Varieties. Lettuce varieties are divided into six groups: buttery-headed, curly, romaine, leafy, cut, and asparagus. For cultivation in protected ground, varieties belonging to three groups are used: with an oily head, curly and cut. In Russia, curly-type varieties dominate, in particular, Bastion (Enza Zaden), Celtic (Rijk Zwaan) and Tethys (R.A. Komarova) have gained the most popularity. From the group with an oily head, it is necessary to note the varieties: Sanora (Enza Zaden), Omega (Rijk Zwaan), Histor (Rijk Zwaan), Viki (Enza Zaden) and Kristel (Enza Zaden), and from the cut: Azart (R.A. Komarova ), Christine (Rijk Zwaan), Relay (Enza Zaden), Loretta (Rijk Zwaan), Miami (Rijk Zwaan) and Kimura (Enza Zaden). It should be noted that only varieties from the group with an oily head are suitable for autumn-winter turnover, as they are the most shade-tolerant.
Growing seedlings. Seedlings must meet the following requirements: have typical strong leaves for this variety without any signs of elongation, have a squat hypocotyl, healthy roots that completely penetrate the root ball of the earth, uniformity of plants within the lot and be free from diseases and pests.
Sowing dates - from mid-December to September. December sowing makes sense if the optimal temperature regime for growing lettuce in low light is maintained. In the event that lettuce is grown as an additional crop, it does not make sense to sow before January, because with a sowing difference of 1 month, the difference in harvesting time will be no more than 7 days.
Planting seeds is carried out in peat pots or 5x5 cassettes filled with seedling mixture. The mixture for seedlings is prepared on a peat basis. The acidity of the water should be pH=6.0-6.3. For fertilizer, "Kemira Super" is used in the proportion N:P:K - 11:12:20, as well as a set of microelements with a ratio of 0.8 kg/m3. In this composition, the concentration of the soil solution should be 1.5 - 2.0 mS/cm. The finished mixture should be added to the cassettes and watered. Then spread the seeds and cover them with a layer of peat chips. The daytime temperature before the appearance of the first sprouts should be 16-18°C, and the night temperature 12-15°C with seedling mixture humidity of 90-95% HB. Then, when the first leaf appears, the temperature is reduced to 8 - 12 ° C during the day and 6 - 10 ° C at night, and the humidity is up to 85 - 90% HB. It is important to strictly adhere to the established temperature limits. Also, it is not recommended to grow lettuce together with cucumbers and tomatoes, in which case the plant will have a long subcotyledon knee of poor quality. With the advent of the first shoots, they begin to illuminate, first around the clock - the first three days, then for 12-16 hours with a power of about 5000-6000 lux. CO supplementation can be used to increase growth rate and yield. Seedlings are grown for about 45 days, until 4-6 leaves appear. From the 2nd decade of March, illumination is no longer needed, and the growing period is reduced to 35 days.
Agrofirm ''Belaya Dacha'' uses a slightly different technology. On special Dutch equipment, 5x5 peat cubes are produced, then they are sprinkled with peat chips. The soil itself is kept at a temperature of 15-17 ° C, air t at 18-20 ° C, and they begin to highlight with the emergence of seedlings, around the clock. Growing terms with this technology are 3-4 weeks (3-4 leaves) in December-January and 2-2.5 weeks (2-3 leaves) in February-March.
Soil preparation. Lettuce produces good yields on highly fertile greenhouse soils, however, it is sensitive to salt content, especially chlorides. According to VNIIO, with a nitrogen content of 18 - 22, phosphorus 8 - 10, potassium 25 - 30, magnesium 15-20 mg / 100g of dry soil, there is no need for basic fertilization and top dressing. The total salt concentration in the soil should not exceed 0.5 - 0.8 mS/cm.
In Holland they do it a little differently. They use high doses of nitrogen fertilizers (N - 20-30 g/m2), long-acting type. Phosphorus fertilizers are used only if there is a lack of phosphorus in the soil. Potassium is used in the amount of about 12 - 13 g/m2, in potassium magnesia form, since magnesium improves the color of the leaves. After using the fertilizer, it is necessary to keep the ratio of potassium and calcium at 1:1.1 to prevent leaf edge burns. In the soil in winter, the salt concentration should not exceed 1.6 mS/cm. However, introducing a lot of salts into the soil at the initial stage of the growing season allows plants to better tolerate watering and prevents leaf burns. It is possible to create a high concentration of salts by adding 1 g/m2 60% potassium salt to the irrigation water.
Landing. Before the seedlings are brought into the greenhouse prepared for planting, non-standard plants are culled. If the seedlings turned out to be heterogeneous, then the smallest plants are discarded, and the rest are sorted into groups, which are then planted separately to ensure subsequent continuous harvesting.
CO2-based top dressings are best done in good weather at a concentration of 0.10-0.12% (concentration of more than 0.12% harms yields). If the illumination is below 4000 lux, top dressing should not be done.
Acceptable air humidity should be about 80% at night, 70-80% during the day in clear weather, and 60-70% in cloudy weather. Moisture in the soil should be no more than 65-70% of the WPV. Before planting, the seed should be watered with moisture, to supply water with a drainage effect throughout the root layer. The second time such watering should be done before closing the rows of lettuce. This will create a sufficient supply of moisture for stable growth. But It is necessary, however, to continue maintenance watering. Fine watering is best suited for this purpose. In the absence of mechanical irrigation, you should not try to grow lettuce, as it is very difficult to obtain high quality products. An example of this is the author's personal experience in growing lettuce in the greenhouse complexes "Matveevskoye", "Kosino" and "named after M.S. Kirov", where there was manual watering.
Lettuce is removed in one go, cutting off the head and removing rotten leaves. Then they are densely placed in boxes in several rows, the lower rows go heads up, the top ones go down. Product quality is checked by several criteria: 1) weight 21-65 kg / 100 heads, 2) head density, 3) freshness of leaves, 4) color of the core. After evaluation, the plants are packed either in polyethylene or in a crate. The shelf life is about a week at a temperature of 5-8°C, and at a temperature of 0-1°C and RH 98-100% of the plants can be stored for up to a month.
Before removing the christened heads of cabbage for 5-7, you can lower the air temperature to 4-5 degrees for intense coloring.
The yield of lettuce depends on the period of sowing and planting seedlings, varieties and growing conditions. So in the experiment, when sowing from August 1 to 5 and harvesting on October 5-10, the yield was 3.2 kg/m2, and when sowing from August 15 to 20 and harvesting on November 15-30, the yield was only 1.1 kg/m2. According to the GSU of the protected ground "Moskovsky" for 1997, when seedlings were planted on February 11 and harvested on April 1, the yield of the Tethys variety was 3.58 kg / m2, and in the Belaya Dacha greenhouse plant in industrial plantings, only 2.2 kg / m2.
Diseases. Due to poor growing conditions, plants can get sick, for example, gray or white rot - the most unpleasant diseases. Of the physiological ailments, one can single out the marginal leaf burn (marginal burn), the danger of which exists throughout the entire growing period, and a massive lesion can occur by the time of maturity. The disease can be caused by changes in air temperature, high salt content in the soil, or a lack of calcium against the background of a powerful vegetation. This disease can reduce the presentation and lead to rot, so the fight against it is very important. Control methods: strict adherence to technology, treatment with saline solution with calcium and hybridization of varieties resistant to this disease. During the day, it is necessary to lower the RHV value - this can be done with the help of ventilation, and it is also necessary to supply the plants with sufficient water. At night, a decrease in osmotic pressure due to abundant watering and an increase in RH up to 95%.