1. Role of canopy shading
(1) Shading effect: the shading net can significantly reduce the light intensity in the shed. The larger the density specification, the better the shading effect. The same specification of black is better than silver gray. Generally, the blackout rate of black is 42% - 65%, and that of silver gray is 30% - 42%.
(2) Cooling effect: the temperature in the greenhouse is decreased due to the coverage of the shading net, especially the surface and soil tillage layer. From 10:00 a.m. to 2:00 p.m., the temperature in the upper part of the greenhouse is as high as 37-40 ℃, while the temperature around the surface plants is 22-26 ℃, and the soil temperature is 18-22 ℃, which is suitable for crop growth.
(3) Moisture conservation and rainstorm prevention: evaporation in the shed is reduced, the soil moisture content is higher than that of the open land, and the topsoil is wet. Because the shading net has certain mechanical strength and is relatively dense, it can decompose the rainstorm into fine rain, avoid the vegetable leaves from being damaged by the rainstorm, and the soil is not easy to harden, with large porosity and good ventilation. It is better to cover the shading net with plastic film in the greenhouse.
2. Precautions for shading and covering cultivation
(1) The shading net with appropriate specifications shall be selected according to the types of vegetables. Generally, the black shading net shall be selected for the short-term cover of green leafy vegetables in summer and autumn, and the silver gray shading net shall be selected for the summer seedling of autumn and winter vegetables, which can avoid aphids. Eggplants and fruits should be kept or cultivated later. It is best to use both plastic and plastic film.
(2) Covering period: generally from July to August, the light intensity in other times is suitable for vegetable growth. If there is no heavy rain, it is not necessary to cover.
(3) Shading Management: the shading net can not be covered on the scaffold for a long time, especially the black shading net. Only at noon on a sunny day in summer and autumn, the near saturated light intensity can be reached under the net. It is best to cover it from 10:00 a.m. to 11:00 a.m. and open it from 4:00 p.m. to 5:00 p.m. 3-4 days before opening the net, the net covering time shall be gradually shortened to make the seedlings and plants gradually adapt to the open environment.