Hydrangea is a genus of roughly 70–100 flowering plants in the family Hydrangeaceae, native to Asia and the Americas with by far the greatest species diversity in eastern Asia — China, Korea, and Japan — and a secondary center in eastern North America, where the type species, Hydrangea arborescens, was first described from Virginia. The genus is the namesake of its family and the type of the tribe Hydrangeae within subfamily Hydrangeoideae, order Cornales. Modern phylogenetic work has expanded Hydrangea in the broad sense to absorb several formerly separate genera, including Broussaisia, Cardiandra, Decumaria, Deinanthe, Dichroa, Pileostegia, Platycrater, and Schizophragma.
Most species are deciduous shrubs roughly 1 to 3 metres tall with opposite, rarely whorled leaves, though the genus also includes small trees and woody lianas that can climb to 30 m using aerial roots. The flowers are borne in crowded cymes — broad, often hemispherical or conical flowerheads — that combine numerous small fertile flowers with much larger, showy sterile flowers whose colorful sepals form the visible "petals" most gardeners recognize. Floral structure varies across the genus, ranging from 4- to 5-merous flowers with minute sepals and short petals to 7- to 10-merous flowers with spreading oblong petals and enlarged sepals that form a sterile ray. The fruit is a capsule that develops from a strongly ribbed hypanthium and opens between persistent styles.
A defining quirk of several species, most famously Hydrangea macrophylla, is that soil pH and aluminum availability alter the pigment chemistry of the sterile florets: strongly acidic soils below pH 5.5 yield blue flowers, alkaline soils above pH 6.5 yield pink, and intermediate pH produces lavenders and purples. This trait, along with the long bloom season from spring through autumn, has made hydrangeas among the most heavily cultivated ornamental shrubs in the world; H. macrophylla alone has more than 600 named cultivars. The bigleaf, panicle, smooth, oakleaf, climbing, and mountain hydrangeas (H. macrophylla, H. paniculata, H. arborescens, H. quercifolia, H. petiolaris, and H. serrata) account for most garden plantings. All parts of the plant contain cyanogenic glycosides and are considered moderately toxic if eaten in quantity.
Etymology
The genus name Hydrangea was coined from the Greek roots hydōr, meaning "water," and angos, meaning "vessel" or "jar," in reference to the small cup- or urn-shaped seed capsules that develop after flowering. The type species, Hydrangea arborescens, was first described from material collected in Virginia.
Distribution
Hydrangea is naturally distributed across temperate and subtropical Asia and the Americas. The center of species diversity is in eastern Asia — particularly China, Korea, and Japan — with a secondary radiation in eastern North America; the family Hydrangeaceae as a whole has a wide distribution across Asia and North America with localized populations in southeastern Europe. NCSU's horticultural treatment summarizes the genus as native to North America, South America, Asia, and Malesia. In the wild, plants typically occupy shady ravines, dense forests, stream banks, and rocky slopes at elevations from roughly 500 to 2,900 metres. Outside the native range, several species are widely planted and locally naturalized; the Swiss flora checklist, for example, records four cultivated Hydrangea species (H. aspera, H. macrophylla, H. petiolaris, and H. quercifolia).
Taxonomy
Hydrangea is the type genus of Hydrangeaceae, a family of nine accepted genera (Carpenteria, Deutzia, Fendlera, Fendlerella, Hydrangea, Jamesia, Kirengeshoma, Philadelphus, and Whipplea) placed in order Cornales within the Asterids clade. Within Hydrangeaceae, Hydrangea anchors the tribe Hydrangeae in subfamily Hydrangeoideae. Recent phylogenetic work has substantially expanded the genus circumscription: Hydrangea s.l. now subsumes the formerly separate genera Broussaisia, Cardiandra, Decumaria, Deinanthe, Dichroa, Pileostegia, Platycrater, and Schizophragma. Species totals reported by different sources accordingly vary — Wikipedia cites 98 currently accepted species, SEINet recognizes about 80.
Ecology
Hydrangea inflorescences are open, crowded cymes containing both fertile and sterile flowers, with the showy sterile florets serving as visual signals to pollinators. Plants flower from spring through autumn depending on species, and the genus is described as attracting pollinators to garden plantings.
Cultivation
Hydrangeas are among the most widely cultivated ornamental shrubs in temperate gardens. They grow well in full sun (six or more hours of direct sunlight) through partial shade (two to six hours), with some species tolerating deeper shade. Soils ranging from clay through loam to sand are accepted provided drainage is reasonable and moisture is consistent — plants resent prolonged root dryness — and the genus tolerates occasionally wet sites. Hardiness across the genus spans USDA zones 3a to 9b depending on species. A signature horticultural trait of several species, especially H. macrophylla, is the soil-pH-driven shift in sepal color: acidic soils below pH 5.5 give blue flowers, alkaline soils above pH 6.5 give pink, and intermediate pH produces purple tones. The six species most widely planted in gardens are H. arborescens, H. macrophylla, H. paniculata, H. petiolaris, H. quercifolia, and H. serrata; H. macrophylla alone accounts for more than 600 named cultivars, most bred to maximize the showy sterile florets. Hydrangeas are used in mixed borders, foundation plantings, hedges, mass plantings, cottage gardens, cutting gardens, and pollinator gardens, and the inflorescences attract pollinators.
Propagation
The genus is straightforward to propagate by several techniques. Seed can be surface-sown in a greenhouse in spring. Vegetative options include half-ripe wood cuttings about 8 cm long taken in summer, mature wood cuttings taken in autumn, and softwood or leaf-bud cuttings. Mound layering — heaping soil over the base of a parent plant to induce rooting from buried stems — is also used and takes roughly twelve months to produce rooted offspring.
Toxicity
All parts of Hydrangea plants contain cyanogenic glycosides and are considered moderately toxic if ingested; NCSU rates H. macrophylla as low-severity, "toxic only if large quantities eaten," with symptoms including nausea, stomach pain, vomiting, and sweating attributed to hydracyanosides in bark, flowers, leaves, and stems.
Cultural uses
Hydrangeas have a long history of cultural and culinary use, particularly in East Asia. The leaves of Hydrangea serrata are processed to make ama-cha, a sweet tea served in Japanese Buddhist ceremonies, and a related preparation, sugukcha, is brewed in Korea. PFAF notes that the leaves have a cucumber-like flavor and are traditionally crushed fresh with miso in Japanese cooking or made into leaf syrups, while sweet sap from some species is used as a beverage. In North America, Hydrangea quercifolia is the state wildflower of Alabama, and Cherokee communities used hydrangea preparations as medicine.
