The Soap bark tree or Soapbark (Quillaja saponaria) is an evergreen tree in the family Quillajaceae, native to warm temperate central Chile north to Peru. In Chile it occurs from 32 to 40° South Latitude approximately. Populations are found even 2000 m (6500 ft) above sea level. It can grow to 15-20 m (50-65 ft) in height. The tree has thick, dark bark, smooth, leathery, shiny, oval evergreen leaves 3-5 cm long, white flowers 15 mm diameter borne in dense corymbs, and a dry fruit with five follicles each containing 10-20 seeds.
The inner bark of Quillaja saponaria can be reduced to powder and employed as a substitute for soap, since it forms a lather with water, owing to the presence of a glucoside saponin, sometimes distinguished as quillaia saponin. It also applied as an agricultural spray adjuvant. The same, or a closely similar substance, is found in soapwort (Saponaria officinalis), in senega root (Polygala senega) and in sarsaparilla; it appears to be chemically related to digitonin, which occurs in digitalis.
Soap bark tree has a long history of medicinal use with the Andean people who used it especially as a treatment for various chest problems. It is the source of quillaia, the extract of which is used as a food additive and as an ingredient in pharmaceuticals, personal care products, and fire-fighting foam. It is used as additive for photographic films and foaming for drinks. The saponin content of the bark helps to stimulate the production of a more fluid mucous in the airways, thus facilitating the removal of phlegm through coughing. The saponins of this tree are also considered to have adjuvant properties for vaccine solutions.
The plant is drought resistant, and tolerates about -12°C (10°F) in its natural habitat. It is often used for reforestation on arid soils. It has been introduced as an ornamental in California. Trees have been acclimatized in Spain but are rarely cultivated there. The wood is used cabinetry, and scents derived from the tree are used in perfumes and cosmetics.