Pachyrhizus tuberosus aka Yam Bean

Pachyrhizus tuberosus is a perennial vine in the bean family, Fabaceae. Its common names include yam bean and potato bean. Kew lists its native range as Venezuela to Ecuador. Useful Tropical Plants also records it from Peru. It grows in tropical regions with a dry season. Wild plants climb trees and shrubs, often on disturbed coastal land.

The plant grows from a tuberous rootstock. Each year, it sends out new stems that twine or trail. These reach about 16–20 feet [5–6 metres] long. The roots store starch and can become very large. PFAF records roots weighing up to 20 kilograms. Growers raise the plant for its roots and young seedpods. Its main garden role is as a food crop in warm regions.

PFAF lists flowering in July, though timing may depend on climate. Each flower has both male and female organs. Insects pollinate the flowers, which give rise to seedpods. Root bacteria form nodules and fix nitrogen from the air. Growers may remove flowers when the main aim is root production. Day length matters for the crop. Plants need about 11–13 hours of daylight to start forming tubers. This can prevent a useful root crop in Britain.

Give this vine full sun and moist soil that drains well. It prefers rich, sandy soil but accepts a wider range. Waterlogged ground does not suit it. Useful Tropical Plants gives a preferred soil pH of 5–5.5. It reports tolerance from pH 4.3 to 7.3. Warm lowland conditions suit the plant best. PFAF lists USDA zones 9–11 but also calls it frost tender. In Britain, it needs greenhouse care rather than year-round outdoor planting. Seeds need warmth, and young plants should go outside only after frost. Divided tubers must stay frost-free through winter.

Sources describe roots as edible raw or cooked, including in starch-based desserts. Yet Useful Tropical Plants also warns that roots can be toxic. These reports conflict, so root safety needs caution. Young seedpods require thorough cooking because they contain rotenone. Leaves, mature pods, and seeds contain poisonous compounds. Stems also appear in the source's warning about human toxicity. Do not treat the whole plant as edible. Rotenone gives the plant potential use as an insecticide. It can harm helpful insects, fish, and amphibians. PFAF records no known medicinal use.