Giant rose attracts colonel from afar

The giant rose, Rosa gigantea, has the largest flowers of all roses and arguably the thickest trunks. Mature plants in nature have been described as having stems 'as thick as a man's forearm', with at least some measuring a metre and a half in girth. 

It's a climber or large shrub, reaching thirty metres or more when let go. The flowers, with soft white petals, are 10-15 cm across. 

This I know from a lovely prose and illustrated portrait of the species in a recent Curtis's Botanical Magazine (volume 48, issue number 2).

I have Terry, our curator of the Southern China Collection and extraordinary horticulturist at Royal Botanic Gardens Victoria, to thank for showing me a live flower of this species in late August. We have two specimens in the Melbourne Gardens, one near the old elms above The Terrace cafe, the other out the back of one of our office buildings.

According to Martyn Rix, a colleague of mine when I was working at Kew Gardens and author of the article on Rosa gigantea in that magazine, most (95) of the 'normally sexual' rose species grow in the Himalaya and China. The so-called dog roses are mostly from Europe. 

As Terry told me, and Martyn confirms, most modern roses were bred from hybrids formed by Rosa gigantea and a form of Rosa chinesis (called spontaea). The first hybrids were grown in China, then introduced into Europe in the eighteenth century. 

The giant rose was only formally described and named in 1888, from specimens collected on the border of Myanmar and Thailand by Colonel Sir Henry Collett, who also proposed the species name. It's said Collett saw the plant in flower from over three kilometres away through his military binoculars.

Sometimes, including in our own plant census at Royal Botanic Gardens Victoria, the species is treated as a variety of Rosa odorata. This is a helpful connection given the beautiful perfume of these giant flowers but there is some uncertainty about how to best apply Rosa odorata, with Martyn suggesting we keep it for cultivated hybrids.

Rosa gigantea, as we'll call it here, is under threat from habitat loss where it grows on the edge of forests and in ditches and hedges of north-eastern India, through Myanmar to the Yunnan Provence of China. Thankfully, I guess, it grows well in cultivation.