In the UK, it is not possible to successfully grow sweet melons outside. This is where a polytunnel really comes in handy – in increasing the range of crops that it is possible to grow in our climate. Melons require a temperature of around 25 degrees C. throughout the growing season – an unheated polytunnel may provide this in the south, though further north a degree of heating in the polytunnel will likely be required to grow melons equal to those that can be grown in warmer climes.
Melons seeds require around 16 degrees C. to germinate, and since melons have a relatively long growing season, seeds should usually be sown indoors before the end of April. Indoors sowing will prevent any problems which could arise from a late cold-snap. You should also make sure that the area you prepare for your melons in your polytunnel is deep, rich and fertile, retains water well and yet is free draining. Add organic matter and make sure the area is free of weeds and there is no competition.
Melons like a humid environment and so will grow well with cucumbers and other such plants, though since you will be keeping the tunnel closed, with high humidity, it is best not to plant melons with crops that will not do so well in those conditions.
Plant your melons out in May or June as long as the soil in your tunnel has warmed sufficiently. Be sure to plant shallowly as if the plants are too deep they tend to rot. Always take care to water the roots and not the stems or foliage. Keep the roots consistently moist and never allow the soil surrounding your melon plant to dry out too much.
To maximise yield, tip off the growing tips of your melons to create side shoots. Using supports, train each of the four strongest growing shoots in a different direction. It is best to retain heat and humidity by keeping the tunnel closed until the flowers begin to form. When melons are in flower, however, you must make sure that you open the tunnel to allow insects to pollinate your plants. Sowing plants to attract pollinators nearby will help make sure that this occurs.
When the first fruits start to form, this will inhibit the growth of further fruit. When fruits are around 2cm in size, add a good quality organic feed that is rich in potassium to encourage good fruit formation. To ensure that the plants have enough energy to grow fruits to maturity, thin the fruits to 2-4 melons per plant when they are around 2.5cm in diameter. Nip back fruiting shoots to two or three leaves beyond the fruit to make sure the energy goes into fruit not foliage. As the fruits grow, place them on a tile, slate or piece of wood to stop them from rotting on the soil.
Your nose will tell you when your melons are ready to harvest. You will be able to tell when it is time to cut melons off the stalk when cracks start to appear near the stem – the telltale sign however, is the sweet, melon smell which will come from the ripe fruit.