Hydrangea (/haɪˈdreɪndʒ(i)ə/; 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.
Having been introduced to the Azores, H. macrophylla is now very common, particularly on Faial, which is known as the "blue island" due to the vast number of hydrangeas present on the island.
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.