Getting into a lather over Quillaja

Plenty of plants provide a soap substitute. Lindera from China, Agave from Mexico, and Acacia or Alphitonia from Australia, for starters.

So when my colleague Peter Symes tweeted about a large specimen of Quillaja saponaria in our Melbourne Garden, I was drawn to it as much by the charming flowers and evocative genus name as the saponin content of the species name and bark.

Saponin, by the way, is both toxic and the usual lathering agent you find in plants. It can be used as soap or even, as in this case, in fire extinguisher retardants and some agricultural sprays. It has medicinal uses as well.

I also remembered seeing this plant (conveniently labelled, as photographed at the top of this post) in the Parque Nacional la Campana, just north of Santiago, where it grows alongside these stunning Chilean Wine Palms (Jubaea chilensis).

As Peter noted, Quillaja saponaria comes from dry places in central Chile, coping with rainfall down to 100-300 millimetres a year. That makes it more than suitable for the hotter and dryer climate predicted for Melbourne, given that it grows here already so clearly doesn't mind a little extra water.

There are two or three other species of Quillaja, all from middle to southern South America, and the genus now resides in its own family, Quillajaceae, more closely aligned with the pea family than where it used to be classified, the Rosaceae. All this is supported by DNA as well as some other chemical constituents.

Quillaja comes from the local Chilean name for this species and for its bark, Quillai or Quillay, so pronounced something like kill-aya. In cultivation Quillaja saponaria is called either Soap-bark Tree or simply Soap Tree. The saponin is found throughout the plant but mostly extracted from bark.

The flowers, part of the attraction for me, have dainty petals squeezed out between each of the green sepals (the next outer layer of the flower).

As John Grimshaw and Ross Bayton declare in their New Trees book (Kew Publishing/International Dendrology Society, 2009), this is 'not an outstandingly beautiful tree'. They admit persistent five-lobed fruits are attractive, noting their resemblance to fruit of the (unrelated) Star Anise.

You can see the developing fruits in the images above and, elsewhere on the tree, these dry fruits. The fruits were all I saw in Chile back in April 2015 so I'm glad I was able to complete this part of my botanical education in Royal Botanic Gardens Victoria, in December last year ...

... as I stood beneath this perhaps not outstandingly beautiful, but acceptably attractive, single specimen of the species growing in Melbourne Gardens.

Postscript: One of our Ambassador Guides at Royal Botanic Gardens Victoria, Rosemary Cotter, sent me a lovely email (1 April 2019) about a coincidence of thought. Canberra's National Arboretum are growing a forest of both species - the Qullaja and the Wine Palm - and provide a very nice webpage with more information about each.