Watermelon

Watermelon (Citrullus lanatus) is a sprawling, vine-forming annual in the Cucurbitaceae family, originating from the Kalahari Desert of southern Africa. It produces large, sweet, juicy fruits with red or pink flesh, ranging from compact 3 kg mini types to giants exceeding 10 kg. It thrives in long, warm growing seasons with ample sunshine, making it a challenging but rewarding crop in Nordic climates. A greenhouse or a warm, sheltered outdoor site with fleece cover is strongly recommended for successful fruiting.

Growing

Start seeds indoors 4 weeks before transplanting. Sow one seed per cell at 1 cm depth, keeping the growing medium consistently moist and the environment at around 24 °C for optimal germination. Germination takes 7–14 days. Grow seedlings on at 20–22 °C with strong light. Transplant outdoors only once all frost risk has passed and night temperatures stay reliably above 15 °C. Space plants 100 cm apart in rows 150 cm apart. Plant into warm, well-drained, fertile soil in the sunniest spot available, ideally under fleece or in a greenhouse.

Care

Water deeply and consistently throughout vine development and early fruit set — watermelons need substantial moisture during growth. Once fruits begin to colour and ripen, reduce watering significantly to concentrate sugars and improve flavour. Feed with a potassium-rich fertiliser once fruits are setting. Train vines and remove excess lateral shoots to direct energy into 2–4 fruits per plant. In cooler climates, lay black plastic mulch to warm the soil and retain heat. Protect with horticultural fleece during cool spells.

Harvest

Each plant can yield 2–4 fruits. Early varieties like Blacktail Mountain mature in 70–75 days from transplant; standard varieties take 80–85 days. Signs of ripeness: the tendril nearest the fruit dries and browns, the underside of the fruit turns from white to creamy yellow, and the skin loses its shine. Tap the fruit — a deep, hollow thud indicates ripeness. Harvest by cutting the stem with a sharp knife. For storage, pick slightly before full ripeness so fruits can finish ripening off the vine.

In the kitchen

Cut watermelon is best enjoyed fresh, eaten raw in slices or cubed into fruit salads. The sweet flesh pairs well with salty cheeses such as feta, fresh mint, lime juice, and chilli flakes. Blend into cold gazpacho or refreshing drinks. The rind can be pickled or candied. Seeds can be roasted and salted as a snack.

Storing

Whole, uncut watermelons store well at room temperature (around 18–21 °C) for up to 2 months if harvested slightly underripe and kept in a cool, dark place. Once cut, wrap tightly in cling film and refrigerate for up to 5 days. Cut cubes can be frozen for use in smoothies, though texture softens considerably after thawing.