Hyacinth bean is a multipurpose legume that grows as an annual or a short-lived perennial in the summer. It is a Lablab purpureus herbaceous plant that twines, climbs, trails, or stands upright. Young seeds, pods, and leaves are grown for food across the tropics. The mature, dry beans can be eaten if appropriately cooked; otherwise, they may be toxic.
I. Appearance and Characteristics
Lablab purpureus is a species of bean in the family Fabaceae. It is native to sub-Saharan Africa and India and it is cultivated throughout the tropics for food. English language common names include hyacinth bean, lablab-bean bonavista bean/pea, dolichos bean, seim or sem bean, lablab bean, Egyptian kidney bean, Indian bean, bataw and Australian pea. Lablab is a monotypic genus.
The plant is variable due to extensive breeding in cultivation, but in general, they are annual or short-lived perennial vines. The wild species is perennial. The thick stems can reach 6 m (20 ft) in length. The leaves are made up of three pointed leaflets, each up to 15 cm (5.9 in) long. They may be hairy on the undersides. The inflorescence is made up of racemes of many flowers.
Some cultivars have white flowers, and others may have purplish or blue. The fruit is a legume pod variable in shape, size, and color. It is usually several centimeters long and bright purple to pale green. It contains up to four seeds. Depending on the cultivar, the seeds are white, brown, red, or black, sometimes with a white hilum. Wild plants have mottled seeds. The seed is about a centimeter long.
II. How to Grow and Care
Sunlight
For the most prolific flowers, choose a site for your purple hyacinth bean that gets full sun. This plant will also grow in partial shade, but will have fewer blooms and can become susceptible to fungal diseases in these conditions. Most varieties of purple hyacinth bean are considered “day-length neutral,” meaning the plant will mature or flower regardless of its exposure to alternate periods of darkness and light. Do note, however, that some varieties of purple hyacinth bean may require shorter days and cooler temperatures to flower. Consult with your nursery to choose the variety best for you.
Temperature and Humidity
Purple hyacinth bean flourishes in temperatures around 70 F, yet this plant can withstand high heat (as long it gets enough water) and short periods of cold no lower than 37 F. This plant is commonly cultivated throughout Asia in temperate climates where humidity levels are high.
Watering
This plant likes lots of water, but make sure that the ground stays consistently moist, rather than wet. To do so, slowly and deeply water the plant’s entire root system when the top 3 inches of soil become dry. During a midsummer drought, you may find the need to water every other day; but with sufficient rainfall, you may be able to go weeks without dry soil.
Soil
Purple hyacinth bean grows best with a fairly neutral soil pH of 6.0 to 6.8. This plant prefers rich, loamy soil, so working organic matter or compost into the soil before planting will give it a good start. Choose a well-drained site for this plant, as its roots will rot in standing water.
Fertilizing
Because this vine grows so vigorously, it’s best to give it some extra food every four to five weeks throughout the summer. A monthly dose of your favorite organic fertilizer, like liquid fish fertilizer, will encourage blooming. Please refer to product directions for amounts, and always use just less than what’s recommended to avoid overfertilization. Choose a fertilizer with a higher phosphorus content than nitrogen. As a member of the legume family, the plant fixes nitrogen in the soil.
Pruning
Purple hyacinth bean will need pruning when grown as a perennial in USDA hardiness zones 10 and above. After a few years, the plant may develop woody stems which may break or look unsightly. In the early spring, remove any dead or broken stems by cutting them back with pruning shears. Then, during the summer, trim your plant to your desired shape. If your hyacinth bean starts to slow its flower production, rejuvenate the vine by cutting it back to 6 inches above the ground.
Even if you’re growing purple hyacinth bean as an annual, you may want to do some regular pruning to keep its rapid growth under control. Once the vine has reached the top of its support structure, use a pair of clean shears to snip off the growing tip. This will encourage the growth of more lateral stems for a balanced spread. Continue to monitor its growth and prune the vines as needed throughout the growing season.
Propagation
You can sometimes find purple hyacinth bean starts for sale in nurseries, however, this plant is most commonly started from seed and rarely propagated, due to its rapid growth. It is also very easy to collect the large seeds from your plant at the end of the season to save for next year.
Here’s how to start this vine by seed:
- Gather a glass of water, a watering can, and a vining support.
- Soak the hard seeds in the glass of water overnight so that the seed will soften and hasten germination.
- Wait until all danger of frost has passed, and then sow the seeds directly in the warmed ground, placing them 1 to 2 inches deep and no more than 6 inches apart.
- Water the seeds thoroughly with a watering can and keep the soil consistently moist until germination (two to three weeks).
- Once the seeds sprout, thin them, as needed, and place your support next to or behind the seedlings, training them to find it as they grow. This will prevent the plants from tangling themselves on the ground, and instead, train them to grow vertically.
- Alternatively, you can start the seeds indoors four to six weeks before the last frost, then transplant them outside after hardening off the plants for a week. That said, sowing seeds outdoors is an easier process, and outdoor plants will catch up to one’s sprouted indoors in no time, anyway.
Pests and Diseases
Common Pests & Plant Diseases
Purple hyacinth bean rarely fall victim to insects or pests. However, it is the host plant for longtail skipper butterflies, and the larva may eat the leaves. Japanese beetles find the flowers and leaves scrumptious. An occasional munch from one of these critters usually poses no overall threat to the plant. Yet, if you want to deter them, a homemade soap and water concoction sprayed on the leaves will do so.
Blight, Fusarium wilt, and rust can all affect purple hyacinth bean. Blight, a bacterial disease, presents as water-soaked spots on the leaves that eventually move onto the stem. Fusarium wilt, an opportunistic fungal disease, will cause the lower leaves to yellow. And rust, a fungal parasite, may make the plant look like it’s burnt or scorched. Prevention tends to be much easier than the eradication of these diseases. Tactics include keeping the soil moist, but not wet, and thinning the plants to allow for ample airspace in between each one.
Common Problems
Growing purple hyacinth bean in a shaded location with indirect sunlight may make it susceptible to fungal disease. This plant loves moist soil, yet it needs the sun’s rays to dry it off between waterings. In a shaded location, it is tricky to provide ample water, while also assuring the soil isn’t consistently soaked. Powdery mildew can be a problem, as well, if grown on a structure that doesn’t allow air to circulate around the plant.
III. Uses and Benefits
- Food in South Asia
In Bangladesh and West Bengal, the green pods along with the beans, known as sheem (শিম), are cooked as vegetables or cooked with fish as a curry.
In Gujarat, lablab is called surti papdi.
In Kerala, it is known as amarakka, avara or amara payar (Malayalam: അമര പയർ). The beans as well as the bean pods are used in cooking curries. The bean pods are also used (along with spices) for preparing a stir-fried dish known as thoran.
In Tamil Nadu, it is called avarai or avaraikkaay (Tamil: அவரைக்காய் / அவரை). The entire bean is used in cooking dry curries and in sauces/gravies such as sambar. The seed alone is used in many recipes and is referred to as mochai (Tamil: மொச்சை / மொச்சைக்கொட்டை).
In Maharashtra, dry preparations with green masala are often made out of these green beans (ghevda varieties; Shravan ghevda (French beans), bajirao ghevda, ghevda, walwar, pavta sheng) mostly at the end of monsoon season during fasting festivals of Shravan month.
In Karnataka, the hyacinth bean is made into curry (avarekalu saaru) (Kannada: ಅವರೆಕಾಳು ಸಾರು), salad (avarekaalu usli), added to upma (avrekaalu uppittu), and as a flavoring to Akki rotti. Sometimes the outer peel of the seed is removed and the inner soft part is used for a variety of dishes. This form is called hitakubele avarekalu, which means “pressed (hitaku) hyacinth bean,” and a curry known as hitikida avarekaalu saaru is made out of the deskinned beans.
In Telangana and Andhra Pradesh, the bean pods are cut into small pieces and cooked as a spicy curry in the Pongal festival season. Sometimes the outer peel of the seed when tender and soaked overnight is removed and the inner soft part is used for a variety of dishes. This form is called pitakapappu hanupa/anapa, which means “pressed (pitaku) hyacinth bean, and a curry known as pitikina anapaginjala chaaru/pitaka pappu is made from the deskinned beans and eaten along with bajra bread.
- Food in Southeast and East Asia
In Myanmar, lablab beans are used to make a braised Burmese curry hnat. They are also crisp-fried and served in Burmese pickled tea leaf salad.
In Huế, Vietnam, hyacinth beans are the main ingredient of the dish chè đậu ván (Hyacinth Bean Sweet Soup).
In China, the seeds are known as Bai Bian Dou. They are usually dried and baked before being used in traditional Chinese herbal remedies to strengthen the spleen, reduce heat and dampness, and promote appetite.
- Food tradition in East Africa
In Kenya, the bean, known as njahe or njahi, is popular among several communities, especially the Kikuyu. Seasons were actually based on it, i.e., the Season of Njahe (Kīmera kīa njahī). It is thought to encourage lactation and has historically been the main dish for breastfeeding mothers. Beans are boiled and mashed with ripe and/or semi-ripe bananas, giving the dish a sweet taste. Today the production is in decline in eastern Africa. This is partly attributed to the fact that under colonial rule in Kenya, farmers were forced to give up their local bean in order to produce common beans (Phaseolus vulgaris) for export.
- Medicinal use
Taiwanese research found that a carbohydrate-binding protein (i.e. a legume lectin) from lablab beans effectively blocks the infections of influenza viruses and SARS-CoV-2.
