Hydrangea (pronounced /haɪˈdreɪndʒ(i)ə/, common names Hydrangea and Hortensia) is a genus of about 70-75 species of flowering plants native to southern and eastern Asia (China, Korea, Japan, the Himalayas, and Indonesia) and North and South America. By far the greatest species diversity is in eastern Asia, notably China, Korea, and Japan. Most are shrubs 1-3 m tall, but some are small trees, and others lianas reaching up to 30 m 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 Islands of Portugal, they are now very common there, particularly on Faial Island, which is known as the "blue island" due to the vast number of hydrangeas present on the island, and on Flores 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.