How to Grow Bitter Melon in Australia

Bitter melon (Momordica charantia), also sold as bitter gourd or karela, is a frost-tender vine of the gourd family, Cucurbitaceae, grown for its warty fruit. It thrives in a humid northern Australian summer, and on a sunny trellis can often crop in warm southern gardens too. Most failures start at sowing: the seed coat is thick and hard, and dry seed in cold soil often never comes up.

Three mistakes that matter

1. Sowing dry seed into cold soil. The hard coat lets water in slowly, so dry seed in cool, damp soil can sit for weeks and rot. Soak it first, as in Sowing Seeds, and wait for the soil temperature given in When to plant.
2. Leaving the vine to sprawl. Fruit lying on the ground rots and marks, and a tangled vine stays damp, which suits mildew. Put up a trellis 1.8 to 2 m tall before sowing, so the vine climbs from the start.
3. Picking too late. A fruit left on the vine soon turns yellow or orange, goes spongy and splits open. Check the vine every two to three days and pick at the stage described in Harvesting Fruit.

When to plant

Warm soil decides the start. Sow once the soil holds at least 20°C; seed comes up fastest at 25 to 35°C and slowly, if at all, in cold ground. One sowing usually crops for months, because a healthy vine can keep flowering for about six months in warm weather.

At the cool edge of the window, start seed in pots on a seedling heat mat and plant out once nights stay warm. In the tropics, vines that flower into the wet season often set poorly, because bees stay home in overcast weather.

Site, soil and setup

Choose the sunniest, most sheltered spot, where the vine can climb a sturdy frame, in soil that holds moisture yet drains freely.

Vines on 2 m stakes crop better and are easier to manage than on a 1 m string line. Once seedlings are up, mulch with straw, clear of the stems.

Sowing Seeds

Soak the seed, then either start it in small pots or sow it straight into warm soil at the foot of the trellis.

Preparing the seed

The seed is flat, ridged and 10 to 15 mm long, with a thick coat that lets water in slowly. Soaking softens it and has the strongest support: in one field trial, seed soaked for 12 to 48 hours came up better than dry seed.

Cutting the coat is optional: in one laboratory trial, sanded seed sprouted less than a day sooner, but slightly fewer seeds germinated. Cracking the tip with pliers has no trial behind it and can injure the root. More on soaking and scarification.

Starting in pots

Start seed under cover where spring is slow, 2 to 4 weeks before planting out.

• Sow one soaked seed per small pot of seed-raising mix, 20 to 25 mm deep.
• Keep the mix warm and moist but never wet; seedlings usually show in 1 to 2 weeks.
• Water the mix, not the leaves, to avoid damping off (seedlings rotting and collapsing at soil level).
• Harden off (accustom seedlings to sun and wind over about a week) before planting out.
• Plant out in the late afternoon at 10 to 15 cm tall, 10 to 20 days after sowing, with the root ball intact.

Sowing direct

In warm gardens, sow at the foot of the trellis once the soil is warm enough.

• Sow two or three soaked seeds per station, 20 to 25 mm deep.
• Space stations 40 to 60 cm apart along the trellis, up to 90 cm for vigorous vines.
• Leave 1.2 to 1.5 m between rows.
• Keep the surface just moist, and thin to the strongest seedling at four true leaves.

Pollination

Each vine carries separate male and female flowers, and bees carry the pollen. Females sit on a tiny, warty fruit. Males stand on bare stalks and outnumber them, often by about 25 to 1, and in long summer days open up to two weeks earlier.

Each flower opens at sunrise and lasts one day, and its pollen fades by midday. Where bees are scarce, pollinate by hand early in the morning.

• Pick a male flower that opened that morning and peel back its petals.
• Touch its pollen-coated centre to the centre of each open female flower.
• Repeat each morning while female flowers keep opening.

Care by growth stage

Steady water and potassium decide the crop, because the vine flowers and fruits for months and does not tolerate drought. Weeks count from sowing.

If you prefer a conventional feed, use each at its label rate. At planting, rake a controlled release granular (NPK around 14-5-10) into the bed. While the vine runs, give an all-purpose soluble (NPK around 23-4-18) fortnightly. From the first flowers, switch to a fruiting and flowering soluble (NPK around 15-4-26).

Pruning and Training

Early training sends the vine upward, and clearing the lowest laterals (side shoots) puts more female flowers on the upper ones.

• Tie the young leader loosely to the frame until its tendrils take hold, from about the fourth to sixth leaf.
• Rub out side shoots from the lowest 10 leaf joints, roughly the first 50 cm of stem.
• Keep removing side shoots until the leader reaches the top of the trellis.
• Then pinch out the leader's tip and let 4 to 6 laterals run along the top.
• Cut out yellowed or mildewed leaves and bin them, to keep air moving.

Companion planting

Good neighbours feed the predators that keep sap-suckers down; poor ones share this vine's pests and diseases.

Grows well with

• Dill and coriander left to flower, whose nectar feeds adult hoverflies; their larvae eat aphids.
• Sunflowers, which draw hoverflies and minute pirate bugs, both predators of thrips.
• Basil and alyssum left to flower, which draw bees and other beneficial insects.

Keep apart

• Cucumber, zucchini and pumpkin, which share some of the same pests and diseases, such as powdery mildew and melon thrips.
• Okra and snake bean, hosts of the same root-knot nematodes, so neither should follow the other.
• Tomato, which also hosts bacterial wilt and root-knot nematodes in northern gardens.

Harvesting Fruit

Pick 15 to 20 days after a female flower sets, usually 8 to 11 weeks from sowing. The skin should be glossy and firm, and the seeds soft and white.

• Pick in the cool of the morning.
• Cut the stalk with scissors or secateurs rather than pulling.
• Remove any fruit showing yellow or orange, so the vine keeps setting.

Fruit keeps two to three weeks at 12 to 13°C in humid air, but chills below about 10°C. Eat or freeze home crops within a few days, and store them away from other produce, since the fruit gives off ethylene (a ripening gas).

Scoop out the seeds and white pith, then stir-fry, curry or add the slices to soup; tender vine tips are cooked too.

Common problems

Flowers but no fruit. Early flowers are mostly male, and a female that is not pollinated yellows and drops; see Pollination.

Fruit too bitter. Bitterness varies between fruit on one vine, and darker fruit is often the most bitter; salting or blanching tames it.

Sudden wilting at fruit set. In northern soils, bacterial wilt (Ralstonia solanacearum) enters through damaged roots and kills the vine within days. Pull and bin it, and raise the next seedlings in clean pots of fresh potting mix.

Pests and diseases

All six occur in Australia, most often in the warm, humid north.

Powdery mildew (Podosphaera xanthii). White powder on older leaves and stems, then yellow patches; dew alone is enough to start it. Cut out the worst leaves and spray 1 part full-cream milk to 10 parts water in the morning. Read more

Downy mildew (Pseudoperonospora cubensis). It can affect bitter melon in wet weather. Yellow patches boxed in by the leaf veins, with grey-purple growth beneath, after long spells of wet leaves and high humidity. Water the soil, space plants for airflow, and use a copper fungicide registered for cucurbits in a wet spell. Read more

Melon aphid (Aphis gossypii). Colonies curl and yellow the shoot tips and spread mosaic viruses that mottle leaves and deform fruit, which no spray stops. Hose them off or use insecticidal soap, and pull nearby weeds. Read more

Melon thrips (Thrips palmi). Pale orange-yellow insects about 1 mm long; their feeding silvers leaves along the veins and scars young fruit. Minute pirate bugs and lacewings eat them. Read more

Cucumber moth (Diaphania indica). Green caterpillars up to 15 mm long with a white side stripe. They web young leaves and chew holes in fruit touching the ground. Bt (Bacillus thuringiensis), a caterpillar-only spray, spares beneficial insects.

Root-knot nematode (Meloidogyne species). Microscopic soil worms that stunt and wilt vines and gall the roots, worst in warm soil cropped year after year. Rotate beds, and dig in cover crops or plenty of compost. Read more

Prevention first

• Train vines up a trellis with space between plants, so leaves dry quickly.
• Water at the base in the morning, never over the leaves at night.
• Check shoot tips and leaf undersides twice a week for thrips, aphids, caterpillars and the first mildew.
• Pull weeds and bin spent vines, which shelter aphids, thrips and virus.

Tips

1. Melon fly (Zeugodacus cucurbitae), a major pest of bitter melon overseas, is exotic to Australia but reaches the Torres Strait in the monsoon. In Queensland, report a suspect to the Exotic Plant Pest Hotline on 1800 084 881.
2. Cucumber green mottle mosaic virus infects bitter gourd and spreads on hands, tools and possibly seed. Buy seed from a reliable source, and wash hands and secateurs between plants.
3. To save seed, grow one variety not labelled F1, since bees cross varieties, and leave a fruit on the vine until it turns orange. Wash the red coating off the seeds, dry them and store them cool; they keep 2 to 3 years. Neither the seed nor its red coating is food, so keep both out of children's reach.