The ratio of the window area (ventilation area without door) to the base of the greenhouse should be between 18 to 24 percent. This is the only way to achieve an air exchange in the summer with high radiation, which allows a temperature difference between inside and outside temperatures of at least 3 ° C. The exchange of air is also dependent on the wind speed.
When buying or building a greenhouse, you should pay attention to adequate ventilation flaps in all roof areas, if possible, in order to be able to ventilate the side facing away from the wind. Additional side vents are always helpful. The best location to install the windows is usually specified by the manufacturer.
Using a vent or opening on the the lowest side with a corresponding roof vent, optimal air circulation is achieved because the warm air rises quickly and cool air is easily tracked from below. Any natural ventilation is preferable to forced ventilation. Ventilation is required so that the temperature does not rise above a maximum value.
There is also a feel-good temperature for plants
For most edible and ornamental plants, such as tomatoes or balcony flowers, 0 ° C or less means that these plants will simply freeze. Up to 10 ° C - growth is vey limited and the plants suffer from stress. They will also be more vulnerable to other pathogens.
Respiration and too much respiration
The main components of air are oxygen, nitrogen, carbon dioxide and water vapour. During photosynthesis, plants can produce sugar, starch and cellulose with chlorophyll in the leaves, as well as many other chemical substances. These substances must be transported throughout the plant.
For this, plants use transpiration, which is caused by a difference in the water pressure between leaf and air. The evaporation of water also has a cooling effect that can prevent overheating of the leaves. Important here are the adjustable openings in the lower leaf surface of the plants (stomata). Water is also released slightly over the leaf surface because the surface is protected by the cuticle, a layer of wax. The release of water is controlled by humidity, wind speed, light intensity, temperature, plant density, nutrient deficiency and soil structure.
Humidity in the greenhouse
It is well-known that warm air absorbs more moisture than cold air. At night, the humidity in a greenhouse is higher than during the day. At high humidity, water evaporates more slowly. The stomata of the plant will then open itself, something critical for nutrient transport. At low humidity (open ventilation, heat) water evaporates faster. The plant then closes its stomata and does not grow anymore.
High humidity may be desirable or dangerous depending on the plant species. In any case, the air supply (temperature) must be regulated by means of ventilation flaps. If the soil and substrate of the greenhouse are well moistened, cooling is additionally impacted. On the other hand, a rapid flow of air (heat) can accelerate evaporation excessively. The plants evaporate more water than they can track over the roots: then they will wither. In addition, this creates an increase in the salt concentration in the root areas, which can also have a negative effect on the roots to the point of total loss. By contrast, instead of even more ventilation, it helps to reduce the amount of air and humidity.
Even something as simple as shading your greenhouse can do a lot to prevent overheating. In Autumn and Winter, but especially in Spring, moisture levels must be kept as low as possible, if the air is still cool. When daylight is low, many fungal diseases easily spread in humid environments. These include mould and mildew, as well as fungi, which inevitably lead to rot of stems and storage organs (onions, sprouts, tubers and the like). Of course, everything depends on which plants you use. At temperatures of 5-8 ° C in winter, the air can also be kept dry simply by sparing only those plants that really grow in the greenhouse. Plants in their resting stage hardly need any water.
Automatic window openers
These are sometimes indispensable because they prevent the greenhouse from overheating and ensure optimal ventilation. Automatic window openers for small greenhouses operate independently of any electricity sources. The drive is based on a wax base contained in a cylinder. Thanks to its physical properties, this wax contracts strongly at low temperatures and expands as temperatures rise. The wax liquid moves a piston rod, which moves forward and opens the window via a lever mechanism, or retracts through the weight of the window and a steel spring. Without the force of the preloaded spring, the weight of the window would be insufficient to push back the wax and piston rod. This process happens slowly. By turning the guide tube on and switching the fan off, the time of opening and closing can be set to the correct temperature, depending on your plants' needs. Depending on the manufacturer, the load capacity for one of these is between 7 and 18 kg.
Suitable greenhouse and garden accessories can be found in the world of Bio Green.
The original version of this article was published in the greenhouse post, issue 06/2016, text and image: Jörn Pinske.