The greenhouse climate is one of the main (if not the main) growth factor for the plant. Therefore, it is worth optimizing the greenhouse climate to further enhance production.
Recent research has shown how plants can be activated and that plants perform better at the combination of higher temperatures and, at the same time, a higher relative humidity (RH) than we have been used to so far. When a higher greenhouse temperature is kept, the greenhouse is ventilated less frequently and more CO2 stays inside. This has the advantage of increasing assimilation. For this increase to take place, more light (=energy) is needed. More light can be allowed in, but the plant must be able to effectively use this extra light. By combining the knowledge of physics with that of the plant, cultivation can be even further optimized.
Activating the crop
During the day, the sun delivers the energy which is necessary to keep the plants active. Photosynthesis takes place and there is sufficient evaporation. At night, and when there is too little incoming radiation during the day, the situation is different. Sunlight has to be compensated to activate the plants. This implies that crop evaporation has to start and has to be kept going. In the case of low relative humidity there is relatively greater evaporation and this is referred to as an active climate. In the case of a higher relative humidity, the possibility of evaporation is more limited and the climate must be activated artificially.
Types of evaporation
Most evaporation is caused by radiation. This can be compared to the steam that is released by a whistling kettle. Radiation heats up the water in the plant, which is released through the stomata.
Air that is warmed up and gets warmer than the surrounding air tends to rise. This ascending movement is called convection. Evaporation by convection is the ‘wet bulb’ evaporation, which can be compared to drying laundry. Convection evaporation is independent of radiation. This evaporation can take place during the day as well as at night. The heat required for this is delivered by convection.
The plant can absorb and emit energy by convection. This occurs when the air in the greenhouse flows along the leaves of the plant and there is a difference in temperature between the leaf and the air. When the leaf is warmer than the greenhouse air, energy is released; when the leaf is colder than the greenhouse air, energy is supplied. The air movement is an important factor in this whole process. When the air is still, the temperature of the leaf and that of the directly surrounding air (the so-called boundary layer) are identical and there is no longer any net energy exchange. The higher the airspeed, the larger the energy transfer.
In the case of insufficient radiation, evaporation must be guaranteed through convective evaporation. At low relative humidity, quicker sufficient evaporation is possible. Thanks to air movement, it is possible to enable energy transfer through convection. This will allow the crop to keep evaporating at a higher relative humidity as well. An active growth climate starts with sufficient air movement around the plant. An additional advantage is that temperature differences are reduced and that no fluid accumulation around the plant can take place.
Higher temperature and higher relative humidity (RH)
It has become clear that plants perform better with the combination of a higher temperature and a higher relative humidity. In other words: plant stress can often be better controlled with a higher RH. Intensive sunlight combined with a low RH quickly causes stress in a plant because the plant cannot make up to a sufficient extent the loss of water through evaporation. At a higher RH a plant can often grow better because evaporation is reduced and CO2 can be absorbed better. This generates more assimilation and thus more growth. Anthurium is not able to absorb much water. Therefore it is important to limit evaporation.
Most plants release radiant heat for approximately 60% through evaporation. Plants can also release excess energy by outgoing radiation, reflection and convection. As long as the balance, i.e. the difference between supply and release, is positive, the plant experiences the climate as ‘active’. The energy balance is positive and the plant can use the surplus energy to keep the evaporation going.
The plant temperature is normally higher than the room temperature in the case of incoming radiation. In such a situation, not enough water is evaporated in order to release all the solar radiation. At a higher plant temperature, more energy is released by convection and radiation, thus creating a new balance.
With the position of the stomata, the plant itself regulates the leaf temperature and, at the same time, the proportion between convection/radiation and evaporation. When the stomata close, the plant temperature will increase too much. The photosynthesis will be inhibited when the stomata are closed because the plant cannot absorb enough CO2.
With a higher greenhouse humidity level, the plant can process more incoming radiation. This is a consequence of the fact that a plant will evaporate less water in this situation. It creates less rapidly a shortage of water as a result of which the crop is able to continue regulating the release of temperature through evaporation for longer. By ‘squeezing’ instead of closing the stomata, more incoming radiation can be used for assimilation and a larger share of the heat will be emitted by convection.
By using high pressure atomization, amongst other things, more heat is extracted from the air in the greenhouse by the water applied. Keeping a higher temperature and RH in the greenhouse implies that ventilation is required later and that more CO2 stays in the greenhouse, as a result of which assimilation is increased. If it is not possible to keep the RH up to standard, screens will have to be used earlier in order to prevent the plants from suffering stress (source: *).
Outgoing radiation and closing of screens
Radiation is the heat you feel when you are standing in front of a stove. Outgoing radiation is the phenomenon that means that something can cool down by emitting long-wave radiant heat. You can feel outgoing radiation when you are standing in a warm room/greenhouse close to a cold window. Due to outgoing radiation, the plant temperature drops below the temperature in the greenhouse. This increases the risk of the plant temperature dropping below the dew point of the greenhouse air and the crop getting wet (source: *).
When, in case of low incoming radiation, the screens are open, there is quickly more outgoing radiation than incoming radiation. Bear in mind that with clear weather (irrespective of the outside temperature) the outgoing radiation can quickly amount to 50-100 watt/m2. If the outgoing radiation is 100 watt and the incoming radiation is 100 watt, then you still have a loss of heat of 30 watt with an incoming radiation transmission percentage of 70% as a consequence of the outgoing radiation. Heat is also lost if the greenhouse temperature is higher than the outside temperature (by convection). This means that the plant temperature will drop below the greenhouse temperature, which will lead to limited or no evaporation.
Therefore, in the case of lower outside temperatures, for Anthurium the outgoing radiation is a reason for closing the screens when the incoming radiation drops below ±150 watt/m2. For Phalaenopsis, radiation of approximately 120 watt/m2 will be needed to be able to open the screens without extra heating at an outside temperature of 12⁰C and the minimum use of tubes.
It is therefore often important to keep the screens longer closed at the beginning of the day and to close the screens earlier at midday/evening in order to avoid too much outgoing radiation. With foil the outgoing radiation is pretty much identical, but less heat will be lost by convection. It is also possible to leave other screens open with lower incoming radiation and to achieve more incoming radiation on the crop, especially on darker days.
Position of the sun and allowing light to enter
The amount of sunlight received by the earth’s surface per square metre depends on the distance of the earth from the sun and the position of the sun. This amount of radiation varies during the day as well as during the year. In areas around the equator there is mainly a variation during the day which is fairly constant throughout the year.
De stand van de zon is gedurende de winter lager zodat er per m2 minder zonnestraling wordt ontvangen. Daarnaast zal door een lagere zonnestand meer licht worden onderschept door de atmosfeer, maar ook zullen kas en installaties relatief meer licht onderscheppen.
The position of the sun during the winter is lower so that less solar radiation is received per square metre. In the case of a lower position of the sun, more light will be intercepted by the atmosphere, but also the greenhouse and the installations will intercept relatively more light.
In Holland, the radiation sum in the summer months is approximately ten times higher than in the winter months. The amount of direct radiation in the winter months is also lower than in the summer. The amount of direct radiation in the winter is on average around 20%, while it amounts to around 40% in the summer.
The amount of PAR light in global radiation varies. On cloudy days, the amount of PAR light increases. The higher the cloud cover, the lower the total incoming radiation becomes (see table 1). Both the altitude of the sun and the cloud cover have a great influence on the intensity and composition of the daylight.
|Tabel 1: Irradiance of daylight in W/m2 and percentages.|
|Height of the sun||Cloud degree||Incoming|
Only part of the global radiation and the PAR light enters the greenhouse. In particular, the glass or foil on the greenhouse effects the total transmission of the greenhouse. Because the amount of natural diffuse radiation is large, the transmission of the glass or foil for diffuse radiation is also important throughout the year. Yet the transmission for direct radiation is also a determining factor for the total light sum in the greenhouse and on the crop.
Winter climate and winter leaf
In the winter, the incoming radiation until the shortest day decreases considerably because of the lower position of the sun and the higher cloud coverage in Holland and in the countries situated further away from the equator. As a result, at the beginning of the winter more incoming radiation can be allowed, but light sums will decrease nevertheless. Because of lower light sums, growth decreases substantially, as a result of which weaker leaves are formed. Because Anthurium and Phalaenopsis do not grow that fast, the leaves that are formed in the winter will start to develop towards the spring.
A few weeks after the shortest day, the incoming radiation increases considerably by a combination of the higher position of the sun and decreased cloud cover. This means that around February much more energy reaches the crop than at the time when the leaf is formed. The winter leaf that is already present on the crop is often not able to process this increase. Therefore, screens should be used at this time already with lower radiation in order to allow the crop to familiarise itself with the greater amount of incoming radiation. With the changeover to more radiation, the crop will be able to process the radiation better if the moisture content in the greenhouse can be kept high.
A better understanding of physics together with knowledge of the plant will allow you to improve climate adjustments and thus optimize cultivation.
This article was written in collaboration with Bureau IMAC. If you require more information or additional advice, please contact Bureau IMAC.
* Bron: Van Weel, P.A. en van Voogt, J.O. 2012, ‘Physical analysis of the moisture and energy balance of a greenhouse.’ Wageningen University Cultivation under glass (report GTB1185).