In the winter and spring seasons, off-season lettuce is cultivated using a simple bamboo and wooden plastic arch greenhouse covered with mulch film and other protective structures. This method allows for an earlier harvest, typically 30 to 40 days ahead of conventional methods. The yield reaches approximately 3,500 kg per mu, with an output value of around 3,000 yuan per mu.
1. **Selecting High-Quality Varieties**: Choose varieties such as white duck egg lettuce, cold-resistant white-skinned lettuce, and purple-skinned lettuce. These types are well-suited for early season cultivation due to their resilience and high yield potential.
2. **Cultivating Strong Seedlings**: Start seedling production in late September. Prepare 15–20 square meters of seedbeds per mu, using 50 grams of seeds. Soak the seeds in water, then cover them with a thin layer of fine soil. Use straw or shade nets to keep the soil moist. Remove the cover once the seedlings emerge. At the two-leaf-one-heart stage, perform weeding and apply 10% human and animal manure water when the seedlings reach 4–5 cm in height.
3. **Proper Planting Density**: Select clay or sandy soil rich in organic matter, with good water retention and fertility. Plow the soil to a depth of 20–30 cm and create raised beds 2 meters wide, covering them with a thin plastic film. From late October to early November, plant seedlings that are 30–40 days old, with 4–6 leaves. Choose healthy, pest-free plants and space them 33 cm apart in rows and 30 cm between plants. After planting, apply 5–10% livestock manure water to help the seedlings establish quickly.
4. **Timely Sheltering**: When nighttime temperatures drop below 3°C, erect a greenhouse that is 1.7 meters high, 4.5 meters wide, and 30–50 meters long. Install columns every 3 meters and cover the structure with plastic film. Ensure proper ventilation on sunny days to prevent excessive growth and premature bolting. Spray a 50% BCST solution diluted 1,000 times or a 150 mg/L paclobutrazol solution 1–2 times based on plant development. When fleshy stems begin to swell and cold weather arrives, cover the plants at night with plastic film or shade net to protect against frost damage.
5. **Fertilizer Management**: Apply all base fertilizer before planting, using 4,000–5,000 kg of well-rotted organic manure and 40–50 kg of 45% NPK fertilizer per mu. After planting, apply 1,500–2,000 kg of 20% livestock manure water mixed with 5–7.5 kg urea or 10–15 kg ammonium bicarbonate. During the early stage of fleshy stem enlargement, top-dress with 20–25 kg urea per mu combined with irrigation. Avoid over-fertilizing, as this can cause cracking of the fleshy stems. Fertilize during the morning to reduce humidity in the greenhouse at night.
6. **Pest Control**: After setting up the greenhouse, ensure proper ventilation and moisture control to prevent downy mildew. Monitor for botrytis and sclerotinia diseases during the fleshy stem expansion phase. Regularly check for aphids and whiteflies and take timely action to manage infestations.
7. **Harvesting at the Right Time**: Harvest in batches from late February to early March. The harvesting standard is when the inner leaves are level with the outermost leaves, the plant has a flat top, and the fleshy stem is fully developed. Timely harvesting ensures optimal quality and marketability.
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