How to Grow and Care for Purple Hyacinth Bean

Purple hyacinth bean (Lablab purpureus) is a vining annual typically grown for its flowers, although most of the plant is edible. Shades of purple decorate the leaves, stems, flowers, and showy seed pods, making this garden inclusion undeniably eye-catching. Purple hyacinth bean is a vigorous grower and a popular choice for those needing a quick cover over an arbor or as a foundation planting. The plant's bright green leaves have purple-hued veining and are pointed, growing in leaflets of three. The flowers resemble those of sweet peas, but without the scent, often showing in the late summer in loose clusters of pale purple and white. Once the blooms drop, glossy purple pods appear that resemble snow peas.

While the young shoots, young leaves, and flowers are edible, as well as the cooked, young pods, the mature, hardened beans contain cyanogenic glucoside levels that are toxic to both humans and pets.

Purple Hyacinth Bean Care

Purple hyacinth bean climbs by twining around structures or other plants. Use it to add height to a border or grow it up a teepee or trellis. Because of its fast growth, this vine makes an excellent choice for camouflaging an eyesore, like an air conditioning unit or garbage can storage. While this vine may need some initial training to reach the support, once attached, this low-maintenance plant will flourish with very little care.

Plant purple hyacinth bean in the spring when the danger of frost has passed, with consistent evening temperatures of 50 degrees F. Once purple hyacinth bean blooms, your garden will have a continual display of color, as purple bean pods will come along soon after the flowers fade. Deadheading is not required to keep this cycle going. However, if you want to encourage a second flush of blooms for fall, cut the plant down to 6 inches. It will quickly grow and bloom again.

Light

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.

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.

Water

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.

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.

Fertilizer

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 phospherous content than nitrogen. As a member of the legume family, the plant fixes nitrogen in the soil.

Types of Purple Hyacinth Bean

Generally, most seeds of this species are simply labeled "hyacinth bean" or "purple hyacinth bean." Yet, a few named varieties feature subtle differences when compared to the generic seed. These favorite varieties include:

• Ruby Moon hyacinth bean produces color all season long with its purple stems, lilac blossoms, and shiny magenta pods. This cultivar boasts an average mature height of 10 to 20 feet.
• An heirloom variety, Purple Moon hyacinth bean, matures in 90 days and is grown mostly as an annual. The bean pods are a reddish-purple in color, with a flat, curved shape. 
• Silver Moon hyacinth bean bears white flowers, bright green leaves and stems, and shiny silver-green seed pods. This cultivar can easily cover a 20-foot trellis in just one growing season.

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.

How to Grow Purple Hyacinth Bean From Seed

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:

1. Gather a glass of water, a watering can, and a vining support.
2. Soak the hard seeds in the glass of water overnight so that the seed will soften and hasten germination.
3. 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.
4. Water the seeds thoroughly with a watering can and keep the soil consistently moist until germination (two to three weeks).
5. 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.

Alternately, you can start the seeds indoors four to six weeks before 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 ones sprouted indoors in no time, anyway.

Common Pests & Plant Diseases

Purple hyacinth bean rarely falls 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.

How to Get Purple Hyacinth Bean to Bloom

If your vine has few to no flowers, it may not be getting enough sun, or your soil may contain too much nitrogen. Beans are nitrogen fixators (meaning they make their own nitrogen) and too much in the soil encourages lush foliage with few flowers. To fix this issue, add a soil amendment that's high in potassium to encourage flowering. If all else fails, you may need to relocate your perennial to a sunnier area, or plant your seeds in a more sun-soaked location next year.

Common Problems With Purple Hyacinth Bean

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.