By Daniel Musyoka
Watermelons, botanically called Citrullus lanatus belong to the family of Cucurbitaceae, they originated in Africa.
Watermelons grow large, but how do you know when they are ready for picking? There are close to a hundred different varieties of watermelons with new hybrids being developed all the time as tastes change and markets evolve. The main types grown in South Africa are All Sweet, Crimson and Seedless varieties. With the many different types of watermelons and the many different sizes, it can be confusing. There are some signs to watch for that can indicate your watermelon is fully grown. We’ll show you how to grow and harvest watermelons here.
Watermelons prefer a hot, dry climate with mean daily temperatures of 22 to 30°C. Maximum and minimum temperatures for growth are about 35 and 18°C respectively. The optimum soil temperature for root growth is in the range of 20 to 35°C. Fruits grown under hot, dry conditions have a higher sugar content compared to those grown under cool, humid conditions. The crop is very sensitive to frost. The length of the total growing period ranges from 80 to 110 days, depending on climate.
Watermelon vines require considerable space. You can sow seeds in hills or rows. Seeds are placed on hills spaced 1m x 1m and planted 2cm deep. One-two weeks after the seedlings are established, thin to the best three plants per hill.
Plant watermelons in good, well-drained soil. The crop prefers a sandy loam soil texture with pH of 5.8 to 7.2. Clay soils do not raise a good crop. Cultivation in heavy textured soils results in a slower crop development and cracked fruits. If your soil is difficult, create raised beds by adding organic matter to improve the drainage and aeration.
Watermelons are heavy feeders. Add generous amounts of manure, compost and leaves to your garden. Work the soil well. Make sure it drains well. Fertilize with a phosphate fertilizer at planting, 3-4 weeks later, top dress with LAN for leaf development and 4 weeks after this, top dress again with NPK for fruits and flowering. You may also apply recommended foliar feeds for vegetative growth and flowering.
Micronutrients: Apply as determined by soil test. Micronutrients that should be tested for are zinc, manganese and boron.
Watermelons need adequate water for productivity, though their requirements are slightly lower than those of other vegetables. Plant stress from limited water availability will cause them to stop growing and reduce fruit size and quality. All the same, be careful not to over water. Excessive irrigation can reduce crop yields by leaching crop nutrients or promoting disease, it can also cause fruiting plants to collapse from lack of oxygen. Watermelons have extensive root systems and can obtain available ground moisture, thus reducing irrigation requirements. Check the soil moisture regularly and water as necessary before the melons start to wilt or go into stress. Allow the top 25 to 50mm of soil to dry between watering.
Plant Development and Care
Maintaining a healthy plant is the first step in disease control. This includes weeding, pruning and proper spacing to allow good air circulation, especially in wet and humid weather.
Sunlight - Watermelons need full sun and heat to grow healthy vines and big fruit.
Weeding - Watermelons are not good competitors and do not flourish if weeds shade them or compete with them for moisture and fertilizer. Remove small weeds to avoid stunting or stressing the melons. Avoid disturbing the root by cultivation as this weakens vines and keeps them from producing fruit. Rotate planting locations and use resistant varieties to avoid pest and disease accumulation.
Pruning - Remove deformed and melon fly-stung fruits. Deformed fruits result primarily from water stress and/or insufficient pollination. These fruits are removed at an early stage in order to obtain uniformly well-shaped fruits. In some instances, growers remove well-formed fruits if there are more than two or three melons already developing on the plant. Reducing the number of melons per plant, concentrates the plant’s producing power in a smaller number of fruits, thereby increasing size and perhaps quality.
Insects and Pests
Early use of insecticides is important. While constant scouting is crucial in controlling pests and diseases. Fungicides can be effective if used early. Major insect pests include aphids, melon fly and mites. Powdery and downy mildews, Anthracnose, Alternaria leaf spot, gummy stem blight and Fusarium wilt are some of the common diseases affecting watermelon. However, some cultivars are resistant to some diseases.
It is difficult to tell if a watermelon is ripe by just looking; it must be examined. Watermelons will not continue to ripen after harvest. For best quality, be sure to scout the crop daily so that melons can be picked when they are at their best. Here are indications you can look for:
• Thump it. When thumped an immature melon will give a metallic “plank” and a mature one a dull “plunk”, if the harvester can determine the difference. This method should only be used when the melons are cool; an immature melon will sound mature if it has become warm throughout.
• Look at the colour on the top. The fruit is ripe when there is little contrast between the stripes. Another indication is when the surface colour of the fruit turns dull.
• Look for the spot where the melon rested on the ground; a yellow or a cream-yellow coloured spot suggests ripeness and a white or pale green spot indicates immaturity.
• The curly tendril immediately opposite where the melon is attached to the vine will be brown and withered when the melon is ripe.