Note the unusual square pods and the wings that emerge from the corners of the uniquely shaped Goa bean.
The winged bean plant looks pretty much like a typical green bean plant. It's a high-climbing, twining vine with trifoliate leaves. The leaflets are more or less triangular, tapering to an acute point, about 3-6 in (7.6-15 cm) long and almost as wide at the widest point. Flower color varies among cultivars; some are a pale sky blue, some are white, and others are reddish brown. They are larger than green bean flowers, a little more than 1 in (2.5 cm) long, and hang in loose clusters of 2-10 flowers. The pods are very distinctive. They have four leafy wings with frilly edges running lengthwise on the pods. In cross section, the pod is square with the four corners tapering out into the thin wings. The pods are pale green, 6-9 in (15-23 cm) long and about 1 in (2.5 cm) wide at maturity. When fully ripe they turn brown and split open, often with a loud popping noise. (Psophocarpus is from the Greek for "noisy fruit.") The seeds are round and look a little like those of soybeans. Some winged bean cultivars produce large tuberous roots from which they can resprout if the top is killed.
It is not clear exactly where winged bean originally came from. Papua New Guinea and Indonesia are possible candidates because there are so many different strains of winged beans there. Winged bean is widely cultivated in the tropics, especially in Myanmar, India, Malaysia, Indonesia, Thailand, Bangladesh, West Africa, the West Indies and South Florida.
Most winged bean cultivars will not flower until daylight length is less than 12 hours, yet they are sensitive to frost. Therefore they will not produce flowers and fruit in most of Europe or the U.S., because by the time the days are short enough, autumn frosts are nipping at the buds. However there are some recently developed cultivars that are day length neutral and these can be grown in the higher latitudes. I have been growing and saving seeds of winged beans in my Zone 8 garden for several years and my plants now produce pods from July until the first frost. You can get seeds of daylength-neutral cultivars (and others) from the non-profit organization, Educational Concerns for Hunger Organization (ECHO).
Light: Full sun. Moisture: Winged beans can tolerate short periods of drought, but they need regular water for best production. They thrive in well drained soil in hot, humid climates with plenty of rainfall. Hardiness: USDA Zones 8 - 11. Winged bean is a perennial but it is usually grown as an annual, even in tropical regions. It will come back from the big starchy roots if they don't freeze. You need at least 180 frost-free days from planting seed to harvesting mature pods. Flowers and immature pods can be harvested in as little as 60 days. Propagation: Sow seeds in place one inch deep, and 4-6 in (10-15 cm) apart in rows 3-5 ft (0.9-1.5 m) apart after all danger of frost has past. Nick the seed coats with a file or sandpaper before planting. Provide a trellis or poles as for pole beans.
The leaves, flowers, pods, green seeds, dried seeds, and (in some varieties) tuberous roots of winged bean are all edible and nutritious. In Indonesia, tempeh and tofu are made from winged bean seeds. A winged bean "milk" and flour are used as dietary treatments for protein-deprived children. Winged bean stems and leaves are used as cattle forage. Winged bean is a powerful nitrogen-fixing legume, and it is used as a "green manure" for intercropping with bananas, sugarcane, taro, and other tropical crops. The tender young shoots and leaves of winged bean may be eaten raw or cooked as green vegetables. Add young stems and leaves to soups and curries. The half-ripe seeds can be removed from the pod and cooked like peas or kidney beans. The flowers can be eaten raw, fried or steamed. Tubers should be dug at the end of the season. Air dry for a few days, then peel before cooking. Roasting is said to be the best way to prepare the tubers. (If growing winged bean for the tubers, keep the flowers pinched off to promote tuber development.) See the ECHO web site for some interesting winged bean recipes.
The winged bean seeds resemble soy beans
The dried seeds of winged beans are about 35% protein, which is higher than that of soybeans. The green pods are about 2% protein, raw leaves 5%, and the dried roots about 25%. It is reported that no other leguminous plant fixes more nitrogen per plant than winged bean.
Proteins are composed of amino acids which contain large amounts of nitrogen. All plants must have nitrogen, but most cannot get it from the air even though the air is 78% nitrogen. Instead, they must get it indirectly from nitrogen containing compounds (ammonia, urea, etc.) that are excreted by other organisms or supplied in chemical fertilizers. Legumes (members of the pea or bean family, Fabaceae) literally make their own fertilizer. Legumes have nodules on their roots which, with the help of a soil micro-organism (Rhizobium spp.), absorb and use gaseous nitrogen from the air. The bacteria convert ("fix") the elemental nitrogen into a form (ammonium ions) that the plant can use. In return, the plant's roots supply the bacteria with energy rich carbohydrates. The alliance is beneficial to both the plant and the bacteria, and is one of the most important symbiotic relationships in all of nature.
Winged bean is a great curiosity in the vegetable garden if for no other reason than to show it off and tell your friends about its many uses in tropical developing countries. The flowers are tasty (some say like mushrooms) and the immature pods are acceptable sliced and cooked like green beans. I have yet to develop a taste for the mature seeds, although I have not tried roasting which is said to make them more palatable.