What actually makes a watermelon sweet
Magnesium does not sweeten watermelons. Epsom salts correct a confirmed magnesium shortage and do nothing more, so the old habit of watering them on every month can be dropped. Use magnesium only where a soil test or older leaves yellowing between green veins point to a shortage, at about one tablespoon in 4.5L of water. Sweetness is built over a long hot season by healthy leaves feeding fruit in full sun, and what a gardener controls is the variety, the length of the warm run and how water is handled in the last fortnight.
Ease off the water as the fruit finishes
This is the one harvest trick that holds up. Once a melon has reached full size and the skin has dulled, cut watering back for the last seven to ten days, so less water dilutes the sugars already in the fruit and the flesh is less likely to split. Keep the vine well watered before that point, because a plant that has been dry all season will only carry small fruit.
How to tell a watermelon is ripe
Read three signs together rather than waiting for the vine to die off, which usually means the melon has sat too long. The tendril nearest the fruit stem turns brown and dry. The ground spot where the melon rests turns from white to creamy yellow. A dull, hollow note when you tap it is a useful third check, although some varieties never sound hollow.
When to sow across Australia
Watermelons need a long warm run, roughly 80 to 100 days, and full sun. Sow direct once the soil holds 20°C at about 5cm deep, because the seedlings resent root disturbance.
- Tropical, the Top End, far north Queensland and the Kimberley: the dry season, April to May and August to early September.
- Subtropical, south-east Queensland and the NSW north coast: September and October.
- Arid and semi-arid, central Australia, western Queensland and the inland: late September to early November, where the long dry heat suits them.
- Temperate, coastal NSW, Victoria and the southern districts: October to December, with the earlier sowings finishing best.
- Mediterranean, Perth, Adelaide and the south-west of WA: the same months, and the dry summer suits them if the vines are watered deeply while they run.
- Cool and alpine, Tasmania, the ACT and the tablelands: marginal, so start in pots undercover in October, plant out after the last frost and choose a small bush type for the warmest bed you have.
These are broad groupings, so gardeners between two zones should lean on local frost dates.
Soil and bed preparation
Watermelons want free draining soil, so work compost, aged manure and a little blood and bone through the bed a few weeks ahead. Coconut coir helps a sandy bed hold moisture: keep it to a modest share of the top 20cm and use a buffered or well rinsed product, since unrinsed coir can be salty. Potassium supports fruit quality and the movement of sugars, but most established beds hold enough, so test before adding more, and put banana peels in the compost rather than under the plant.
Sowing, spacing and pollination
Sow 2cm deep, two or three seeds per station, and thin to the strongest. Allow 1m to 1.5m between plants and about 2m between rows. Bush types can go at 1m and suit a small bed, so pick a variety to match your space from the watermelon seeds range. Mulch straight away, 5cm to 7cm thick, leaving the soil above each seed clear until the seedling is up. Every fruit depends on bees carrying pollen from male flowers to female ones, so keep flowers nearby and avoid spraying while the vines are in flower.
Watering and feeding
One common mistake is watering too lightly. Water deeply two or three times a week rather than a little each day, and water at the base early in the day so the leaves dry, which helps keep powdery mildew in check. Alternate a seaweed concentrate and a fish emulsion every two weeks: seaweed works as a biostimulant, while the fish emulsion supplies nitrogen and phosphorus. Ease off the nitrogen once fruit has set, or the vine will put its energy into leaf rather than melons.
Companions, and what to keep apart
Alliums such as onions, spring onions and garlic make a useful border, and flowers and herbs bring in the bees the crop depends on: marigolds, lavender, dill and oregano all work. Nasturtium works too, but it self seeds freely and has naturalised in parts of Australia, so deadhead it and keep it away from bushland and creeks. Mint is best sunk in a pot, or it will run through the bed. Keep watermelons away from other cucurbits and rotate the bed each year, not because they compete for nutrients but because cucurbits share pests and diseases. One cucurbit in a bed is fine, which is why a Three Sisters planting works with a single vine.
Related growing guides
- Pumpkin Growing in Australia: Timing, Space, Pollination, and Curing for Storage
- Water-wise gardening: getting more from every drop in a dry climate
- When the heat sets in: vegetables that thrive in an Australian summer
Ready to plant? Browse our watermelon seeds.