How to Grow Lablab in Australia

Lablab (Lablab purpureus, pea family, Fabaceae), or hyacinth bean, is a fast, twining summer legume grown as a cover crop for resting soil. It handles heat and dry spells, smothers weeds and fixes its own nitrogen, so it suits a bed left empty over the hottest months. Where gardeners go wrong is timing: sown into cold soil it stalls, and left past flowering it sets seed.

Three mistakes that matter

1. Sowing into cold soil. Lablab is a tropical plant, and seed sown into cool, damp soil comes up slowly and patchily, if at all. Wait for the soil temperature When to plant gives, with the frosts behind you.
2. Letting it flower and set seed. Once the crop is in full flower its nitrogen content falls, and ripe pods drop seed that comes up in the next crop. Cut it as it nears flowering, as Slashing and Digging In describes.
3. Sowing seed straight into the dug-in crop. Fresh green material breaks down fast, and for several weeks that burst of activity can hold back germinating seed. Slashing and Digging In gives the wait, and a way round it.

When to plant

Soil temperature decides it. Sow after the last frost, once the soil holds a steady 18°C at sowing depth at 9 am for three or four days running. One sowing covers a bed for the summer. In the tropics lablab grows year round wherever there is moisture.

At the cool edge of a window, sow early, because a late sowing can meet frost before it reaches cutting size. At the hot, dry edge, soak the bed a day or two before sowing, because the seed needs moist soil to come up.

Site, soil and setup

Pick a sunny bed that is resting over summer, clear of fences, shrubs and bushland. The stems run 3 to 6 m and twine through anything nearby.

Sowing Seeds

Sow Green Manure Lablab seeds straight into the bed: the seed is large, quick to come up and needs no tray.

Preparing the seed

Coat the seed with a Group J inoculant (a peat culture of the rhizobia bacteria that let lablab fix nitrogen). Moisten the seed with a little milk or methyl cellulose glue (5 g per litre of water), not plain water, so more rhizobia survive. Stir in the inoculant until every seed is coated, then sow within 24 hours, keeping it out of the sun. Store the inoculant in the fridge and use it before its expiry date. No soaking or nicking is needed, because the common annual forage types have soft seed.

Sowing direct

• Clear weeds, then dig and rake the bed to an even surface.
• Scatter 50 g of seed over 25 square metres, about eight seeds per square metre, or sow in rows 20 to 40 cm apart.
• Cover the seed 30 to 50 mm deep in moist soil and firm it down.
• Water in well, and lay shade cloth over the bed if birds dig for the seed, lifting it once seedlings appear.
• Expect seedlings from about 5 days in warm soil, and up to 3 weeks when it is cooler.

Some growers sow up to twice as thick for quicker cover. Where lablab or cowpeas grew recently, the right rhizobia may already be in the soil, though inoculating is still the safe choice.

Care by growth stage

Moisture in the first month decides this crop. Once its taproot is down, lablab draws water from 2 m deep. It makes its own nitrogen, so it needs no nitrogen feed at all.

To check the inoculant, lift one plant at 8 to 10 weeks. Plenty of pink nodules near the top of the root means it is fixing nitrogen. If a soil test shows a trace element shortfall, a soluble trace element mix at the label rate corrects it.

Companion planting

Lablab pairs best with tall summer grasses and worst with low crops it can climb over.

Grows well with

• Corn, which lablab can climb; sow the lablab about four weeks after the corn so it does not smother the young stalks.
• Millet or forage sorghum, summer grasses that add bulk while the legume adds nitrogen.
• French marigolds (Tagetes patula), a summer cover crop dug in to help control root-knot nematode.

Keep apart

• Beans, because bean fly moves from older legume plantings onto young ones.
• Tomatoes, in a bed with root-knot nematode (see Pests and diseases).
• Pumpkins and other low, sprawling crops, which the vines climb over and shade out.

Slashing and Digging In

Cut the crop as the first flower buds appear, or before the first frost if that comes sooner. Most types flower in autumn as the days shorten, whatever the sowing month. The canopy is usually full 8 to 12 weeks after the seedlings emerge, so it can be cut then if the bed is needed.

• Slash or mow the plants low: cutting below 15 cm kills many plants, though some regrow from the base.
• Chop the tops finely, so they break down faster.
• Dig them into the top 15 cm of soil, or leave them on the surface as mulch.
• Wait 4 to 6 weeks before sowing seed into a dug-in bed.
• If the bed is needed sooner, leave the tops as surface mulch and plant through it.
• Dig out any regrowth before it flowers.

In northern New South Wales, early types flower in April, while late types are often killed by frost before they flower. Leftover seed stays viable for two to three years if kept dry, cool and dark.

Weed lists, state by state

Lablab has naturalised in Queensland, New South Wales and Western Australia, including the Kimberley, and the Northern Territory record is doubtful. No state or territory declares it, and Western Australia lists it as permitted. Pasture references rate its weed potential low, because volunteers (self-sown plants) rarely return beyond one year. Cutting before seed set keeps it in the bed.

Common problems

Gaps in the bed. Seed sown too deep, into dry soil or before the soil warmed often fails to come up (see Three mistakes that matter). If warm, moist soil still leaves gaps, dig for wireworms, shiny cream to tan larvae up to 35 mm long that eat germinating seed. Pick out any you find, and resow once the soil is warm and moist.

Pale, slow plants. Few or no pink nodules on the roots means the rhizobia did not take, often because the inoculant was old or sun-baked. Wet, poorly drained soil gives the same look.

Blackened leaves in autumn. Frost has caught the crop, which tolerates only very light frost. Cut and dig in what is left, and sow earlier the next summer.

Pests and diseases

Young lablab is most at risk in its first four weeks. Once established it is fairly free of root diseases, though caterpillars and locust swarms can chew the growing tips and leaves.

Bean fly (Ophiomyia phaseoli). Tiny shiny black flies leave pale pinprick marks in the leaves, and their maggots tunnel into the stems, sometimes killing seedlings. It is worst in tropical and subtropical coastal gardens. Hill soil around damaged stems 2 to 3 weeks after germination, so new roots form.

Cowpea aphid (Aphis craccivora). Shiny black aphids with black and white legs cluster on shoot tips and can stunt seedlings. Hose them off with a firm jet of water. Read more

Cutworms (Agrotis species). Smooth grey-brown to black caterpillars that hide in the soil by day and cut seedlings off at ground level at night. Scratch the surface near a felled seedling to find and remove them. Read more

Root-knot nematode (Meloidogyne species). Microscopic soil worms that swell roots into galls, which, unlike nodules, cannot be rubbed off. Lablab is a host, so it can keep numbers up in an infested bed. Read more

Sclerotinia stem rot (Sclerotinia sclerotiorum). A stem rot recorded on lablab in Queensland, most likely in wet conditions. Rotting stems carry white, cottony growth and small black lumps (sclerotia) that survive years in soil. Bin affected plants rather than digging them in, and follow with a grass crop such as corn.

Prevention first

• Clear volunteer legumes and weeds such as phasey bean before sowing.
• Rake off debris and clods, where cutworms hide by day.
• Check seedlings twice a week for leaf pinpricks, aphid colonies and cut stems.
• Choose a free-draining bed and do not overwater.

Tips

1. Keep lawn weed sprays well away: lablab is highly sensitive to hormone herbicides such as 2,4-D and dicamba, and drift can damage it.
2. Before planting a new garden bed, grow one crop of lablab to cover the ground, suppress weeds and ease compaction first.
3. Sowing with millet or forage sorghum, cut the lablab rate by about a quarter and the grass rate by half, so neither swamps the other.