(1) Seedlings are typically started right after the beginning of autumn, with the latest planting date being no later than August 15th. If the nursery is delayed, it will push back both the planting and fruiting periods, which can negatively affect the watermelon's ripening process. Before sowing, watermelon seeds are first soaked to enhance germination, especially since the temperature is now suitable. After soaking, the seeds are covered with mulch, and a new plastic rainproof membrane and shade net are installed. The shade net is put up at 8 a.m. on sunny days and removed at 4 p.m., but not used on rainy days. Common seedling diseases include blight and damping-off. To manage these, a mixture of chlorothalonil (600 times diluted) and mancozeb (1000 times diluted) can be sprayed every 5–7 days, sometimes up to 2–3 times. At the seedling stage, pests like larvae, diamondback moths, and noctuid moths may appear. These can be controlled using a 1500-fold dilution of dimethoate mixed with an insecticide. Seedlings are ready for transplanting when they have 3 leaves and 1 heart, usually around 20–25 days old.
(2) Soil preparation is crucial for growing watermelons in a greenhouse. The soil should be plowed, leveled, and ridged, ensuring good drainage. The ridge width should be between 2.5 to 3.0 meters, with ditches 35 cm wide. A nutrient ditch 50 cm wide and 40 cm deep is prepared, where 3,000 kg of well-decomposed organic fertilizer, such as pig manure, is applied per 667 square meters. Then, 75 kg of NPK fertilizer is added, mixed into the soil, and the ditch is covered. Additionally, 1500 kg of composted manure is spread over the field, as watermelon vines can develop adventitious roots that absorb nutrients from the soil, supporting overall growth.
(3) Autumn watermelon planting usually occurs at the end of August or early September. About 800 rows are planted per 667 square meters, directly on the nutrient zone. When planting, the seedlings are carefully removed, and the planting depth should be slightly deeper than the original soil. After planting, the area is watered immediately, and a herbicide like Dore is sprayed—100 ml per 667 square meters diluted in 50 kg of water—to prevent weed growth. The area is then covered with plastic film. To protect against heavy rain, the greenhouse's top cover is pulled down right after planting.
(4) Optimal temperatures for post-planting care range between 25–30°C. If temperatures drop too low, growth slows; if too high, virus diseases may occur. Nighttime temperatures should not fall below 18°C, and daytime temperatures must stay under 32°C. From late August to early September, nighttime temperatures begin to drop, so the greenhouse films are closed at night. On sunny mornings, ventilation starts at 8 a.m., and the sides are closed at 4 p.m. As temperatures continue to decline, morning ventilation is gradually delayed, and evening closures are advanced. About a week after planting, the vines are laid out, and two main vines are selected. Since there are no pollinators in the greenhouse, artificial pollination is necessary. This should be done between 6 a.m. and 10 a.m., preferably using the second female flower. After pollination, the vine tip is pinched to redirect nutrients to the fruit. When the melon reaches about 1 kg, side branches are pruned. The shape and size of the melon are fixed during this phase. Once the melons are mature (usually 6–7 days), they are harvested. During the growing period, two rounds of fertilization (for vine extension and fruit swelling) and two rounds of watering are needed.
(5) Watermelons generally take about 40 days to mature after flowering, which is 10 days longer than those grown in open fields. Autumn watermelons are more durable and can be stored at room temperature for 15 to 20 days after harvest.
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