thinner leaves than evergreen rhododendrons. They are generally medium-sized shrubs with smaller funnel-shaped flowers that usually have 5 stamens: Deciduous hybrid azaleas: Exbury hybrids – derived from the Knap Hill hybrids, developed by Lionel de Rothschild at the Exbury Estate in England. Ghent (Gandavense) hybrids – Belgian raised Knap Hill hybrids – developed by Anthony Waterer at the Knap Hill Nursery in England. Mollis hybrids – Dutch and Belgian raised New Zealand Ilam hybrids – derived from Knap Hill/Exbury hybrids Occidentale hybrids – English raised Rustica Flore Pleno hybrids – sweet-scented, double-flowered Evergreen hybrid azaleas: Gable hybrids – raised by Joseph B. Gable in Pennsylvania. Glenn Dale hybrids – US raised complex hybrids Indian (Indica) hybrids – mostly of Belgian origin Kaempferi hybrids – Dutch raised Kurume hybrids – Japanese raised Kyushu hybrids – very hardy Japanese azaleas (to −30 °C) Oldhamii hybrids – dwarf hybrids raised at Exbury, England Satsuki hybrids – Japanese raised, originally for bonsai Shammarello hybrids – raised in northern Ohio Vuyk (Vuykiana) hybrids – raised in the Netherlands Azaleodendrons – semi-evergreen hybrids between deciduous azaleas and rhododendrons Planting and care Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5–5.5; some tropical Vireyas and a few other rhododendron species grow as epiphytes and require a planting mix similar to orchids. Rhododendrons have fibrous roots and prefer well-drained soils high in organic material. In areas with poorly drained or alkaline soils, rhododendrons are often grown in raised beds using media such as composted pine bark. Mulching and careful watering are important, especially before the plant is established. A new calcium-tolerant stock of rhododendrons (trademarked as 'Inkarho') has been exhibited at the RHS Chelsea Flower Show in London (2011). Individual hybrids of rhododendrons have been grafted on to a rootstock on a single rhododendron plant that was found growing in a chalk quarry. The rootstock is able to grow in calcium-rich soil up to a pH of 7.5. Hybrids Rhododendrons are extensively hybridized in cultivation, and natural hybrids often occur in areas where species ranges overlap. There are over 28,000 cultivars of Rhododendron in the International Rhododendron Registry held by the Royal Horticultural Society. Most have been bred for their flowers, but a few are of garden interest because of ornamental leaves and some for ornamental bark or stems. Some hybrids have fragrant flowers—such as the Loderi hybrids, created by crossing Rhododendron