Dioscorea mexicana aka Mexican Yam

Dioscorea mexicana, the tortoise plant, belongs to the Dioscoreaceae family. Its native range runs from Mexico to northern Colombia. It grows in tropical regions with a dry season. LLIFLE also records it in semiarid sites. This climber has a lasting, swollen stem base called a caudex. Leafy vines grow from its top each year. The caudex stays close to the ground as shoots climb.

The caudex forms a broad, low dome with deep cracks. Thick, grey-brown bark splits into raised, many-sided plates. These plates become more prominent as the plant ages. Their shell-like pattern gives the plant its common name. The base looks woody but has succulent tissue inside. Its bark helps retain moisture and guards against animals. In cultivation, the base usually stays below 1.6 feet [0.5 metres] wide. Larger examples can reach 3 feet [0.9 metres] across. The exposed base can stand about 1 foot [0.3 metres] high. It grows slowly, and LLIFLE reports lifespans of centuries.

The vines grow much faster than the swollen base. New shoots emerge in late spring and bear leaves. They can reach about 20 feet [6 metres], sometimes more. The stems usually die back during winter. Some plants keep growing beyond this season or start early. Small purple flowers form in clusters during August. Each flower measures no more than about 0.007 feet [0.002 metres] across. The flowers are subtle rather than a main display.

The plant suits containers, with its base as the main feature. Train long shoots over a hoop or trellis. Pinch back the shoot tip to limit its length. LLIFLE describes this species as robust and suitable for beginners. It tolerates long dry spells but needs some moisture. Use a large pot and soil that drains well. Keep the soil slightly moist, then increase moisture as roots form. Choose a bright position; partial shade also suits it. Follow active growth rather than a fixed seasonal schedule.

Roots emerge from the caudex sides, not its underside. Raising the base too far can prevent roots from forming. The plant may then dry out and die. Seeds provide a means of propagation. Sow them shallowly, keep them warm, and provide indirect light. Young plants form their caudex below the soil surface. Leaving it buried for the first few years speeds growth. The supplied evidence gives no frost limits or pest records. It also does not establish food uses or safety.