In the winter and spring seasons, off-season lettuce is cultivated using a simple bamboo and wooden structure with plastic arches covered by mulch film and other protective facilities. This method allows for an earlier harvest, typically 30 to 40 days ahead of conventional planting. The yield reaches approximately 3,500 kg per mu, with an output value of around 3,000 yuan per mu. This technique not only improves market timing but also enhances profitability.
1. **Selecting High-Quality Varieties**: Choose suitable lettuce varieties such as white duck egg lettuce, cold-resistant white-skinned, or purple-skinned types. These varieties are known for their resilience to colder conditions and high yield potential.
2. **Cultivating Strong Seedlings**: Start seedling production in late September. Prepare seedbeds covering 15–20 square meters for every mu of farmland, using about 50 grams of seeds per mu. Soak the seeds after watering the bed, then cover them with a thin layer of fine soil. Place straw or shade nets on top to maintain moisture. Remove the cover once the seedlings emerge. During the weeding stage, when the seedlings have two leaves and one heart, apply 10% diluted human and animal manure water to promote healthy growth.
3. **Proper Planting Density**: Choose clay or sandy soil rich in organic matter with good water retention and fertility. Plow the field to a depth of 20–30 cm, then create raised beds 2 meters wide and cover them with a thin plastic film. From late October to early November, plant seedlings that are 30–40 days old, with 4–6 leaves. Select strong, disease-free plants and space them 33 cm apart within rows and 30 cm between rows. Immediately after planting, apply 5–10% livestock manure water to help the plants establish quickly.
4. **Timely Shelter Installation**: When nighttime temperatures drop below 3°C, construct a shed that is 1.7 meters high, 4.5 meters wide, and 30–50 meters long. Install vertical posts every 3 meters and cover the structure with plastic film. Ensure proper ventilation during sunny midday hours to prevent excessive humidity and premature bolting. Apply a 50% BCST solution diluted 1,000 times or a 150 mg/L paclobutrazol solution 1–2 times depending on plant development. If the 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**: In double-layer film cultivation, apply all base fertilizer at once. Use 4,000–5,000 kg of well-rotted organic manure and 40–50 kg of 45% NPK fertilizer per mu. After planting, when the leaves start to open, apply 1,500–2,000 kg of 20% livestock manure water along with 5–7.5 kg urea or 10–15 kg ammonium bicarbonate per mu. During the early stage of fleshy stem growth, top-dress with 20–25 kg of urea per mu combined with irrigation. Avoid over-fertilizing, as this can cause the stems to crack. Fertilize in the morning to reduce nighttime humidity inside the shed.
6. **Pest and Disease Control**: After setting up the shelter, ensure proper ventilation and moisture control to prevent downy mildew. Monitor for botrytis and sclerotinia diseases during fleshy stem expansion. Regularly check for aphids and whiteflies and implement timely control measures.
7. **Harvesting at the Right Time**: Harvest in batches from late February to early March. The ideal harvesting standard is when the inner leaves are level with the outermost leaves, the plant has a flat top, and the fleshy stem has fully developed. Timely harvesting ensures optimal quality and marketability.
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