Hydrangeas are perennial shrubs or climbers in the family Hydrangeaceae, native across North America, South America, Asia, and Malesia. The genus name comes from the Greek for water vessel, a reference to the cup-shaped seed capsule, and the connection to moisture is apt: most species perform best with consistent water and repay it with the generous flower clusters that have made them garden staples across temperate climates worldwide. Deciduous in most settings, they carry their flowers in terminal clusters — round, umbrella-shaped, or the flattened lacecap form — in white, pink, blue, or purple depending on species and soil chemistry.

That soil chemistry relationship is one of the genus's most distinctive traits. In acidic soils below 5.5 pH, aluminum becomes available to the plant and flowers shift toward blue; in alkaline soils above 6.5 pH, the same flowers open pink. The middle range produces purple. This is specific to the bigleaf types and does not apply to all species — smooth hydrangea, for instance, flowers cream-white regardless of what the soil reads. Hydrangeas attract pollinators reliably and range in hardiness from zone 3 to zone 9 depending on species. All parts of the plant are considered mildly toxic if ingested.