The natural diversity of woody species as a source of ornamental plants.The collection of the Kalmthout Arboretum contains 4 000 plants, representing 170 généra of woody plants and 240 généra of herbaceous plants. An additional 6 000 plants are kept on the site of Hemelrijk, in Essen. The material originated partly from other institutions but also from collecting trips in Japan, South and North Corea, Russia and China. A part of thèse introductions was evaluated or used for hybridizing with a view to produce woody plant cultivars of ornamental value, especially in the généra Hamamelis, Hydrangea, Prunus, Malus, Pyrus and Rhododendron. A large number of thèse new cultivars are now grown in commercial nurseries in Europe, North America and Japan.