Generally, leaf vegetables planted in winter and spring cover sunshade net directly on the surface of leaf vegetables (covered by levitated) to prevent low temperature damage. Because of its light weight, it is only about 45 grams per square meter, for the grown tall leaf vegetables it will not overwhelm, bend, and reduce commodity. And because it has certain air permeability, the surface of the leaves is still dry after being covered, which reduces the occurrence of diseases. It also has a certain degree of light transmission, and it will not "cover yellow and rot" after being covered ". 1997 henan Agricultural University in linying County longtang vegetable Park has been basically grown in winter greenhouse celery, in the shed levitated cover for cold protection, it is found that in the case of outside-6 ℃ for sunshade net levitated cover, celery leaves did not find a large amount of ice. However, the single-layer shed control, especially the middle and upper leaves, had been frozen hard, and the measured cover was 1.5 ℃ higher than the control.

Sunshade net is mainly used in summer, especially in the South. Some people describe that winter in the North is white (covered by film) and summer in the South is black (covered by sunshade net). Using sunshade net to cultivate vegetables in the south in summer has become a major technical measure for disaster prevention and protection. Northern application is only limited to summer vegetable seedling. In summer (June-August), covering sunshade net is mainly used to prevent the exposure of hot sun, the impact of rainstorm, the harm of high temperature, and the spread of diseases and insect pests, especially to prevent the migration of pests.

After summer coverage, it plays a role of blocking light, rainproof, moisturizing and cooling; After winter and spring coverage, it also has a certain effect of heat preservation and humidification.

Moisturizing principle: after covering up sunshade net, due to the cooling and windproof effect, the communication speed between the air in the covering area and the outside world is reduced, which significantly increases the relative humidity of the air. At noon, the added value of humidity is the largest, generally, it reaches 13-17%, with high humidity and decreased soil evaporation, increasing soil humidity.

Sunshade net uses polyethylene (HDPE), high-density polyethylene, PE, PB, PVC, recycled materials, new materials, polypropylene and other raw materials. After UV stabilizer and anti-oxidation treatment, it has strong tensile strength, aging resistance, corrosion resistance, radiation resistance, light and other characteristics. It is mainly used for protective cultivation of vegetables, xianggu, flowers, edible fungi, seedlings, medicinal materials, ginseng, Ganoderma lucidum and other crops and aquatic poultry breeding industry, which has obvious effect on increasing yield and so on.