Borderline-hardy beauties from South America
The sky is grey, the leaves have mostly fallen and the temperature is dropping, but one plant is currently bringing a splash of tropical colour to the garden: Tibouchina urvilleana (also known as the glory bush). The large royal purple flowers and neatly arranged silky leaves make this an extremely elegant plant which looks stunning poking up through the Cyathea medullaris and Cestrum ‘Newellii’ in a sheltered raised bed near the house.
Tibouchina is one of around 165 genera in the mostly tropical Melastomataceae family, which is in the order Myrtales. The family is one of the easier to recognise and includes a number of characteristic features (see Melastomataceae family page), which are all found in Tibouchina.
One of the most distinctive features of the family are the stamens (the male part of the flower in which anthers, containing pollen, are held at the end of a filament), which are sickle-shaped and often with strange sterile appendages the presence, size, shape and position of which can help in identification. In Tibouchina, these appendages extend on the underside of the anther and are bi-lobed. The stamens are also unequal in size, with five small stamens and five which are much larger. Tibouchina also have fruit which are capsules (rather than berries) and hairs on the calyx, bracts, leaves and stems.
Tibouchina grow in the wild from Mexico and the Caribbean to north eastern Argentina and Paraguay. They are mostly middle to high altitude shrubs and small trees with large, conspicuous purple flowers, although there are also some more weedy herbs with white flowers, such as Tibouchina paratropica. It is the largest genus of Melastomataceae in the Americas with around 240 species. However, a recent analysis of their DNA showed that several other genera are nested within the same genetically-related group which divides naturally into several sub-groups, but not along existing genus lines, so the genus may be split and some species be re-named at some point in the future (probably as the earlier-described genus Pleroma).
I grow two species outside in the garden. Tibouchina urvilleana originates from the Atlantic rainforest of southern Brazil. Tibouchina paratropica grows in Argentina, Bolivia and Brazil. Both grow in a raised bed near the house, which I constructed to house my Cyathea medullaris. Tibouchina urvilleana grows up through the Cyathea, using it for support (it can get quite tall and straggly – to avoid this pinch out young shoot tips to encourage bushiness, and cut back flowered shoots to two pairs of buds in spring) while the smaller Tibouchina paratropica grows under the canopy and requires no pruning. The raised bed is filled with a mix of ericacous compost and topsoil, with some fine bark mixed in to improve drainage, and this part of the garden gets morning light but is in shade for the rest of the day. They seem quite happy with this set-up.
Another species, Tibouchina organensis is listed by some UK nurseries, but unfortunately these plants seem to be in fact Tibouchina urvilleana. True Tibouchina organensis reportedly has much larger flowers which are a deeper blueish-purple colour. I’d love to see it. Another plant, sold as Tibouchina grandifolia or Tibouchina hetermalla is more compact and has smaller flowers, but borne in much greater numbers; it’s a gorgeous plant and I’ll definitely be trying it next year.
According to the books, Tibouchina will not tolerate temperatures below freezing. Both my species have spent over a week at temperatures just below freezing last winter and, although they dropped their leaves, they recovered in time to flower by early autumn this year. It seems that a sheltered position near the house, the canopy provided by the Cyathea, and a loose wrapping in horticultural fleece on cold nights may be enough to see them through winter. Alternatively, you could grow them in containers and move them into the house, garage or even the shed when cold nights are forecast.
These fast-growing plants are definitely worth experimenting with and if, like me, you take some cuttings (they strike very easily) in late summer to keep on a windowsill as insurance against losses, you’ll have nothing to lose.
Thanks to contributors to growingontheedge.net for assistance in writing this post.
Tibouchina urvilleana distribution
Countries where Tibouchina urvilleana grows in the wild
Countries where Tibouchina paratropica grows in the wild