Planting Watermelon: Soil Warmth, Spacing and How Much Room One Plant Needs

Plant for the space it will fill, not the plant in your hand

A watermelon transplant is a few inches across and looks like nothing. By midsummer the same plant has runners six to ten feet (2 to 3 m) long in several directions and it will have covered a bed you thought was generous. Almost every disappointing watermelon crop in a home garden comes from planting too many plants too close, so decide the footprint before you decide the number of plants.

Crowding costs you in two ways. The leaves close into one canopy, the humidity inside never falls, and anthracnose and powdery mildew move through the planting weeks earlier than they otherwise would. And with fewer square feet of leaf per fruit, the sugars simply are not there: a crowded plant makes fruit of ordinary size that taste watery.

Two well spaced plants outyield five crowded ones and the fruit taste better. If the space is genuinely limited, grow a small type or grow it vertically rather than squeezing standard plants together.

Wait for the soil to reach 65F

Watermelon is the least cold tolerant of the common summer crops. Set out into soil below about 65F (18C) it sulks, the roots do not extend, and a check at this stage costs two weeks that the plant never gets back. Waiting an extra fortnight for warm soil almost always produces an earlier harvest than planting into cold ground, which is counterintuitive enough that most gardeners have to learn it once.

Measure rather than guess. Push a soil thermometer four inches (10 cm) down at mid morning for three consecutive days and take the lowest reading. Warm afternoons over cold nights mean nothing to a root system, so the run of readings is what matters. If you are impatient, warm the ground instead of planting early: black plastic laid down two weeks ahead lifts soil temperature several degrees and it is the single most effective thing you can do in a cool climate.

1. Lay black plastic early Put black plastic or a woven black mulch down two to three weeks before you intend to plant, tucking the edges in. It raises the soil several degrees and holds the warmth overnight.
2. Plant through the mulch Cut a cross in the plastic and plant through it rather than lifting it. The mulch then keeps weeds down all season and stops rain splashing soil onto the leaves.
3. Use a cover for the first fortnight A cloche or row cover over each plant for two weeks after planting protects it from wind and cold nights. Take it off as soon as flowers appear so pollinators can get in.
4. Do not plant an overgrown transplant Watermelon resents root disturbance and a pot-bound plant never recovers properly. Two or three true leaves is the right size; a plant already running is worse than a smaller one.

Soil, drainage and the base dressing

Watermelon wants a warm, free draining, sandy soil with a pH between 6.0 and 6.8, and it will tolerate poorer ground than most fruiting crops as long as the drainage is right. Wet feet are what kills it: on heavy ground the roots suffocate in the first thunderstorm after planting and the plant either dies outright or grows on stunted all season.

Keep the nitrogen modest. Heavily fed watermelon makes enormous leaves and long runners and sets almost nothing, and if it does set, the fruit are bland. Aim for moderate nitrogen at planting, then move towards potassium once the fruit start filling, because potassium is what sugar and flesh texture actually depend on.

Do not apply lime and compost on the same day. Leave several days between them so the lime does not drive nitrogen off the compost as ammonia.

Build a hill and plant high

Plant on a raised hill or a ridge eight to twelve inches (20 to 30 cm) above the surrounding ground, three feet (90 cm) across. The hill drains, it warms faster in spring than flat ground, and it keeps the crown of the plant out of standing water after heavy rain. On genuinely sandy, fast draining ground you can plant flat, but on anything with clay in it the hill is not optional.

Set the rootball level with the surface of the hill or a fraction proud of it, never buried. A watermelon planted deep sits with its stem in damp soil and rots at the collar, and collar rot is the most common way to lose a plant in the first month.

1. Water the hole first Fill the planting hole with water and let it drain away completely before you set the plant. Watering the hole rather than the surface puts the moisture where the roots will go.
2. Handle the rootball whole Do not tease out the roots the way you would with a tomato. Watermelon roots break easily and the plant checks badly; slide the rootball out intact and set it down.
3. Two plants per hill if you like Setting two plants on one hill and thinning to the stronger after a fortnight is good insurance in cold springs. Do not leave both: a doubled hill halves the fruit size.
4. Water in and then hold back Water generously at planting, then keep the soil merely moist for the first two weeks. Sodden soil around a plant that is not yet drawing water is how you get root rot.

Where you plant it matters as much as how

Full sun, at least eight hours of it, and as much heat as your garden has. A watermelon in partial shade will grow perfectly good foliage and produce fruit that never sweeten, because sugar accumulation is driven by light on the leaves in the last three weeks before harvest. A south facing slope, or a bed against a wall that reflects heat, is worth more than any amount of feeding.

Keep it away from ground that has grown squash, cucumbers, melons or watermelon in the last three years. The soil-borne diseases of the cucurbit family build up in a way that no amount of good management overcomes, and fusarium wilt in particular will take a healthy plant down in a week just as the fruit are filling.

1. Give it the hottest spot Choose the sunniest, most sheltered, best drained corner you have. Watermelon repays a good position more visibly than almost any other crop in the garden.
2. Plan the runners' direction Plant so the runners can travel away from paths and other crops. Runners walked on or repeatedly moved lose roots at the nodes and set fewer fruit.
3. Record the position of the bed Write down which bed grew watermelon and which grew any other cucurbit each year. Three years of that record is what keeps the rotation honest when the seasons blur together.
4. Shelter it from wind A young watermelon rocked by wind loses its fine roots and stalls. A temporary windbreak on the exposed side for the first month is worth the trouble in an open garden.