Air potatoes in glasshouse nook

Every botanic garden has a few stray plants around that may or may not be in their records but are part of the botanical fabric. This is often true of mosses and liverworts, which are seldom cultivated for themselves but frequently left to add some warmth and texture to potted specimens in the nursery.

This vigorous flowering plant emerges each year from near a bench post and then proceeds to occupy one end of a glasshouse. In late winter it dies back to the ground, but when there is room, we let it sprawl the rest of the year.

I notice the Botanic Garden at University of Oxford, the oldest botanic garden in the UK, has an unnamed species of Dioscorea that also dies back naturally each year. We don't have a confirmed name for ours but it's probably a form of Dioscorea bulbifera. It flowered this year, which might help in identification, but I wasn't quick enough to capture them on camera.

Elsewhere in the collections we have a couple of (confirmed) Dioscorea elephantipes, the Elephant's Foot from South Africa, and Dioscorea transversa, the Australian native Pencil Yam, from New South Wales, Queensland and Northern Territory. 

There are more than 600 species of Dioscorea, or yams, and the underground tubers of many species are edible and widely cultivated. The one that is rampant in our glasshouse, the so-called Aerial (or Air) Yam or Aerial (Air) Potato, also produces sometimes edible 'bulbils' on the above-ground parts of the plant. 

I gather the bulbils and possibly tubers of many wild forms of this species are toxic, and can only be consumed with careful treatment and great care. 

The Aerial Yam is native to tropical Australia, as well as Asia and Africa, but has spread outside its native range as a weed in places like Queensland. The tubers are prepared and eaten by the Tiwi people (as is the Long Yam, by Indigenous peoples in Arnhemland and Melville Island).

Back to our glasshouse bench-surfer. The bulbils drop to the ground and send out roots and a shoot, the beginnings of another giant plant.

The leaves are elegantly heart-shaped with long drawn out tip, which is called a 'drip tip'. This is a common feature of plants growing in wet rainforests and has been shown to channel water from the leaves quickly, stopping them from getting covered in algae, mosses and fungi that would thrive on a constantly moist surface.

The other thing to note is the 'wing' at the base of the leaf stalk (pedicel), where it is attached to the stem. There is usually a twist in this flattened part of the stalk, allowing the plant to position its leaf to collect as much of that limited sunlight penetrating through the rainforest canopy as it can. Maybe the wing is part of that engineering, but I'm not sure.

All this combined help the Aerial Yam to survive and spread in tropical forests, or be tolerated in the corner of a botanic garden glasshouse. In fact a little more than tolerated. Nursery horticulturist Chris says it is one of his favourite plants and a companion since he began work at the nursery twenty years ago.

We think this plant began life under a bench in our old Cacti House, nearby, but it's now contained within what we call our Orchid House. It does need a little care and attention, mostly to stop it heading out through the wall vents. Although it doesn't like a draft so might not enjoy like outside the glasshouse as much as it thinks!

This is the scenic location where it all starts each year...