Detailed Explanation of Key Lettuce Cultivation Techniques in Summer
Lettuce is the common name for leaf lettuce, belonging to the genus Lactuca in the Asteraceae family. It is an annual or biennial herbaceous crop, and is also a popular vegetable in European and American countries, deeply loved by people. Lettuce originated along the Mediterranean coast of Europe, domesticated from wild species. The ancient Greeks and Romans were the first to eat it. Lettuce has a long history of introduction into our country, with more cultivation in the southeastern coastal areas, especially in the suburbs of large cities and the Guangdong and Guangxi regions, and it is particularly widely grown in Taiwan, China. In recent years, the cultivation area has expanded rapidly, and lettuce has also moved from hotels and restaurants onto the dining tables of ordinary people.
Morphological Characteristics of Lettuce
Lettuce is an annual or biennial herb, 25-100 cm tall. The root extends vertically straight down. The stem is erect, solitary, with the upper part branching into a paniculate inflorescence, all stems and branches white. The basal leaves and lower stem leaves are large, undivided, oblanceolate, elliptic, or elliptic-oblanceolate, 6-15 cm long, 1.5-6.5 cm wide, with acute, short-acuminate, or rounded apex, sessile, with a cordate or sagittate base semi-clasping the stem, with wavy or finely serrate margins; the upper leaves gradually become smaller, shaped the same as the basal and lower stem leaves or lanceolate; the leaves at the lower part of the paniculate inflorescence branches and the leaves on the paniculate inflorescence branches are very small, ovate-cordate, sessile, with a cordate or sagittate base clasping the stem, with entire margins; all leaves glabrous on both surfaces.
The capitula are numerous or very numerous, arranged in a paniculate inflorescence at the apex of stems and branches. The involucre is ovoid in fruit, 1.1 cm long, 6 mm wide; the involucral bracts are in 5 layers, the outermost broadly triangular, about 1 mm long, about 2 mm wide; the outer layer triangular or lanceolate, 5-7 mm long, about 2 mm wide; the middle layer lanceolate to ovate-lanceolate, about 9 mm long, 2-3 mm wide; the inner layer linear-oblong, 1 cm long, about 2 mm wide; all involucral bracts acute at the apex, glabrous outside. There are about 15 ligulate florets. The achene is oblanceolate, 4 mm long, 1.3 mm wide, compressed, light brown, with 6-7 fine vein lines on each face, acute at the apex forming a slender beak, the beak filiform, about 4 mm long, nearly equal in length to the achene. The pappus is in 2 layers, slender, slightly scabrous-hairy. Flowering and fruiting period is February to September.
Summer Cultivation Techniques for Lettuce
1. Selecting Good Varieties
For summer cultivation of lettuce, choose varieties that are heat-tolerant and not prone to bolting, such as Caesar, Emperor, Xialü, and Lühu, and sow them in 3-4 batches from May to June.
2. Raising Strong Seedlings
1. Seed soaking and germination. Lettuce seeds are small, direct sowing techniques are difficult to master, and seedling emergence uniformity is poor, so seedling raising and transplanting are generally used. The seedling bed should be leveled and raked fine; 25 grams of seed are needed per mu, requiring a seedling bed area of 10 square meters. Soak the seeds to promote germination before sowing. First soak in clean water at about 20°C for 4-6 hours; after draining the water, wrap the seeds in gauze and place them under conditions of 15°C-20°C to promote germination. They can also be suspended above the water surface of a well for low-temperature germination promotion, or placed in a refrigerator at 5°C-10°C for 24 hours, then placed in a cool indoor spot with moisture maintained to promote germination, and sown after the seeds show white.
2. Sowing and seedling raising. Before sowing, water the seedling bed thoroughly; after the water seeps down, mix the seeds with fine sand and broadcast them, cover with sifted fine soil 0.3-0.5 cm deep, and cover with a shade net or reed curtain. The seedling age is generally about 25 days; during the seedling stage, keep the soil moist. Remove the shade net when the young seedlings show their heart leaves, and water lightly once; to prevent washing out the young roots of the seedlings and causing lodging, cover with a thin layer of fine soil after watering. Thin the seedlings when two true leaves appear, and at the same time pull out weeds in the bed and water once; transplant promptly when the seedlings grow to 4-5 true leaves.
3. Field Management
1. Cultivation. Carry out timely cultivation 3-5 days after transplanting to conserve soil moisture; before the rows close, generally carry out 1-2 shallow cultivations to promote early root development.
2. Watering. During the growing period, water evenly, with frequent light watering, keeping the bed surface moist, and neither too dry nor too wet.
3. Topdressing. Because lettuce is mainly eaten raw, do not use human excrement or chicken manure for topdressing; mainly use chemical fertilizers. Generally topdress 3-4 times: once 15 days after the seedlings recover, once at the rosette stage, and once at the heading stage, applying 5-8 kg of urea per mu each time, flushed with water, but do not apply too much to prevent lettuce tipburn. During the growing period, foliar spraying of monopotassium phosphate may be done appropriately to improve disease resistance and eating quality.
4. Pest and disease control. Lettuce has very few pests and diseases, only a small amount of aphids, which can be controlled promptly with high-efficiency, low-toxicity pesticides.
5. Harvesting in batches
Because lettuce is planted at a relatively high density and is mainly eaten for its crisp tender leaves, loose leaf heads can be harvested successively for market 30 days after transplanting.