Detailed Explanation of Key Techniques for Winter Solar Greenhouse Cultivation of Lettuce

Lettuce (var. ramosa Hort.), commonly known as leaf lettuce, also called goose lettuce, wheat lettuce, or stem lettuce, belongs to the Asteraceae family, genus Lactuca. It is an annual or biennial herbaceous crop with long obovate leaves that densely form cabbage-like heads; it can be eaten raw, is crisp and refreshing, and slightly sweet.

Morphological characteristics of lettuce: Lettuce can be divided into two types by leaf color: green lettuce and purple lettuce. If divided by leaf growth habit, there are two types: loose-leaf lettuce and head lettuce. The former has scattered leaves (20 photos of lettuce preparation), while the latter has leaves that wrap together into a head shape. If further subdivided, head lettuce also has three types: one is butterhead lettuce, with obovate leaves, smooth leaf surface, soft texture, and slightly wavy leaf margins; another is crisphead lettuce, with obovate-round leaves, wrinkled leaf surface, crisp and tender texture, and serrated leaf margins.

The latter is more commonly cultivated; bitter leaf lettuce has thick, long elliptical leaves with entire margins and is semi-heading. The single plant weight of loose-leaf lettuce is 250—500 grams, while head lettuce has a longer growth period than loose-leaf lettuce and a larger single plant weight, generally 400—750 grams, and late-maturing varieties with a long growth period can reach more than 1000 grams. Lettuce has a shallow root system with well-developed fibrous roots, and the main root mass is distributed within the 20 cm soil layer on the surface. The stem is shortened, and the leaves are alternate, lanceolate, elliptical, ovate, etc. The leaf color is green, yellow-green, or purple, the leaf surface is flat or wrinkled, the leaf margin is wavy or shallowly lobed, the outer leaves spread out, and the heart leaves are loose or wrap together to form a head. The seeds are grayish-white or dark brown, with a thousand-grain weight of about 1 gram.

Medicinal value of lettuce: Lettuce is sweet and cool in nature. Because its stems and leaves contain lactucin, it tastes slightly bitter. It has effects such as clearing heat and refreshing the mind, relieving pain and inducing sleep, lowering cholesterol, and assisting in the treatment of neurasthenia. Lettuce contains active ingredients such as mannitol, which has diuretic effects and helps promote blood circulation, clear the liver and gallbladder, and nourish the stomach.

Edible value of lettuce: Lettuce is rich in nutrients, containing large amounts of β carotene, antioxidants, vitamin B1, B6, vitamin E, vitamin C, as well as large amounts of dietary fiber and trace elements such as magnesium, phosphorus, calcium, and small amounts of iron, copper, and zinc. It strengthens the digestion and absorption of protein and fat, and improves gastrointestinal blood circulation, so lettuce can be eaten regularly.

Key points for winter solar greenhouse cultivation of lettuce

Variety selection

Choose mid- to late-maturing head lettuce varieties with strong cold resistance for cultivation.

Cultivation date

For solar greenhouse cultivation, planting can be done from mid-to-late November to late January of the following year, and harvest from early-to-mid January to early March.

Seedling raising

Choose fertile sandy loam soil, break it up finely, and make flat beds. Then apply 100 kg of screened high-quality manure and 0.5 kg of potassium dihydrogen phosphate per 10 square meters, mix thoroughly, level the bed surface, and water. After the water seeps down, scatter the seeds. Generally, dry seeds are used for sowing, but seeds can also be soaked and pregerminated before sowing. For dry seed sowing, first treat the seeds with 75% chlorothalonil wettable powder equivalent to 0.3% of the seed weight, but it should not be left for too long. For soaking and pregermination, first soak in warm water at 20°C for 6-7 hours, remove, wrap in a cloth bag, and place at 20°C for pregermination, rinsing once daily with clean water; after 2-3 days, the sprouts will be uniform. Sow on a windless, sunny morning, sowing 3-5 grams of seeds per square meter of seedbed. After sowing, cover with a 3-5 mm layer of fine soil.

Land preparation and fertilization

Before planting, apply 5000 kg of decomposed organic fertilizer and 30-40 kg of compound fertilizer per mu. After deep plowing, make flat beds; if using plastic film mulch, small raised beds can be made.

Planting

For solar greenhouse cultivation, the row spacing is generally 40 cm and the plant spacing 30 cm. Water before lifting seedlings and cut into blocks, carrying more soil. Plant in furrows or holes, close the furrows and level the beds, then water thoroughly after planting.

Management after planting

After planting, keep the daytime temperature at 20°C-24°C and the nighttime temperature above 10°C. During the seedling recovery period, cultivate and loosen the soil 1-2 times. After seedling recovery, lower the temperature appropriately, with 15°C-20°C suitable during the day, and water for seedling recovery 7-10 days after recovery, while lightly applying chemical fertilizer, about 10 kg per mu. After that, cultivate again and appropriately restrain seedling growth. About 15 days later, when the rosette leaves have grown and the heart leaves begin to head, apply 15-20 kg of compound fertilizer or ammonium nitrate as a top dressing, and apply top dressing once more during the heading stage. In winter, watering should not be too frequent to prevent lowering the ground temperature or causing excessive humidity. During the heading stage, water supply should be even, and abrupt drying and wetting must be avoided to prevent head cracking or spreading growth of head leaves. Stop watering 3-5 days before harvest.