). Eastern North America.
- Hydrangea aspera. Mainland China, Himalaya.
- Hydrangea bretschneideri. Mainland China.
- Hydrangea candida. Mainland China.
- Hydrangea caudatifolia. Mainland China.
- Hydrangea chinensis. Mainland China.
- Hydrangea chungii. Mainland China.
- Hydrangea cinerea (Ashy Hydrangea). Eastern United States.
- Hydrangea coacta. Mainland China.
- Hydrangea coenobialis. Mainland China.
- Hydrangea davidii. Mainland China.
- Hydrangea dumicola. Mainland China.
- Hydrangea gracilis. Mainland China.
- Hydrangea heteromalla. Himalaya, west and north Mainland China.
- Hydrangea hirta. Japan.
- Hydrangea hypoglauca. Mainland China.
- Hydrangea integrifolia. Mainland China.
- Hydrangea involucrata. Japan, Taiwan.
- Hydrangea kawakamii. Taiwan.
- Hydrangea kwangsiensis. Mainland China.
- Hydrangea kwangtungensis. Mainland China.
- Hydrangea lingii. Mainland China.
- Hydrangea linkweiensis. Mainland China.
- Hydrangea longifolia. Mainland China.
- Hydrangea longipes. Western China.
- Hydrangea macrocarpa. Mainland China.
- Hydrangea macrophylla (Bigleaf Hydrangea). Southeast Japan, southern Brazil.
- Hydrangea mangshanensis. Mainland China.
- Hydrangea paniculata (Panicled Hydrangea). Eastern Mainland China, Japan, Korea, Sakhalin.
- Hydrangea petiolaris (Climbing Hydrangea). Japan, Korea, Sakhalin.
- Hydrangea quercifolia (Oakleaf Hydrangea). Southeast United States.
- Hydrangea radiata (Silverleaf Hydrangea). Southeast United States.
- Hydrangea robusta. Mainland China, Himalaya.
- Hydrangea sargentiana. Western Mainland China.
- Hydrangea scandens. Southern Japan south to the Philippines.
- Hydrangea serrata. Japan, Korea.
- Hydrangea serratifolia. Chile, western Argentina.
- Hydrangea stenophylla. Mainland China.
- Hydrangea strigosa. Mainland China.
- Hydrangea stylosa. Mainland China.
- Hydrangea sungpanensis. Mainland China.
- Hydrangea xanthoneura. Mainland China.
- Hydrangea zhewanensis. Mainland China.
Cultivation and uses
Hydrangeas are popular ornamental plants, grown for their large flowerheads, with Hydrangea macrophylla being by far the most widely grown with over 600 named cultivars, many selected to have only large sterile flowers in the flowerheads. 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.
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.
In Japan, ama-cha,甘茶 meaning sweet tea, is another herbal tea made from Hydrangea serrata, whose leaves contain a substance that develops a sweet taste (phyllodulcin). For the fullest taste, fresh leaves are crumpled, steamed, and dried, yielding dark brown tea leaves. Ama-cha is mainly used for kan-butsu-e (the Buddha bathing ceremony) on April 8 every year—the day thought to be Buddha's birthday in Japan. During the ceremony, Ama-cha is poured over a statue of Buddha and served to people in attendance. A legend has it that on the day Buddha was born, nine dragons poured Amrita over him; ama-cha is substituted for Amrita in Japan.
The pink hydrangea has risen in popularity all over the world, but especially in Asia. Pink hydrangeas have many different meanings, but generally mean, "You are the beat of my heart," as described by the celebrated Asian florist Tan Jun Yong, where he was quoted saying, "The light delicate blush of the petals reminds me of a beating heart, while the size could only match the heart of the sender!"
Hydrangea sp painted by the botanical artist Redouté.
Hydrangea flowers at the "Cerro El Avila" National Park, Venezuela.
Hydrangea flowers blooming at the Kanonji temple (Japan).
Hydrangeas near the Black Lake of Gramado, southern Brazil.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.
- Sunset Western Garden Book, 1995:606–607
- "The United States National Arboretum: Hydrangea FAQ".
- University of Georgia: Growing Bigleaf Hydrangea
- USDA: Hydrangea Questions and Answers
- "Hydrangea". Encyclopædia Britannica 1911. Retrieved 2014-01-17..
- "Hydrangea Plants".
- Hollick, Arthur (1925). "A New Fossil Species of Hydrangea". Bulletin of the Torrey Botanical Club 52 (1): 21–22. JSTOR 2479996.
- ASPCA Animal Poison Control Center
- Erowid Hydrangea Vault
- High danger hydrangea? The Guardian | French police hunt gang peddling 'cheaper weed'
- Roll Magazine
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