Temperature And Light
There are many issues related to temperature in cultivation of crops. The temperature can be measured with a thermometer, but knowing how it goes up or down, is important to be able to manage the temperature in the green house to a certain extend.
Sunlight is the overall cause of temperature change. Sunlight consists of a range of different types of light. For cultivation we divide light in 3 groups.
1. Ultra Violet (UV). This light (around 400 nm) is not much used by plants, although it is important for keeping the plants shorter and for taste, aroma and colour. The negative effect is the break down effect on plastics (green house films, trays, crates, pots, etc.) The blue light in this range keeps the plants shorter.
2. PhotosyntheticallyActiveRadiation (PAR). The large middle range of light (400-700 nm) is used by the plants to grow. Most of this light range starts at some blue, full range of green and through yellow to the red, which is the real growth light.
3. Infra Red (IR).Dark red to black.This range of light (> 700nm) is the heating radiation.

Through led lights we can give the light range most desired at certain periods of development of the crop, however it is still a subject under study. Moreover light is not a stand-alone factor, but related to temperature, day length, etc. Therefore according to nature, the full range of light is most secure for balanced growth.
However the light at the extremes of the range, the UV and IR light is not much used by plants. Plants shall be protected against excessive infra red light to avoid dehydration and burning of leaves.Diffused plastic does avoid direct radiation on the plants, the quality of light improves and the penetration is deeper into the crop reaching also the lower leaves. Blocking part of the IR radiation with certain green house plastics will reduce the temperature duringhours of heat. Keep in mind that when the weather is cold, you would like the green house to warm up quickly and then the IR comes in handy. In general, reducing sunlight in the green house will moderate the rise in temperature, but keep in mind that light is the number one factor for growth. The screening can be done with shade nets, which can be spread or folded according to need. The nets may block light from 30% up to 80%. There is also an option of shadenet with strips of aluminum to reflect sunlight, which has a further cooling effect. Ideally we would like to put shade nets over the green house, but two issues: wind may get hold of the nets and we will not be able to open and close the nets. Painting the plastic is another option. Mardenkro is a company that specializes in this. Some paints wash off with rain others not, some paints become transparent during rainfall allowing light to enter the green house.

So far, all this is regarding heating by radiation.
Another form of warming is through convection, which takes place in the movements of air that may have absorbed heat from the soil that was heated up by radiation.
In temperature management we have to differentiate between the two causes of heating. Radiation is discussed above. Heat through convection can be reduced by exchange of air or cooling the air. The natural cooling of green house air is the inflow of cooler (outside) air pushing out the warmer air, based on the principle that cool air descends pushing up the warmer air. With all the turbulence taking place in a green house, this does not work perfectly. Another aspect is that, especially during midday in summer, the outside air may also be warm and of low humidity. We have to provide sufficient water by misting, fogging and maintaining good top soil humidity. The somewhat dryer air will help to evaporate water more quickly from the soil and from misters, as well as increase the evapo-transpiration of plants. This evaporation of water has a cooling effect.
The pad ‘n fan system takes care of all this by pulling air through a pad (closed top) that is made wet continuously. The air coming into the green house is saturated with water which is evaporated by the flow through the green house, hence maintaining humidity and cooling the air. Under high humidity conditions this system has a reduced cooling capacity. The largest cooling factor are the plants themselves. Contrarily, in an empty and dry green house, very high temperatures (>60 degr C) can be accumulated in full sun. This is mainly due to radiation, especially the infra-red waves. A large crop in a greenhouse covers the soil and cools the air with the evapo-transpiration and also helps to maintain the humidity level. So, a huge amount of bio-mass in the summer period is a significant help to maintain a reasonable temperature and humidity level. To build up bio-mass in a rose crop over a period we harvest somewhat shorter stems and do green pinching.

Note: Our feelings are not good at measuring temperature. We feel dryer air as cooler, because of increased evaporation from our skin, not because the more humid air is necessarily warmer. A thermometer is required to check this. The most current thermometer is the mercury thermometer, or sometimes substituted by alcohol. Further it is practical to have a wet and dry bulb thermometer to measure also the humidity and a minimum/maximum thermometer for record keeping.

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