The intercropping of sweet potatoes and watermelons under greenhouse sheds is an innovative agricultural technique that maximizes land use efficiency and resource utilization. This method not only boosts yields both above and below ground but also makes full use of the plastic film, reducing costs and minimizing white pollution. By combining the nutrient-rich properties of watermelon with the high-yield potential of sweet potatoes, this system optimizes the use of limited land, light, and heat resources, enabling a double-crop system in suburban areas like Beijing.
First, site selection and shed construction are crucial. Sweet potato cultivation under protection should be rotated for at least three years to avoid soil-borne diseases and nutrient imbalances. Sandy loam soil is ideal, while land used for sweet potatoes for more than two years requires disinfection and balanced fertilization. It's best to choose fields previously planted with wheat, corn, or vegetables, ensuring good drainage and irrigation. The shed design should be adapted to the local topography, typically 4 meters wide and 1.2–1.5 meters tall, with bamboo supports spaced every 1–1.2 meters. The plastic film must be tightly secured, with edges buried 15 cm deep to prevent wind damage and ensure stability.
Next, variety selection and site preparation are key. For mulched watermelons, sorghum varieties are often used, and seedlings are transplanted into small arch sheds or directly under improved mulch films. Early-maturing, high-yield varieties such as Yushu 10 and Jingshu 2 are preferred to maximize light and temperature use. Before covering the shed, the field is leveled and deeply tilled, with 2,000 kg of organic fertilizer and 40–50 kg of sweet potato-specific fertilizer applied per acre. Disinfection and pest control measures are essential to protect the crop. Ridge spacing is set at 75 cm, with a height of 25 cm and a flat top 20–25 cm wide. The mulch is carefully placed over the ridges to ensure even coverage and efficient watering.
Planting timing is critical. Sweet potato seedlings are usually planted in mid to late April, either directly in the furrow or after transplanting. Watermelon seedlings grow slowly, so shading helps protect the sweet potato plants. Seedlings are transplanted to sunny, sheltered spots, watered, and covered with plastic. Within 3–7 days, they adapt well, improving survival rates and promoting healthy growth. Sweet potatoes are planted using either sloping or direct methods, with density adjusted based on variety and harvest time. Watermelon rows are spaced widely, allowing one row of sweet potatoes between them. Plant spacing is reduced to 15–20 cm, achieving a density of 2,000–2,500 plants per mu. Seedlings are placed 30–35 cm from watermelon plants, breaking the film before planting to ensure proper root development.
During management, the two crops coexist without interference. After harvesting watermelons, their removal protects the sweet potato seedlings. Regular weeding and mulch maintenance are necessary. In early stages, maintaining warmth and moisture promotes seedling growth. Temperatures inside the shed should stay below 35°C for the first few days, with no ventilation to preserve humidity. If temperatures drop below 13°C, the shed should be sealed or heated. When it exceeds 35°C, side vents are opened to prevent cold drafts. As temperatures rise, the shed is gradually ventilated, and the film is removed once the outside temperature stabilizes around 30°C. Fertilizer is applied in holes along the ridges before the sweet potatoes are lifted, with 30–40 kg of compound fertilizer per mu. Watering ensures even distribution and prevents drought. Pest control, especially for cabbage caterpillars, is important during this phase.
After 65–70 days, early-maturing varieties can be harvested, packaged, and sold according to market demand. Soil preparation and treatment are done promptly to ensure continuous planting. This integrated approach not only improves productivity but also enhances sustainability and economic returns for farmers.
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