Watermelons are native of African regions, and as such they are susceptible to frost. They require a long growing season, with high temperatures (between 70 and 80°F in daytime, and 65-70°F at night). Watermelons should be grown in semi-arid areas (they grow well in semi-humid areas as well, but their leaves will be more susceptible to diseases).
Fertilizing is generally different depending on your soil: try asking your local agricultural department or local greenhouses first.
Generally, a good starting point is one pound of actual nitrogen, two pounds of P2O5 phosphorus and three pounds of K2O potash every 1,000 square feet.
If your soil is particularly bare, consider a 25% increase in nitrogen, as a sidedress application when vines begin to run. The ideal pH range for watermelons is from 6.0 to 6.8, so don't use too much lime juice, if at all.
As a timing strategy, nitrogen should be applied until flowers form, then switch to a high phosphorous and potassium fertilizer (otherwise, nitrogen would promote the growth of leaves instead of fruits).
Watermelon plants can be purchased from your local greenhouse and started in pellets or peat pots for about a month. You can then plant them, but be sure to do so when the temperature is already fairly high (certainly you should not expose the plant to frost, or it will likely die).
Choose a well-drained soil, and apply lots of compost, leaves and manure.
Mulching is very important to conserver moisture, suppress weed growth and guarantee an earlier harvest due to a warmer soil.
Watermelon seeds should be sowed in hills or rows: three to four seeds per hill, spacing the hills eight to ten feet apart. The rows should be spaced ten feet apart or more, if you have room
The plants will take around 90-100 days to maturity, for regular watermelons, and 80-90 for baby varieties.
Watermelon plants are particularly sensible to beetles, mites, Fusarium wilt, aphids, gummy stem blight, alternaria leaft spot and anthracnose, but some newer cultivars are resistant to several diseases, so ask your local botanist or greenhouse if they are selling disease-resistant plants and opt for them.
A good method to avoid fusarium wilt is to rotate crops each year, planting disease-resistant varieties, and sowing radishes in your watermelon patch (they deter cucumber beetles, which are responsible for transmitting the disease.
The first set of flowers to develop on the vines are male (pollen-bearing), but fruits can only develop from female (or pistillate) flowers. Thus, the earliest blossoms will not fruit!
For pollination, honey bees are one of the most effective aids, and you should ensure their presence in the vicinity of your plant.
Watermelons should be picked from the plant when the ground spot (underbelly) changes from green/white to a creamy yellow. The tendrils near the stem, at this point, will dry and turn brown.
Try patting your finger on the fruit, you should hear a muffled tone, which indicates that the fruit is ripe. On the other hand, unripe fruits will thump with a metallic ringing tone.