Watermelon need 70-90 days to fully mature, so if your summers are short, start seeds indoors up to 2 weeks before your last frost date. Wait until the soil is at least 70oF to help seed germination (or use a heat mat). Watermelons are heavy feeders, so amend your soil with compost, seaweed, manure, or a fertilizer with a higher nitrogen level. Your soil should be on the slightly acidic side. Pile your soil into hills, and plant 1-2 seeds in the top of each hill. Each plant needs 2-6’ around it to spread vines - as a side note, watermelon vines can be trellised, but the fruit will need support as it gets heavier (like old pantyhose), or let the vines stay on the ground.
Watermelons are 90% water, so consistent watering is key, make sure that your soil is well draining, and water 1-2” per week - be sure to only water at the base of the plant. Watermelon flowers come in male and female flowers, like other squash varieties. Male flowers appear first and will be just a normal flower. Female flowers will have a tiny melon at the flower base. If there are nutrient problems or watering issues, blossom-end rot or weirdly-shaped fruit can occur. Blossom-end rot will make the fruit unusable, and the affected fruit should be removed before disease sets in. Since watermelon is a relative of squash, it can be susceptible to squash vine borers.
Try to keep the fruit off the soil by putting cardboard or mulch underneath. Knowing when a melon is ready takes practice. A few signs we look for:
-the little tendril CLOSEST to the watermelon stem will turn brown & shrivel up
-the light mark on the bottom of the melon is yellow/cream colored (NOT white)
-the stripes of the watermelon should start to blend together -gently knock on the watermelon, if it sounds hollow, it should be ready
Watermelons do not sweeten off the vine. When ready to harvest, use a knife and cut the stem closer to the fruit.