Dioscorea (Testudinaria)
Dioscorea is a large, cosmopolitan genus of yams in the family Dioscoreaceae, but to succulent growers the name usually points to a particular group: the caudiciform yams once separated out as the genus Testudinaria. These are the plants that build a woody, corky, above-ground tuber — often deeply cracked into a mosaic of plates like a tortoise's shell — from which thin, twining vines climb each growing season and then die back. Modern classification folds Testudinaria back into Dioscorea, so a plant sold as Testudinaria elephantipes is the same thing as Dioscorea elephantipes.
Description
The succulent Dioscorea share a distinctive two-part architecture. The storage organ is a large, hard, mostly above-ground caudex — smooth and pale when young, developing with age into the corky, angular, cracked surface that makes these plants so prized. This caudex is the perennial part; it holds the plant's water and energy through long dry seasons.
From the growing point at the top of the caudex, one or more slender, wiry vines emerge in the active season. They twine over supports and carry heart- or arrow-shaped leaves and small, greenish, rather inconspicuous flowers (plants are typically dioecious, so male and female flowers occur on separate individuals). At the end of the growing season the vines yellow and die back completely, and the plant rests as a bare caudex until the next cycle. This strong seasonal rhythm — leafy growth alternating with a dormant, leafless rest — is the single most important thing to understand about growing them.
Distribution
The caudiciform members of the genus are concentrated in seasonally dry regions of the Old and New World. The best-known species come from South Africa, where they grow on rocky slopes and in scrub, but the group as a whole ranges across parts of Africa and the Americas. In habitat the swollen caudex often sits partly buried among rocks, with only the upper, weathered portion exposed — a detail worth remembering when deciding how deep to pot them.
Notable species
- Dioscorea elephantipes — the classic "elephant's foot" or "Hottentot bread", with a strongly domed, cork-plated caudex; the species most growers mean when they say Testudinaria.
- Dioscorea sylvatica — a smaller, flatter-caudexed South African species, generally faster and easier from seed.
- Dioscorea mexicana — a New World species with a broad, low, plated caudex and vigorous vines.
Cultivation
The key to these plants is to work with their dormancy rather than against it. Grow the caudex in a very free-draining, mostly mineral mix and give the vines something to climb — a few slim stakes or a small trellis. Position the plant in bright light; the vines are happy in full sun, and good light keeps growth compact.
Water regularly while the plant is in active leafy growth, always letting the mix dry between waterings, and provide a support for the vines to scramble over. As the vines begin to yellow and fade, taper off and then stop watering: during dormancy the bare caudex wants to be kept dry and should be watered little or not at all until new growth reappears. Overwatering a resting caudex is the surest way to rot one. Because some species are winter-growers and others summer-growers, let the plant's own leafing-out and die-back tell you when to start and stop — follow the vines, not the calendar. See Watering and Repotting for general technique.
Growers differ on how much of the caudex to expose: keeping most of it above soil shows off the ornamental plating and reduces rot risk, though a partly buried caudex may fatten faster. When potting up, handle the growing point carefully, as damage there sets the plant back for a full season.
Hobby and cultivar notes
Dioscorea are collected almost entirely for the sculptural beauty of the caudex, which slowly becomes more cracked and characterful with age — these are patient growers' plants, not fast ones. There is little formal cultivar selection; interest centres on wild species and on the individual character of each plant's caudex. Nearly all plants in the trade are raised from seed, since a good caudex takes many years to develop and cannot be shortcut. As with all cactus and succulent collecting, seek out nursery-propagated stock rather than wild-collected plants.
See also
- Dioscorea elephantipes · Dioscorea sylvatica · Dioscorea mexicana
- Soil and potting mix · Watering · Repotting · Propagation - seed · Pests and diseases