In the agricultural production process, some vegetable farmers choose to ventilate in the morning to remove moisture and lower humidity inside the greenhouses. However, the author believes that this method may be acceptable during the hot season but is not ideal during colder periods.
During the winter months, early ventilation for dehumidification is often ineffective. This is because, as temperature rises, air humidity tends to decrease. In a greenhouse during cold nights, temperatures typically range between 10°C and 15°C, with relative humidity reaching up to 95% or even 100%. If farmers lift the cover in the morning without first allowing sunlight to warm the space, the temperature will gradually increase, naturally reducing humidity without the need for ventilation. However, due to the extremely low outdoor temperatures—often below 0°C—the cold air entering the greenhouse can cause condensation, making it difficult to effectively reduce humidity. As a result, ventilation during this time is minimal and has limited impact on moisture levels.
Another issue with early ventilation is its effect on carbon dioxide concentration. In winter, greenhouses are usually tightly sealed, leading to higher accumulation of carbon dioxide from decomposing organic matter in the soil. This gas is essential for photosynthesis, and in the morning, its concentration inside the greenhouse can be several times higher than outside. This high level of COâ‚‚ provides an excellent environment for plants to perform photosynthesis more efficiently. Therefore, it's advisable to delay ventilation for about an hour after uncovering the greenhouse, allowing the plants to fully utilize the accumulated COâ‚‚. Ventilating too early would waste this valuable resource and reduce potential yields.
The correct approach is to let sunlight warm the greenhouse first, which helps naturally lower humidity. After about an hour, controlled ventilation can be introduced to allow fresh air—and necessary CO₂—into the greenhouse, supporting ongoing photosynthesis. This strategy not only helps manage temperature but also ensures that the beneficial CO₂ remains for plant growth. Adjusting the timing of ventilation during colder periods is therefore key to maintaining both optimal temperature and CO₂ levels, ultimately benefiting crop production.
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