Hydrangea

Hydrangea flowers are produced from early spring to late autumn; they grow in flowerheads (corymbs or panicles) 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 (tepals). 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.

Hydrangea flowers, when cut, dehydrate easily and wilt very quickly due to the large surface area of the petals. A wilted hydrangea may have its hydration restored by first having its stem immersed in boiling water; as the petals of the hydrangea can also absorb water, the petals may then be immersed, in room-temperature water, to restore the flower's hydration.[9]

Hydrangea flower color changes based on the pH in soil. As the graph depicts, soil with a pH of 5.5 or lower will sprout blue hydrangeas, 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 manipulated by soil pH, they will always be white because they do not contain pigment for color.

In most species, the flowers are white, but in others (notably H. macrophylla), they can be blue, red, pink, light purple, or dark purple. In these species, floral color change occurs due to the presence of aluminum ions which are available or tied up depending upon the soil pH.[10][11] 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), will have available aluminum ions and typically produce flowers that are blue to purple,[12] whereas an alkaline soil (pH above 7) will tie up aluminum ions and result in pink or red flowers. This is caused by a color change of the flower pigments in the presence of aluminum ions which can be taken up into hyperaccumulating plants.[13] Lowering the pH of potting soils or mixes usually does not change the flower color to blue, because these soils have no aluminum ions. The ability to blue or pink a hydrangea is also influenced by the cultivar. Some plants are selected for their ability to be blued, while others are bred and selected to be red, pink, or white. The flower color of most other Hydrangea species is not affected by aluminum and cannot be changed or shifted. Hydrangeas are also nicknamed 'Change Rose'.

†Hydrangea alaskana is a fossil species recovered from paleogene strata in Jaw Mountain Alaska.[14]

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.[15]

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.[16] 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:[17]

• S. hydrangeoides var. concolor 'Moonlight'[18]
• S. hydrangeoides var. hydrangeoides 'Roseum'[19]
• S. integrifolium[20]

Hydrangea root and rhizome are indicated for treatment of conditions of the urinary tract in the Physicians' Desk Reference for Herbal Medicine and may have diuretic properties.[21] Hydrangeas are moderately toxic if eaten, with all parts of the plant containing cyanogenic glycosides.[22] Hydrangea paniculata is reportedly sometimes smoked as an intoxicant, despite the danger of illness and/or death due to the cyanide.[23][24]

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.[25] Adding organic materials such as coffee grounds, citrus peel or eggshells will increase acidity and turn hydrangea flowers blue, as described in an article on Gardenista.[26] A popular pink hydrangea called Vanilla Strawberry has been named "Top Plant" by the American Nursery and Landscape Association.

The hybrid "Runaway Bride Snow White", bred by Ushio Sakazaki from Japan, was named Plant of the Year at the 2018 RHS Chelsea Flower Show.[27]

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  Hydrangea macrophylla
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  Hydrangea flowers close up.
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  Flowers
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  Hydrangea macrophylla, flowers.
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  Hydrangea macrophylla, flowers
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  Hydrangea sp painted by the botanical artist Redouté.
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  Hydrangea quercifolia
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  Hydrangea macrophylla, leaves
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  Wild Hydrangea v. Annabelle
  Hydrangea arborescens
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  Wild Hydrangea v. Annabelle
  Hydrangea arborescens
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  8" Annabelle Hydrangea Bloom. Hydrangea arborescens.
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  Hydrangea aspera ssp. sargentiana
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  Hydrangea paniculata
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  Hydrangea flowers from Kerala, India
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  Hydrangeas in front of the Office de Tourisme Building in Chartres, France.
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  Hydrangea flowers at the "Cerro El Avila" National Park, Venezuela.
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  Hydrangea flowers, Srinagar, Kashmir, India.
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  Hydrangea flowers
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  Hydrangea flowers in Petrópolis, Brazil.
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  Hydrangeas near the Black Lake of Gramado, southern Brazil.
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  Purple hydrangea flowers
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  Hydrangea arborescens leaf
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  flower of hydrangea
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  flower of hydrangea

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Hydrangea". Encyclopædia Britannica. Vol. 14 (11th ed.). Cambridge University Press. p. 34.

• erowid.org
• Flora of Nepal: Hydrangea species list
• Hydrangea – selecting shrubs
• Hydrangea species and hybrids
• Propagating Hydrangeas
• Red Listing of Threatened Hydrangeas
• Hydrangea all FAQs