The intercropping of sweet potatoes and watermelons under greenhouse sheds is a highly efficient agricultural technique that maximizes land use and resource efficiency. This method not only boosts yields both above and below the ground but also enhances the dual use of plastic mulch, reduces costs, and minimizes white pollution. By effectively utilizing the nutrients from watermelon fertilization and irrigation, sweet potatoes can maintain high fertility, leading to greater productivity and better land and light resource utilization. This system also supports double-season cultivation of sweet potatoes in the suburbs of Beijing, making it an ideal approach for sustainable farming.
First, site selection and shed construction are crucial. Sweet potato cultivation in protected environments should ideally be rotated with crops like wheat, corn, or vegetables for more than three years to avoid soil-borne diseases and nutrient imbalances. If the land has been used for sweet potatoes for over two years, soil disinfection and balanced fertilization are necessary, along with increased organic matter. The chosen area should have good drainage and irrigation systems, and the shed orientation should be determined based on local topography to reduce wind damage and improve working efficiency. A typical four-meter wide shed with a height of 1.2–1.5 meters is suitable, with bamboo poles spaced every 1–1.2 meters. Plastic film must be tightly secured, with edges buried at least 15 cm deep for stability.
Second, variety selection and field preparation play a key role. Mulched watermelons are often grown using sorghum, with seedlings transplanted into small arch greenhouses or directly under improved mulch films. Watermelon and sweet potato intercropping follows similar techniques as traditional melon fields. Early-maturing, high-yield varieties such as Yushu 10 and Jingshu 2 are recommended to make the most of available light and temperature resources. Before covering the shed, the field is leveled and deeply tilled, with 2,000 kg of organic fertilizer and 40–50 kg of special sweet potato fertilizer applied per acre. Disinfection and pest control measures are essential to ensure healthy growth. Ridge spacing is set at 75 cm, with a height of 25 cm, and the entire ridge is covered with mulch to ensure even watering and drainage.
Third, planting timing and methods are critical. Seedlings are typically planted in mid to late April, with watermelon seedlings placed first in the furrow before sweet potato planting. Watermelon has a slow growth phase, so shading helps protect young sweet potato plants. After transplanting, seedlings are watered and covered with plastic to promote adaptation. Sweet potatoes are usually planted using either a sloped or direct method, depending on the variety and harvest time. When intercropped with watermelons, sweet potato density is adjusted based on watermelon row spacing, with 15–20 cm between plants, resulting in about 2,000–2,500 plants per mu. Watermelon mulch is carefully broken, and sweet potato seedlings are placed 30–35 cm away from watermelon plants, ensuring proper spacing and moisture retention.
Fourth, management and harvesting require careful attention. During the early stages, no interference with watermelon is needed, but after harvesting, watermelon vines are removed promptly to protect sweet potato seedlings. Regular weeding and mulch maintenance are important. In the early stage of heat preservation, maintaining moisture promotes healthy growth. Temperatures inside the greenhouse should not exceed 35°C for the first few days, and ventilation is avoided to keep humidity levels high. If temperatures drop below 13°C, the shed should be sealed or heated. At higher temperatures, side vents are opened to prevent cold winds. As temperatures rise, the shed is gradually ventilated until the film is completely removed once the risk of frost has passed. Fertilizer application is done based on plant growth and soil conditions, with 30–40 kg of compound fertilizer applied per mu. Pest control is also essential during this period.
Finally, early-maturing sweet potato varieties can be harvested 65–70 days after planting. After harvesting, the field is prepared for the next planting cycle, ensuring continuous production and maximizing land use. This integrated system not only improves yield and quality but also supports environmentally friendly and economically viable farming practices.
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