To grow sweet tasting watermelons the plants must have plenty of sun, warm temperatures, lots of nutrients, room to grow and a good supply of water.

Water Needs

Moderate

Watermelons need to produce a lot of leaves to make enough sugar to produce sweet fruit. To do this they need a constant supply of water. The best way to water them is with a drip system or soaker hose, as they don't like having wet leaves (this encourages fungus disease).

Fertilizer Needs

Heavy

Not surprisingly for such a large and vigorous plant, watermelons are fairly hungry feeders. Melons don't need a lot of nitrogen as it encourages leaf growth at the expense of fruit. They do like phosphorus (give them colloidal phosphate) and potassium (wood ashes), as well as boron and magnesium.

Thinning, after sowing

18 inch(es) apart, after sowing, 1 time

When all of the seeds have germinated, thin the plants to their final spacing. 

Side Dressing, after sowing

Mulch, 2 inch(es), after sowing, 1 time

Mulch helps to conserve moisture in the soil in hot weather. Don’t apply it until the soil has warmed up however.

Pruning, when 12in tall

when 12in tall, 2 times

The seedlings should be pinched back twice, so they produce a bushy plant with four growing tips. These are then allowed to grow and flower.

Side Dressing, at early fruit set

Compost tea, 2 cup(s) per plant, at early fruit set, every 3 weeks

If the soil isn't very fertile you might want to feed the plants with compost tea regularly once the flowers start to set fruit.

Side dressing should be done cautiously, as too much nitrogen may merely vegetative foliage growth at the expense of fruiting.

Watering, before fruit set

Water, 0.5 gallon(s) per plant, before fruit set, 2 times a week

Melons are quite shallow rooted so they need a constant and even supply of water. Watering also depends on your local weather; don't water if it's raining, or water more frequently if it's dry. Just be sure to keep soil moist but never soggy for the best crop. The best way to know how much moisture is in your soil is to feel 2" below the soil line. If it's dry, water.

Thinning, during fruit production

during fruit production

Ideally you want one fruit to develop simultaneously on each of the four shoots. If one shoot produces a fruit before the others, it should be pinched off, as it may prevent the plant from producing any more.

Once you have four fruits developing, you should pinch off any others that form. The fewer fruit you allow to develop, the bigger they will all get.

Watering, during fruit production

Water, 2 gallon(s) per plant, during fruit production, 1 time a week

Melons prefer a constant and even supply of water, especially when the fruits are sizing up. They should get all the water they can use at this time (in very hot weather this could be as much as 2 gallons a day). Ideally it should be lukewarm so it doesn’t cool the soil.

When the fruits have reached full size you should ease up on watering, otherwise the fruit may split.

The best way to water Melons is with a drip system, as they don’t like having wet leaves (this encourages fungus disease).

Support

Optional

Watermelons are most often allowed to run over the ground, but if space is limited they may be trained up a trellis, just as you would cucumbers (though they don't climb very well). However the fruits are much heavier than cucumbers and will need support (slings made from old panty-hose work well.) Also the trellis itself will have to be pretty sturdy.

If a fruit is growing on the ground you can insert a board, tile or stone underneath it, to keep it off the soil.