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Humidity in cultivation

Besides temperature and light, humidity is the most important environmental factor for a plant to grow. In this article we discuss the importance of humidity and the influence that it has on the anthurium and phalaenopsis plant.

Humidity in the cultivation
Humidity affects the growth of a crop, especially as a consequence of its influence on the total leaf area and on photosynthesis. The leaf surface and a good exchange of CO2 via the stomata are very important for photosynthesis. Humidity influences both of them. In the event of a moisture deficit of 5-10 g/m3, the influence on the CO2 exchange is crucial. However, in the case of a smaller moisture deficit, the adaptation of the leaf area is decisive. The leaf area changes due to the impact of the humidity as regards the formation of leaves and the stretching of cells. It takes some time before you notice this crop adaptation, since the process is quite slow.

The number of stomata on a plant can vary from 60 to 1000 per mm2 and depends strongly on the crop, the age and the place on the leaf together with the growth conditions. In crops with a lot of stomata, these are often small. If there are few stomata, then they are usually large. Stomata are found on the top of leaves, but underneath there are many more. All crops have more stomata per square millimetre when the humidity is high and they are ±30% larger as well. As a consequence, the total surface area of stomata per square millimetre of leaf surface increases considerably. The crop responds to the circumstances that make that evaporation difficult by increasing the total surface at the openings in the leaf.

Higher humidity causes more lavish growth with longer shoots and sometimes larger leaves. When the humidity is increased, plants stretch the main stem, shoots and petioles more. The cell elongation and hence the stretching of the entire crop depends on the cell pressure (Turgor pressure). In the event of low humidity, evaporation is high and the cell pressure of the crops is relatively low. This will slightly slow down the cell elongation. Under conditions with little evaporation (little radiation, high humidity), the cell pressure is generally higher and more stretching can occur. This is why you can see the effect of humidity on stretching best under conditions with little light.

The influence on leaf stretching differs per crop. The plant produces larger, but also thinner leaves, mainly as a result of better cell elongation. The leaf generates approximately the same number of cells. Higher humidity allows the leaf cells to stretch less than the shoots and the petioles. This results in the slightly stretched, lush crop form.

In the event of high humidity, the axillary buds of many plants sprout more easily. Possibly, the axillary buds receive more hormones under the influence of the root pressure which stimulates the sprouting. Pot plants become fuller and hence qualitatively more attractive. For Phalaenopsis this is an important factor in the phase of branch induction.

We often speak of humidity in the form of a relative number like relative humidity (RH) or moisture deficit (MD). RH stands for the amount of moisture expressed in percentages in the air with regard to the maximum amount. MD is the difference in the amount of moisture expressed in grams per cubic metre of air that the air could contain even more up to the maximum.

A moisture level alone does not reveal much. It does not matter if it is represented in RH or MD. More importantly, drainage has to take place. In technical jargon, this phenomenon is also called an ’active climate’. With such an ‘active climate’, evaporation can easily occur because moisture can be constantly drained and extracted from the greenhouse climate. Even in the event of a high RH or a low MD, due to a very active climate there may be a lot of moisture withdrawal, because of which the plant loses a lot of moisture.

Another important influence in this game is convection. Convection is the energy transfer between the plant and the greenhouse air. When the plant is cooler than the air, energy is supplied. This is necessary to enable evaporation without irradiation. Air movement is essential for the energy transfer. This air movement is caused by the warm air that rises. The air is heated by pipes or radiation of the sun or lights. This method of air movement can accelerate the release of moist air, but can also make the greenhouse air around the plant dryer by mixing moist air with less moist air, which will enable it to release moisture more easily. By doing so, a higher moisture level can be pursued. This has some major advantages climatically.

Moist air contains more energy than dry air. We refer to this as enthalpy of the air. When the moisture evaporates, the energy used is ‘stored’ in the air. It will be released again when the moisture condensates, but can also be ventilated. When moist greenhouse air is ventilated outward, more energy is exchanged outward than when the greenhouse air is dry. So, less ventilation is needed to allow the same temperature to decrease. When higher humidity is kept in the greenhouse, the greenhouse temperature can also be kept more stable.

It is clear that good humidity is of great importance for the plant. If you have any additional questions or if you want to receive more information on this or another cultivation-related subject, please contact Bureau IMAC Bleiswijk BV. For more information about humidity in Anthurium and/or humidity in Phalaenopsis in particular, please refer to the other articles in this respect on the website.