How to grow Watermelon in New Zealand

Where to plant

Give watermelons the hottest, sunniest spot you have, sheltered from wind and salty air. They thrive in light, sandy soil and struggle in heavy clay; on clay, add compost and gypsum and plant on a raised mound. Allow plenty of room for the vines to sprawl, or train compact types along a trellis and support the fruit in slings.

They need at least 80 to 100 frost-free days of warmth to mature. In a cool, cloudy, wet summer they take longer, so choose quick varieties in cooler areas.

• Northland, Auckland, Waikato and the Bay of Plenty: plant from October or early November into the warmest spot.
• Central North Island: marginal on the plateau. Plant in December under cover or on black plastic in the warmest spot.
• Wellington, Nelson and Canterbury: plant in November and December, and grow a compact, early variety such as Sugar Baby.
• Otago, Southland and Central Otago: plant in December after the frosts, ideally in a tunnel house or glasshouse. Central Otago's hot summers can ripen small varieties outdoors.

Choosing a variety

• Sugar Baby: compact and sweet, with small fruit that suit smaller gardens and cooler districts.
• Crimson Sweet: a large, round, juicy variety with deep red flesh, best where summers are long and hot.
• Gold in Gold: pale yellow fruit with golden stripes and orange-gold, sweet flesh.

How to plant

1. For a head start, sow seed indoors in biodegradable pots in warmth a few weeks before planting out. Seed may not germinate below about 15°C.
2. Plant seedlings when they have about five leaves and the weather is warm, at least 90 cm apart on low mounds of soil enriched with compost.
3. Set the plant so its stem sits slightly above the surrounding soil, which helps prevent stem rot in moist soil.
4. Water in, then mulch around the plant, keeping the mulch off the stem.
5. Cover young plants overnight with frost cloth or a cloche if a cold night threatens.

Watering and feeding

Keep plants well watered while they grow, with a deep watering once or twice a week depending on the weather. Water at the base, not over the leaves. Ease off as the fruit approaches ripeness, which helps concentrate the sweetness.

Feed regularly with a liquid seaweed or fish fertiliser, especially once flowering begins.

Pruning and care through the year

• Watermelons rely on bees to carry pollen from male to female flowers; only the female flowers, with a small swelling behind them, become fruit.
• Plant borage or lavender nearby, and avoid spraying insecticides while the plants are flowering.
• If bees are scarce, hand pollinate early in the morning with a small paintbrush, from a male flower to a female flower.
• Sit developing fruit on a board or straw to keep it off damp soil.

Harvesting

Watermelons are tricky to judge. The best sign is the curly tendril nearest the fruit: it should be brown and dry, not green. The patch where the melon sits on the ground turns creamy yellow, and a ripe melon gives a deep, hollow sound when knocked. Cut the fruit off with a short stalk.

Growing in pots

Compact varieties such as Sugar Baby can be grown in a large container in full sun, with a frame or trellis to climb and slings to support the fruit. Water often, as pots dry out quickly in summer.