Light and heat are not negotiable
Melons need full sun and sustained warmth. Reservoir at 22–25 °C (72–77 °F), air temperature 25–30 °C (77–86 °F), and as much light as you can provide for 14–16 hours.
Below 20 °C (68 °F) growth is slow; below 15 °C (59 °F) it stops. Fruit will not ripen properly without sustained heat and light, and a melon that does not ripen is a melon you cannot eat.
If you cannot provide those conditions, the honest advice is to grow something else and come back to melons when you have a greenhouse.
Choose small-fruited varieties
A full-size watermelon will not ripen in a bucket. Look for varieties described as icebox or personal-size, typically 1–3 kg fruit, or small melons like charentais types at 500 g to 1 kg.
Dwarf and bush-habit varieties exist and suit containers far better than vining types. The catalogue carries plant height and spacing for each entry — filter before you commit.
Training and fruit support
Train the vine up a strong vertical support and remove side shoots beyond the fruit you intend to keep. A melon plant allowed to run freely will set six fruit and ripen none of them properly.
Limit the plant to two or three fruit. Pinch out the growing tip two leaves beyond the last fruit you are keeping, so energy goes into ripening rather than more vine.
Melons are heavy. Support the fruit itself with a sling — a net, a piece of cloth, a stocking — tied to the frame. Otherwise the stem snaps as the fruit swells.
Pollination by hand
Melons have separate male and female flowers, like squash, and no bees indoors. Identify a female by the swelling at the base of the flower, pick a male in the morning when both are open, and press the centres together.
Do it early — melon flowers open for a single morning. If a female flower closes unpollinated, the small fruit behind it yellows and drops within days.
Saving seed
Melons cross within their species: Cucumis melo includes cantaloupe, honeydew and charentais, and they will all interbreed. Watermelon is a different genus and does not cross with them.
Isolation is 500 metres (1,640 ft) to a kilometre, or bagging and hand pollination. Seed is embedded in the central cavity — scoop it out, rinse free of pulp, and ferment for two days before drying, the same process as cucumber and tomato.
Seed from a fully ripe fruit only. An under-ripe melon gives seed that will not germinate.