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Humidity Control in greenhouses
 
 

Humidity refers to the quantity of water present in the air as vapour. 

If we set up a fogging system (or boil some water) to continually put water fog/vapour into a closed greenhouse the humidity will continually climb until a point is reached where it is impossible to add any more water vapour.  This is called the saturation point or 100% relative humidity (RH).  More water can only be added after an equal amount has condensed out of the air.  The condensation will occur on the coolest surfaces available.  If no cool surfaces are available then the fog/vapour droplets would combine and fall as water drops onto the floor ie it will rain.

Humidity is most often measured as Relative Humidity (RH).  This represents the amount of humidity present in the air as a percentage of the maximum possible.  100% RH represents saturation of the air and 0% RH represents absolutely dry air.  Greenhouses often operate between 60% and 80% RH

 A few lessons in physics

 1) Temperature and relative humidity are inter-related

If you take a fixed volume of air with a fixed amount of moisture in it and change its temperature the RH will change dramatically.  By heating air you tend to dry it out and the RH falls.  In fact, increasing the temperature by as little as 10^oC  nearly doubles the water holding capacity of the air.  This means that if the RH was 100% at a temperature of say 10^o C then heating to 20^o C would lower the RH to little more than 50%. 

Conversely, by cooling air the RH rises and eventually it will reach saturation condensation will occur.  The temperature at which this occurs is called the Dew Point.  This is another important measure of humidity. (Note that although it is a measure of humidity it is expressed in deg C).  Condensation will appear on any surface whose temperature is at, or below, the dew point.  Normally, the roof and walls cool down first and this is where you will notice most condensation.  

2) Plants transpire a lot of moisture into the air. 

A tomato vine will transpire about 0.2L per hour into the air in summer.   That’s almost 2L per day per plant.  For a 2000 m² house with 2.5 plants per m² that’s a total of 5000 plants and nearly 10,000L per day of transpiration to get rid of.  Plants are the primary humidifiers/coolers of greenhouse air.  It is therefore vital to provide adequate irrigation on hot, sunny days. 

 3) All of this moisture must go somewhere.

It all has to go out the window or else it has to condense on something.  
Ventilation is obviously critical in the control of high humidity.

 Effects of Humidity  

1) Humidity too high. 

Usually at night or in winter when vents are closed the humidity will build up.

• If humidity becomes excessive, the plants stop transpiring, and if warm, the stomata open wide exposing free water that won’t evaporate. 

• Sap flow slows and consequently mineral uptake diminishes.

• Plants lose turgor. 

• In this situation the plant is an excellent haven for fungal spores. 

• Soft growth.

• Mineral deficiencies appear due to low sap movement.

• Condensation dripping from the roof causes fungal diseases

• Early morning condensation on cool fruits and leaves causes fungal diseases.

2) Humidity too low 

Typically in hot dry weather with vents wide open. 

• Plants close stomata to try to stop wilting, therefore they stop absorbing CO² and growth slows. 

• Plants become dry and leaves become hard and may develop dry tip burn.

• Spider mites proliferate

• Stunted plants

• Wilting occurs if conditions are very severe.

So, what level of humidity is ideal?

 The answer to this question depends on the type of crop to be grown.  Plants from tropical climates will need a higher humidity than those native to arid regions.  Most greenhouse crops favour relative humidities between 60% and 80% RH.   However, the ideal RH also depends on the temperature and plants require a higher humidity when the temperature is high and a lower humidity when it is low.   This helps to moderate the transpiration rate of the plant eg when it is hot the plant will tend to transpire heavily so by making the RH high this will tend to reduce transpiration and delay the point at which the plant starts to wilt.  

The converse is also true ie when temperatures are low then RH should be reduced.  At low temperatures the plant will tend to reduce transpiration (with consequently reduced growth) but by reducing the RH this will tend to raise transpiration back up again.

The grower with simple humidity controls can address this temperature dependence by reducing RH set points during winter and raising them in summer.

The following table gives you some idea of the ideal level of RH for a typical crop.  

You can see from the table that the higher the temperature is, the higher the humidity should be.  The above makes it difficult to specify control parameters as different RH settings are required at different temperatures.

Another measure called VPD is gaining popularity in the greenhouse industry as it combines both temperature and humidity effects in a way that better reflects how the plant “feels”.  VPD stands for “vapour pressure deficit” and while we won’t go into the physics of it here it is worth noting that it represents the lack (or deficit) of moisture in the air.  Consequently, the numbers run in the opposite direction to RH ie a VPD of zero represents saturation or 100% RH and larger VPD numbers represent increasing “dryness”. 

 Now look how much simpler the above table is made by using VPD as the whole of the above table is contained in just three VPD values as follows:-  

 Controllers are increasingly implementing venting controls based on VPD rather than just RH and temperature.

 Control strategies

 What are the options for dealing with high humidity? 

 We have seen that temperature is intricately entwined with humidity.  Take a greenhouse in the evening with a fairly low RH of 50% and temperature of 25^oC.  Imagine a sudden drop in temperature down to 15^oC.  The vents close tight and, without adding any water to the air, the RH will soar to over 90% and in some corners or where exposed to cold surfaces condensation WILL occur.   

If this happens on the greenhouse roof and if this a good design so that dripping is minimized with internal gutters that collect and lead away the condensation then this can actually help to reduce the humidity in the air.  

However, the RH will still be very high and will be even higher inside the crop canopy.  Radiant heating applied low down by pipes running amongst the crop will help to warm the plants and avoid condensation directly on the crop.  Gentle fan stirring will also help to ease the humid air out of the crop canopy where it will rise and condense out on the cold roof.   If applied correctly, this can be a reasonably cost effective method of humidity control.  

Now imagine that we have a double skinned greenhouse or a single roof skin with a thermal screen below.  The humidity from the crop is now trapped below the screen (or inside cover) and no longer has such a cold surface to condense on.  If the humidity gets very high it will only need to find a surface which is a few degrees cooler to start condensing.  This situation is not quite so easy to deal with.

On the positive side, the thermal reflecting screen will help to prevent the crop from loosing radiant heat.  Always remember that transpiring plants will be adding water vapour to the air which must go somewhere or the RH will continually climb.

 There really are only three options:-

1) Ventilate to remove the moist air
2) Heat the air to dry it out (but eventually the water must go somewhere)
3) Condense the water on the roof (or use a dehumidifier)

Or some combination of the above.

 Lets look at a few scenarios and try to decide on a best strategy.  Obviously the design of the greenhouse, type of crop etc will have significant influence on some of these suggestions.

Scenario 1 – moderate temperature ie temperature either in the opening range of the vents or close to either side of the opening range.

a) high humidity

The strategy normally used here is to change the venting set point a little to cause the vents to open sooner (or wider) in order to allow the humid air to escape and to be replaced by drier, cooler air.  

Another simple strategy that can be employed to lower high humidity is to increase the heating (raise the heating set point). This is especially effective where the heat is applied inside the crop canopy.  In extreme cases the heating and venting setpoints can be overlapped so that heating and some venting occur simultaneously.  The cost benefits of doing this must be carefully weighed up by the grower.

b) low humidity

The strategy normally used for low humidity is to change the venting set point a little to cause the vents to open later (or less wide) in order to reduce excessive humidity.

Obviously, there is a limit to what can be achieved (without cooking or freezing the crop) but these passive control strategies are very low cost and use little energy.    

Scenario 2 – High humidity in very cold conditions

In an attempt to save energy a mode of operation called “purging” may be used to reduce high humidity.  Here, the heating setpoint is raised to help dry air and the crop and then periodically the vents are cracked open to allow the warmed humid air to float up and out and allow cooler air to enter.  Surprisingly, this works even when the outside air is quite damp providing heating is used as the rise in temperature inside will dry the incoming fresh moist air.  However, the transpiring crop will eventually bring the RH back up again and the vents will need to be cracked again.    Providing the crop is not over sensitive to the sudden drops in temperature this is a viable low cost method of humidity control.  Where crops are sensitive to sudden changes in temperature an alternative is to set a “minimum crack size” for the leeward vent and to just elevate the heating set-point.  There is normally an override on the minimum vent crack so that if the temperature drops below a certain value the vents will fully close – this is sometimes referred to as “frost protection”.   A minimum pipe temperature setting will also help in these conditions as there is always some low level heat radiating from the heating pipes keeping the crop warm and dry.

Scenario 3 – High humidity in very cold conditions with a threat of condensation on crop at sun-rise.

This is the situation where the sun comes up after a cold night and the leaves (at least on the sunny side) warm quickly and start transpiring.  The rest of the plant is still cold and below the dew point of the now moist air.  Condensation can occur and in some weather conditions the fruit may stay wet for some time – long enough for disease to set in.

A reflective thermal screen will help to prevent the crop from loosing radiant heat to a clear sky but will also trap the humidity down close to the crop.  By cracking the screen periodically, say 10% open, the humidity will escape to the upper cavity away from the crop.  A controller that automates this is very useful.

A further strategy that may be used to avoid morning condensation on the crop is called pre-dawn heating.  As the name suggests, the heating set point is gradually ramped up in the early hours of the morning so that by dawn the temperature of the plant fruit, stems and leaves are only a little below the day-time venting temperature.  Modern controllers automatically calculate the time to start ramping the heating up in order to achieve the desired dawn temperature.    

Scenario 4 – Low humidity in hot conditions

In this situation it is not desirable to try to close the vents as the temperature would rapidly build up and harm the crop.  The only option is to apply fogging or other humidification.   Luckily, heat and low humidity usually come together and fogging will address both of these issues.  As the fog turns to vapour, it takes a lot of heat out of the air and can result in considerable temperature drop – 5^oC not being uncommon.  Of course it also adds humidity at the same time which is exactly what is required to maintain a reasonable VPD.  Foggers with a very small droplet size are the most desirable as the fog vaporizes almost instantly without any wetting of nearby plants.  Fog controllers with an automatically adjustable “puff” size gives the best results.  These give out small puffs of fog when the temperature or humidity error is small and larger sized puffs as the error increases.  

Manual or automatic

Although it is possible to implement some of these ideas in a manually operated greenhouse, to get the most consistent and accurate control a modern environmental control system is essential.  These will typically allow the grower to select the desired temperatures, humidities, ramp rates and allowable control strategies for each time zone (typically four time zones per day).  The controller will then select and implement the most appropriate strategy for the conditions.  Of course to get the most out of any control system the more the grower understands the processes involved the more likely that they will select the most appropriate settings.

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