Purple Hyacinth Bean Plant Care

The purple hyacinth bean plant (Dolichos lablab or Lablab purpurea) is a beautiful annual vine. Native to tropical African regions, it is also commonly called the tonga bean, Eqyptian bean, or tobacco bean. While typically considered a flowering plant, most parts of purple hyacinth beans are also edible if cooked in a specific way. The leaves, stems, flowers, and seed pods all display varying shades of purple. These fabulous heirloom plants grow quickly and vigorously, making them a popular pick for covering spaces quickly. By flowering late in the season, they provide a burst of color when the rest of the plant life has started to go brown. 

Purple Hyacinth Bean Plant Care

Growth Characteristics

Purple hyacinth bean plants have bright green, pointed leaflets of three leaves. The vibrant green of the top of the leaf contrasts with the purple tinge on the leaf's underside and the stems. The scentless purple hyacinth bean flower grows in pale purple or white clusters, resembling a sweet pea flower. After the flowers fade, the glossy purple seed pods follow and grow to the approximate size of a lima bean pod. The vine climbs by twining around structures rather than using aerial roots. Purple Hyacinth beans can get 10 to 15 feet high and need adequate support through trellising or fencing. 

Light Requirement

Purple hyacinth bean plants require full sun, although most varieties are day-length neutral. This means that while it needs full sun during the day, it doesn't affect the growth pattern, particularly if the days are longer or shorter. There are some varieties of purple hyacinth beans where flowering will not be triggered until the days shorten. Purple hyacinth beans can grow in partial shade, but there will be fewer blooms and a higher risk of fungal diseases.

Soil Requirement

A nice loamy soil helps the purple hyacinth plant thrive and prefers a neutral soil pH between 6.0 to 6.8. They don't require rich soil necessarily, although working some compost through the soil is a good jumpstart to any plant. The soil also needs to be easily drained, as root rot will occur if the soil stays consistently wet. 

Water Requirement

Purple hyacinth bean vines need watering regularly and frequently but never drenched. A light daily mist is the best.

Fertilizer

The vines grow so rapidly that it would be beneficial to give them extra food every month throughout the heat of summer. With a base of the loamy, composted soil mentioned above, add a dose of fertilizer low in nitrogen and high in phosphorus every four to five weeks. This will encourage blooming. If a vine has a low bloom performance, it is either not getting enough sun or given too much nitrogen. Any fertilizer that you choose needs to be low in nitrogen, and high nitrogen supplements like fish emulsion and soy meal should be avoided. 

Temperature and Humidity

In USDA hardiness zones 10 to 11, purple hyacinth bean vines can become perennial. They may develop woody stems. While it's nice that they would return year after year, they tend to be short-lived perennial. The purple hyacinth is more commonly sold as an annual, although they will reseed if the pods are left over the winter. 

Common Pests

Thankfully, purple hyacinth bean vines are rarely prey to disease or pests. Butterflies will sometimes lay eggs, and the caterpillars will eat away at the leaves for a time. It's a short period of time, however, and butterflies come as a result. 

Harvesting

In some parts of the world, purple hyacinth vines are a forage crop. When the pods are still small, similar to snow peas, they cook similarly to green beans. The leaves and flowers are also edible, but care needs to be taken if the mature beans are to be eaten. They have to cook in a particular way, or they can be quite toxic, with high levels of cyanogenic glucosides. This causes vomiting, convulsions, and trouble breathing. The bean pods are easily harvested for future seed, however. The pods will grow in size and plumpness as the flower dies away, and the prime time to harvest them is just before the first frost. Leave the pods to dry, and they'll be easy to open and remove the seeds. 

Growing From Seed Pods

While purple hyacinth bean vines can sometimes be found as seedlings in nurseries, they are usually grown directly from seed. The seeds will need to be soaked overnight before planting to encourage germination, as they are quite hard. After all danger of frost is passed, the seeds can be directly sowed. Must pass the chance of frost, as the seeds will not germinate in cold soil, and young plants can die. Some prefer, then, to start the seeds indoors a month or more before planting outside.

Seeds should plant an inch or two deep, spaced six inches apart. Germination will occur in two to three weeks. The plant will be full and leafy for most of the spring and summer before any blooms appear. Help the vine find support with a trellis or fencing or whatever you want them to cover, and they will train themselves to follow that structure the remainder of the growing cycle. Otherwise, they will become a tangled mess on the ground. 

Toxicity of Purple Hyacinth Bean

Both the leaf and the flower are edible. The mature beans or seeds themselves can be toxic when uncooked. They can cause vomiting, breathing difficulty, and convulsions.