Carbon Negative Solution with Pretty Flowers

We have only one specimen of this attractive tree in the Royal Botanic Gardens and I’m wondering why. In leaf it’s pretty enough, and in flower a show stopper. Millettia grandis is known as the South African Ironwood – its tough wood being suitable for furniture to walking sticks – or the Tree Wisteria – an appropriate nod to its spectacular flowers.

It comes from the warm coastal forests of south-eastern South Africa, but expect to see it or its relatives grown more widely soon. In looking for information on this species on the web I came across a site promoting a Millettia species as a ‘carbon negative solution’.

A company called ‘Palm Plantations of Australia’ are breeding up new varieties of another pretty species of millettia, Millettia pinnata. Also known as Ponge or Pongam (it used to be classified in the genus Pongamia), it’s probably native to India but widely planted throughout Asia.

I can’t vouch for the credibility of the enterprise but they make some interesting points about this species. They like the fact that it becomes a big tree with a long taproot – a ‘substantial carbon sink’, as they put it – and are convinced it can fix more carbon that is used in producing the fuel they will eventually extract.

They say it is a tough plant, tolerant of tough growing conditions and poor soils. It’s a legume, so via friendly bacteria in its roots it can fix nitrogen. When it comes to its oil yield, they calculate it has a higher oil content per seed (40-50% apparently) than oil palm or jatropha (the latter was the subject of extensive research at the South China Botanical Garden in Guangzhou I visited last week), and can produce 800-1000 kg of seed per tree, per year.

I gather (from Wikipedia!) the University of Queensland is also involved in a study of Tree Wisteria as part of an Australian Research Council Centre for Excellence in Legume Research.

You may ask whether the seed oil would be good for cooking. The short answer is probably not because the seeds of Millettia grandis at least are poisonous when eaten in large quantities. However it has been found that ground up and soaked in milk they can help treat roundworm. The powdered roots will make you sleepy.

There ‘used to be’ some Australian species of Millettia but taxonomists have moved them to another genus called Callerya, along with a few from overseas. Callerya is a genus of plants still poorly known it seems, with a new species described from New Caledonia in 2005, the first for that country.

I should make mention here of Callerya cinerea, which also has Wisteria-like flowers but in this case even grows like one and we include it among our Wisteria collection in the HSBC Oriental Garden. Our specimen is from Vietnam but its found right through southern China to India. Unlike Wisteria, it doesn’t loose its leaves in winter. I didn’t check this year but usually flowers in November.

Until Millettia is ready to save us from climate change, enjoy the beautiful purple flowers on display near the Henry Lawson Gate in the Royal Botanic Gardens.

Images: a couple of low resolution pictures of the tree near Henry Lawson Gate, taken a few years ago