How to Grow Vegetables in a Greenhouse? The Latest Planting Techniques and Methods Tutorial

Greenhouse vegetable growing is a fairly common technique with good insulation performance, and it is well loved because you can eat off-season vegetables at any time. Generally, greenhouse vegetables use a structural framework mainly made of bamboo and steel, and then one or more layers of insulating plastic film are laid over it, so that this simple structure creates a complete greenhouse space.

Greenhouse Construction

1. Frame types and structures

There are many types and structures of plastic greenhouses. At present, the two most widely promoted and applied are the assembled galvanized thin-walled steel pipe type (abbreviated as steel pipe greenhouse) and the bamboo-wood round-arch greenhouse. They are mainly used for the spring early-maturing cultivation and winter delayed cultivation of summer vegetables such as tomatoes, sweet (hot) peppers, eggplants, and cucumbers, as well as for seedling raising and hybrid seed production. There is also a small bamboo-frame tunnel, often used alone or together with a large greenhouse (that is, a large greenhouse with a small tunnel inside), for raising seedlings of eggplant and gourd vegetables in winter and spring and for spring early-maturing cultivation.

Steel pipe greenhouses come in two specifications: one has a center height of 2.2 m, a width span of 4.5 m, a length of 20 m, and an area of 90 sq m; the other has a center height of 2.5 m, a width of 6 m, a length of 30 m, and an area of 180 sq m. The service life is generally 15 years.

To reduce production costs, bamboo-frame greenhouses can also be used. The frame is built with moso bamboo or small round logs, about 2 m high and 4-5 m wide, and the general service life is 3 years.

2. Covering materials

Greenhouse covering materials include the following:

① Ordinary film: made from polyethylene or polyvinyl chloride as raw material, film thickness 0.1 mm, colorless and transparent. Service life is about half a year.

② Multifunctional long-life film: made by adding an appropriate amount of anti-aging agent and surfactant during the polyethylene blow molding process. The multifunctional film produced by Zhejiang Xinguang Plastic Factory is 7.5 m wide and 0.06 mm thick, with a service life twice as long as ordinary film, and at night the greenhouse temperature is 1-2°C higher than with other materials. Moreover, the film does not easily form water droplets, the covering effect is good, the cost is low, and the benefit is high.

③ Straw mats and straw fans: woven from rice straw, with good insulation performance; they are nighttime insulation materials.

④ Polyethylene high-foam soft sheets: white plastic soft sheets with many air bubbles, 1 m wide and 0.4-0.5 cm thick, lightweight and rollable, with insulation performance similar to straw mats.

⑤ Non-woven fabric: a kind of polyester filament, a cloth-like material that is not woven. It is divided into black and white types, with different densities and thicknesses; the commonly used specification is 50 g, and besides insulation it is also often used as a shade net.

⑥ Shade net: a kind of woven plastic net. Commonly used types are black and silver-gray, with several density specifications, and different shading rates. It is mainly used for shading and rain protection in summer, and can also be used as winter insulation covering.

3. Greenhouse erection

Choose a place that faces the sun, is sheltered from wind, is high and dry, has good drainage, and has no soil-borne infectious diseases to build the greenhouse.

4. Plastic film maintenance

① When installing the film, try to avoid mechanical damage to the greenhouse film, especially for bamboo-frame greenhouses. Before installing the film, the protruding parts of the frame surface should first be smoothed down or wrapped with old cloth. When fixing with springs, a layer of old newspaper should be added at the clamp groove. Also be careful to avoid long-term contact between old and new film, so as not to accelerate the aging of the new film. Operate carefully during ventilation.

② Film that is frozen or exposed to the sun will age faster. Steel pipes exposed to the sun in summer can rise to 60-70°C, thereby accelerating film aging and breakage.

③ During use, the film will inevitably have holes, which should be promptly patched with adhesive or tape.

Environmental Characteristics and Regulation

Because the greenhouse is covered with plastic film, it forms a relatively closed special microclimate different from the open field. For greenhouse vegetable cultivation, one must understand the characteristics of the environment inside the greenhouse and take corresponding regulation measures to meet the conditions for vegetable growth and development, thereby obtaining high quality and high yield.

1. Environmental conditions inside the greenhouse

(1) Light: depends on the solar radiation intensity outside the greenhouse, the optical characteristics of the covering material, and the degree of pollution. The light transmittance of new plastic film is 80-85%, while the light transmittance of old film polluted by dust and mud is often below 40%. When water droplets condense on the film surface, the diffusion effect of the droplets can reduce the light inside the greenhouse by 10%-20%. The frame, the film-pressing lines, and the trellis materials for tall vegetables all shade light, so greenhouse management should avoid and eliminate factors that weaken the light inside the greenhouse as much as possible.

(2) Temperature

① Temperature change pattern: The daily temperature change trend inside the greenhouse is the same as in the open field, but the day-night temperature difference varies greatly. During the day with sufficient light, if the film is sealed tightly the temperature inside the greenhouse rises quickly, reaching up to 40-50°C, more than 20°C higher than outside. On cloudy and rainy days, the warming effect is poor, and the lowest nighttime temperature inside the greenhouse is generally 1-3°C higher than outside. The soil temperature inside the greenhouse is more stable than the air temperature, usually 10-20°C. The air temperature inside the greenhouse also varies by position; in the horizontal distribution it is high in the middle and low on both sides, so plants in the middle of the greenhouse are often taller than those on both sides. In the longitudinal distribution, during the day when there is sunlight, the temperature is high at the top and low at the bottom, while at night and on cloudy days it is the opposite.

② Temperature inversion phenomenon: In greenhouses covered with polyethylene, on breezy clear nights in winter, the temperature inside the greenhouse sometimes becomes lower than outside. The reason is: at night the air temperature outside the greenhouse is higher at higher places than at lower places; due to wind disturbance, the near-ground area outside the greenhouse can obtain heat replenishment from the upper air layer, but inside the greenhouse, because of the blocking by the covering, this part of the heat cannot be obtained; in winter the daytime is cool, the soil stores little heat, and in addition polyethylene film has a relatively high long-wave radiation rate and slightly poor insulation, so effective ground heat radiation is large and heat loss is great, thus causing the temperature inside the greenhouse to be lower than outside.

③ Temperature regulation: Greenhouse temperature regulation is mainly carried out through ventilation and heating. Using film removal for ventilation is the most commonly used method to lower and control the daytime temperature inside the greenhouse. Using shading materials to reduce the amount of light received by the greenhouse can also prevent the temperature inside from becoming too high.

In winter, in order to reduce heat loss and raise air and soil temperatures, the greenhouse film should be covered as tightly as possible. Windbreaks can be set around the greenhouse, and measures such as small tunnels inside the greenhouse plus multi-layer covering with straw sheets, non-woven fabric, and foam plastic can be used. Heating measures can also be used to raise the temperature, such as using heating wires to raise soil temperature; where conditions permit, factory waste heat, geothermal water, or coal stoves can be used to raise the temperature inside the greenhouse. Placing water bags (plastic bags filled with water) inside the greenhouse takes advantage of water's high specific heat: during the day the water bags absorb a large amount of solar energy and convert it into heat energy for storage, and at night release it gradually, which can raise the greenhouse temperature.

(3) Regulation of air humidity

① Change pattern of greenhouse air humidity: Plastic film is highly sealed, and the exchange between the air inside the greenhouse and the outside air is hindered, so water vapor from soil evaporation and leaf transpiration is difficult to disperse. Therefore, humidity inside the greenhouse is high. During the day, under ventilation conditions, the relative air humidity inside the greenhouse is 70-80%. On cloudy and rainy days or after irrigation, it can reach above 90%. The relative air humidity inside the greenhouse decreases as temperature rises, and at night it is often 100%. The humid air inside the greenhouse, upon meeting cold, condenses into a water film or water droplets attached to the inner surface of the film or on the plants.

② Regulation of air humidity: Excessive air humidity inside the greenhouse not only directly affects the photosynthesis of vegetables and the absorption of mineral nutrients, but also favors the germination and infection of pathogen spores. Therefore, ventilation should be carried out to promote exchange between the highly humid air inside the greenhouse and the low-humidity air outside, which can effectively reduce the relative humidity inside. Heating with ground heating wires inside the greenhouse can also reduce relative humidity. Using drip irrigation technology combined with plastic film mulching cultivation to reduce soil moisture evaporation can greatly reduce air humidity (by about 20%).

(4) Air composition inside the greenhouse

Because of the film covering, air flow and exchange inside the greenhouse are restricted. When vegetable plants are tall and the branches and leaves are luxuriant, the carbon dioxide concentration in the air inside the greenhouse changes dramatically. Before sunrise in the morning, due to crop respiration and soil release, the carbon dioxide concentration inside the greenhouse is 2-3 times higher than outside (about 330 PPM); after 8-9 o'clock, as leaf photosynthesis strengthens, it can drop below 100 PPM. Therefore, after sunrise, ventilation should be carried out as appropriate to timely replenish carbon dioxide inside the greenhouse. In addition, artificial carbon dioxide fertilization can be carried out at a concentration of 800-1000 PPM, used after sunrise and before ventilation. Artificial application of carbon dioxide, under conditions of weak light and low temperature in winter and spring, has a very significant yield-increasing effect.

In low-temperature seasons, the greenhouse is often kept closed for insulation, and toxic gases such as ammonia, nitrogen dioxide, sulfur dioxide, and ethylene are easily accumulated and cause harm. When ammonia in the greenhouse reaches 5 PPM, the leaf tips of plants first produce water-soaked spots, then turn black and die; when nitrogen dioxide reaches 2.5-3 PPM, irregular green-white spots appear on the leaves, and in severe cases the entire leaf except the veins is bleached. The production of ammonia and nitrogen dioxide is mainly caused by improper use of nitrogen fertilizer. The production of carbon monoxide and sulfur dioxide is mainly caused by heating with coal fire, incomplete combustion, or poor coal quality. Because film aging (plastic pipes) can release ethylene, causing premature plant senescence, excessive use of ethylene products is also one of the reasons.

To prevent the accumulation of harmful gases inside the greenhouse, fresh manure must not be used as base fertilizer, nor should undecomposed manure be used as topdressing; ammonium carbonate is strictly prohibited as topdressing; when urea or ammonium sulfate is used as topdressing, it should be applied with water or applied in holes followed by timely soil covering; the amount of fertilizer should be appropriate and not excessive; in low-temperature seasons, ventilation should also be carried out appropriately to expel harmful gases. In addition, the coal used should be of good quality and fully burned. Where conditions permit, hot air or hot water pipes should be used for heating, and the exhaust gas after combustion should be discharged outside the greenhouse.

(5) Soil moisture and salinity

Soil moisture inside the greenhouse is unevenly distributed. The soil near both sides of the frame has greater humidity because more outside water seeps in and because water droplets on the greenhouse film flow down. The middle of the greenhouse is relatively dry. For cucumbers and eggplants planted in greenhouses in spring, especially those under plastic film cultivation, soil moisture is often insufficient and seriously affects quality. It is best to lay soft drip irrigation tape and apply fertilizer and water at any time according to actual needs, which is an effective measure to increase yield. Because the greenhouse is covered for a long time and lacks rainwater leaching, salts move upward from below with groundwater, which easily causes excessive accumulation of salts in the cultivated soil layer and leads to salinization. Therefore, attention should be paid to appropriate deep plowing, applying organic fertilizer, and avoiding long-term application of fertilizers containing chloride ions or sulfate ions. Topdressing should be light, and soil testing for fertilizer recommendation is best. Each year there should be a certain period without film covering, or in summer only a shade net should be used for shaded cultivation, so that the soil can be leached by rainwater. When soil salinization is severe, flooding to press down salts can be used, with very good results. In addition, adopting soilless cultivation technology is a fundamental measure to prevent soil salinization.