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Hydrangea (/ha?'dre?nd?i?/;common labels hydrangea or hortensia) is a genus of 70-75 species of flowering plant life indigenous to southern and eastern Asia (China, Japan, Korea, the Himalayas, and Indonesia) and the Americas. Probably the greatest kinds diversity is within eastern Asia, china notably, Japan, and Korea. Most are shrubs 1 to 3 meters tall, however, many are small trees and shrubs, while others lianas getting up to 30 m (98 feet) by climbing up trees and shrubs. They could be either deciduous or evergreen, although cultivated temperate varieties are deciduous generally.Having been introduced to the Azores, H. macrophylla is currently very common, on Faial particularly, which is known as the "blue island" because of the multitude of hydrangeas present on the island.Life cycleHydrangea plants are produced from planting season to late fall months; they increase in flowerheads (corymbs or panicles) most often at the ends of the stems.

Usually the flowerheads contain two types of flowers: small non-showy blooms in the guts or interior of the flowerhead, and large, showy blossoms with large multi-colored sepals (tepals). These showy blossoms are long in a band often, or to the surface of the tiny flowers. Crops in wild populations have few to none of the showy blossoms typically, while cultivated hydrangeas have been bred and selected to have significantly more of the bigger type flowers.There are two flower arrangements in hydrangeas with Corymb style inflorescens, which include the commonly grown "bigleaf hydrangea"--Hydrangea macrophylla. Mophead blossoms are large round flowerheads resembling pom-poms or, as the name implies, the relative brain of any 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 blossoms of some rhododendrons and viburnums can seem, at first glance, comparable to those of some hydrangeas.Colors and ground acidityIn most species the flowers are white, however in some kinds (notably H. macrophylla), can be blue, red, red, light crimson, or dark crimson. In these species the colour is damaged by the occurrence of metal ions which are available or tied up depending upon the soil pH. For H. macrophylla and H. serrata cultivars, the flower color can be dependant on the relative acidity of the soil: an acidic soil (pH below 7), will have available aluminum ions and produce flowers that are blue to purple typically, whereas an alkaline soil (pH above 7) will tie up aluminum ions and lead to pink or red flowers.

This is the effect of a color change of the rose pigments in the presence of aluminium ions which may be taken up into hyperaccumulating crops.[6] Lowering the pH of potting soils or mixes usually does not change the blossom color to blue, because these soils have no aluminum ions. The ability to blue or green a hydrangea is also affected by the cultivar. Some plants are selected for their ability to be blued, while some are bred and selected to be red, pink or white. The flower color of most other Hydrangea species is not influenced by aluminum and can't be changed or shifted. Hydrangeas also have a nickname called 'Change Rose'.