Hydrangea,[1] commonly named the hortensia, is a genus of more than 70 species of flowering plants native to Asia and the Americas. 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.[2]
The flowers of many hydrangea act as natural pH indicators, sporting blue flowers when the soil is acidic and pink ones when the soil is alkaline.[3]
Hydrangea is derived from Greek and means ‘water vessel’ (from Greek, Modern (1453-);: ὕδωρ húdōr "water" + Greek, Modern (1453-);: ἄγγος ángos or Greek, Modern (1453-);: ἀγγεῖον angeîon "vessel"),[4] [5] [6] in reference to the shape of its seed capsules.[7] The earlier name, Hortensia, is a Latinised version of the French given name Hortense, honoring French astronomer and mathematician Nicole-Reine Hortense Lepaute.[8] Philibert Commerson attempted to name the flower Lepautia or Peautia after Lepaute. However, the flower's accepted name later became Hortensia. This led to people believing Lepaute's name was Hortense, but the Larousse remarks that this is erroneous, and that the name probably came from hortus, garden.[9]
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.[10]
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, (notably H. macrophylla), 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.[11] [12] 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,[13] whereas an alkaline soil (pH above 7) 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.[14]
Hydrangea alaskana is a fossil species recovered from Paleogene strata at Jaw Mountain Alaska.[15] †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.[16] [17]
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.[18]
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.[19] 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:[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.[24] Hydrangeas are moderately toxic if eaten, with all parts of the plant containing cyanogenic glycosides.[25] Hydrangea paniculata is reportedly sometimes smoked as an intoxicant, despite the danger of illness and/or death due to the cyanide.[26] [27]
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.[28] Adding organic materials such as coffee grounds and citrus peel will increase acidity and turn hydrangea flowers blue, as described in an article on Gardenista.[29] 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.[30]
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.
In Korean tea, Hydrangea serrata (hangul:산수국 hanja:) is used for an herbal tea called sugukcha (수국차) or isulcha (이슬차).
The pink hydrangea has risen in popularity all over the world, especially in Asia. The given meaning of pink hydrangeas is popularly tied to the phrase, "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!"[31]
Hydrangea quercifolia was declared the official state wildflower of Alabama in 1999.[32]
Hydrangeas were used by the Cherokee people. A mild diuretic and cathartic, it was considered a valuable remedy for removal of stone and gravel in the bladder.[33]
See main article: article and List of hydrangea diseases.