How to Grow Cucamelons in Australia

Cucamelon (Melothria scabra) is a climbing vine of the family Cucurbitaceae, grown for grape-sized fruit that look like tiny watermelons and taste of sour cucumber. Sold as Mexican sour gherkin, it is botanically neither a cucumber nor a gherkin, which have their own guides, such as Cucumber. It crops from the Top End to Melbourne's outer east, and the usual error is picking the fruit too big.

Three mistakes that matter

1. Planting out into the cold. The young stems and tendrils are easily damaged by cold, and chilled seedlings stall. Wait for the soil warmth given in When to plant, and cover plants if a late cold snap arrives.
2. Letting the vine sprawl. On bare ground the vines knot into a mat, slugs damage fruit touching the soil, and ripe fruit hides. Put up a sturdy trellis before sowing, since the vines reach 3 to 4 m.
3. Picking too big and too seldom. Fruit left past about 2.5 cm turns seedy, and hard seeds inside mean it is overripe. Pick every two or three days, which also keeps the vine flowering.

When to plant

Sow once the soil holds at least 21°C and frost has passed; seed germinates best at about 23°C. The vine starts slowly, and first fruit usually comes 10 to 11 weeks after sowing. One sowing early in the window gives the longest picking season, because a healthy vine keeps bearing until cold weather stops it.

At the cool edge of a window, start seed in pots on a seedling heat mat 4 to 6 weeks before the last frost. In frost-free gardens a vine left from the year before regrows from its tuber in spring, so a new sowing is optional.

Site, soil and setup

Rotate beds: do not follow cucumber, zucchini, pumpkin or melons, because cucurbits grown in succession pass powdery mildew from crop to crop.

Container growing

Use a pot of at least 20 L, 30 cm across and 30 cm deep, which holds one to three plants. Choose a heavy pot, or weigh it down, so a loaded vine and its frame cannot tip it over. Fill it with premium potting mix. The vine also suits a hanging basket, where it cascades over the rim.

• Watering: check pots daily in hot weather and water whenever the top few centimetres of mix are dry.
• Feeding: seaweed solution at 30 mL per 9 L fortnightly until flowering, then liquid potash at the label rate fortnightly.
• Position: full sun against a warm wall or fence, with a trellis or obelisk fixed in the pot at planting.

Sowing Seeds

Start seed in pots where spring is cool or short, and sow direct into warm soil at the base of the trellis elsewhere.

Starting in pots

• Sow two seeds per small pot, 5 to 10 mm deep, laying each seed on its side to lower the risk of rotting.
• Keep the mix warm and barely moist; seedlings appear in 7 to 21 days.
• Thin to the stronger seedling by snipping the other at soil level.
• Harden off (get seedlings used to sun and wind by leaving them outside a little longer each day) before planting.
• Plant out at 3 to 4 weeks old, beside an upright of the trellis, at the spacing in Site, soil and setup.

Sowing direct

• Sow three or four seeds per spot, 5 to 10 mm deep, then thin to the two strongest.
• Water in once and keep the surface just moist until shoots show.
• Guard seedlings from slugs and snails, which chew them at night.

Pollination

Each vine carries separate male and female flowers, so one plant can set fruit on its own. The yellow flowers are only about 4 mm across, and bees carry the pollen between them. Unusually for a cucurbit, the first female flowers open before any male flowers, so the earliest tiny fruit often shrivel.

Fruit fails to set when few bees visit, as in heatwaves or very humid weather, or after insecticide sprays. Drought stress and fleece covers left on after flowering starts have the same effect.

Hand-pollinating so many tiny flowers is slow, so save it for the first weeks of flowering or a spell with few bees.

• Work first thing in the morning, when the flowers have just opened.
• Collect pollen from an open male flower with a soft, fine paintbrush.
• Dab it onto the centre of a female flower, then repeat on the next.

Care by growth stage

Warmth, steady water and regular picking decide the crop. Weeks count from sowing and stretch in cool gardens.

Where frost kills the vine or winter soil stays wet, keep the tuber for next season.

• In frost-free gardens with free-draining soil, cut off the dead vines and leave the tuber in place under mulch.
• Elsewhere, dig carefully once the vine has died back, since tubers can sit about 30 cm deep.
• Store it in a pot of barely damp potting mix, or wrapped in shredded newspaper, somewhere cool, dark and frost-free.
• Replant in spring after the last frost, when seed would be sown.

If you prefer a conventional feed, mix a controlled release granular (NPK around 14-5-10) into the soil at planting. While vines climb, water on an all-purpose soluble (NPK around 23-4-18) fortnightly. From first flowers, switch to a fruiting and flowering soluble (NPK around 15-4-26), each at the label rate.

Pruning and Training

Cucamelon needs no routine pruning, but early training gets it onto the support, and thinning a tangle lets light and air into the vine.

• At planting, cut back any floppy top growth; stronger new shoots soon replace it.
• Set a short cane beside each young plant to lead it onto the trellis.
• Tie young stems loosely with soft twine until the tendrils grip, handling them gently.
• Where vines knot into a dense mass, cut out a few stems so light reaches the inner leaves.
• Trim off dead and yellowing leaves to keep air moving.
• Cut the vines off at ground level once they die back in autumn.

Companion planting

Cucamelon does well beside plants that hold it up or bring bees, and badly beside its own relatives.

Grows well with

• Corn, whose tall stems act as a living stake for the vine.
• Basil, whose flowers draw bees to the tiny cucamelon flowers.
• Calendula, a daisy-family flower that brings bees and the predators of aphids.

Keep apart

• Cucumber, zucchini and pumpkin, which share powdery mildew and aphid-spread mosaic viruses.
• Watermelon and rockmelon, hosts of the same mildew and viruses.

Harvesting Fruit

Pick when fruit reach the size of a large grape, about 2.5 cm long, and still feel firm.

• Twist the fruit off, or snip it with scissors, without tugging the fragile stems.
• Pick larger, older fruit too, since it suits pickles better than eating fresh.
• Pick up fallen fruit, which can self-sow in warm gardens.
• Leave fruit unwashed until you use it, since surface moisture speeds spoiling.

Store fruit in the fridge in a perforated bag or a lidded container, where it keeps for about a week. Eat it whole in salads, as a snack or in stir-fries, or make pickles. Vegetables need enough vinegar to be safe from botulism, so follow a tested Australian recipe and keep pickles in the fridge unless pasteurised.

Common problems

No seedlings after three weeks. Seed rots in cold, wet mix; sow again once the soil is as warm as When to plant describes. Read more

Thin, slow seedlings. Young cucamelons look scrawny and start climbing later than cucumbers do; they soon take off once the weather warms.

Tiny fruit that shrivel and drop. Early on, female flowers open before any males can pollinate them; later drops usually trace to few bees or a dry spell, as Pollination explains.

Pests and diseases

Cucamelon draws fewer pests than cucumber, but check that any spray label covers cucurbits, and keep to its withholding period.

Powdery mildew (Podosphaera xanthii). White powdery spots start on older, shaded leaves, often on ageing vines late in the season, and spread fast in humid weather without rain. At the first spots, spray 1 part full-cream milk to 10 parts water. Read more

Melon aphid (Aphis gossypii). Small green to almost black insects cluster under young leaves and on shoot tips, curling the leaves. Hose them off with a strong jet of water, or spray insecticidal soap on leaf undersides. Ladybirds and lacewings eat them. Read more

Zucchini yellow mosaic virus (Zucchini yellow mosaic virus). Leaves show yellow and green mosaic patterns and distort, and plants stunt and set little fruit. Aphids spread it within a minute of feeding, so sprays cannot stop it. Pull and bin infected plants. Read more

Garden snail (Cornu aspersum) and slugs. They chew seedlings overnight and damage fruit resting on the soil. Hand-pick them after dark, and keep vines off the ground on the trellis. Read more

Prevention first

• Grow on a trellis in full sun with room for air, and water at soil level in the morning.
• Check leaf undersides and shoot tips twice a week, and treat as the entries above describe.
• Pull self-sown cucurbit seedlings and old vines, which carry aphids and viruses into the next crop.
• Spare the bees: if you must use pyrethrum, spray late in the day, away from open flowers.

Tips

1. Grow the vine over an arch or pergola, where the fruit hangs down in plain sight and picking is quick.
2. To save seed, leave a few of the ripest fruit, often those that have fallen, for a few more weeks. Scoop out the seeds, soak them in water for a few days, and keep those that sink.
3. Dry saved seed on a screen until brittle, then store it airtight in a cool place. Cucamelon does not cross with cucumbers or melons, so a mixed vegie patch does no harm.