Quillaja saponaria, or Soapbark Tree as it is commonly referred to, is a Chilean tree previously placed in the Rose family (Rosaceae) but is now considered to be monotypic in the Quillajaceae. It is a slender, upright evergreen tree growing to 30-45 ft. tall with pendulous branchlets. The bright green oval leaves are 1 to 1 ½ inches long and ½ to ¾ inches wide with slightly toothed margins that become almost entire towards the leaf base. Light yellow to cream-white star-shaped flowers, about ½ inch across are freely produced at the branch tips during May and June and are followed by the ¾ inch wide 5 lobed fruit which is of leathery texture and matures in the fall.
Plant Quillaja in full sun, water deeply and infrequently - can withstand drought conditions but looks much better when given fairly regular deep irrigation or when planted in a lawn. It is also tolerant of poor soils. Quillaja is reliably cold hardy down into the low 20's Fahrenheit and went through our December 1990 freeze without damage with short duration temperatures down to 18° F (Sunset suggests zones 8,9,13-24). Young plants tend to be bushy and need occasional pruning when young but eventually form narrow, almost columnar, trees that are ideal for narrow confines. Another usage is as a tall hedge.
This forest tree from Chile, where is found in dry, poor soils up 6,500 feett, is often called Soapbark as the bark is harvested to extract saponins and other foaming agents. The specific epithet is in reference to these compounds while the name Quillaja (pronounced "key - ya - ha") is from the indigenous Chilean name Quillai (pronounced "key ya hey"), a modification of the word ’quillean’, meaning "to wash".
Aside from its ornamental qualities Quillaja has several other virtues. One that can exploited while still enjoying the ornamental qualities is that this tree is an attractant to beneficial insects. Cornflower Farms of Elk Grove, California has been marketing this tree as a "Beneficial Insect Plant", attracting to the garden such insects such as lacewings, hoverflies, lady beetles and wasps.
The other attribute has to do with the substances in the bark that has both caused a medical stir as well as put the plant on poisonous plant lists. Quillaja saponaria is listed in Poisonous Plants of California (Thomas Fuller and Elizabeth McClintock, UC Press 1986) because of toxic saponin glycosides. These toxins characteristically affect cold blooded animals but if the gastrointestinal tract of warm blooded animals has been injured they can be absorbed. Plants containing saponins are typically bitter to the taste and are rarely browsed. It is Quillaja's bark that contains saponin and there is no mention of any known poisoning associated with it. In recent medical studies it is this same saponin that has been determined to have potential. The saponin QS-21, derived from the bark of Quillaja saponaria, has shown great potential as an adjuvant with a number of vaccines and most recently it has been looked at for this purpose for a COVID-19 vaccine. Unfortunately the harvesting of this plant has caused the decline of native stands of the tree in Chile and has also sparked an interest in cultivating the tree as a agricultural crop.