Growing crops in greenhouses has several advantages. At its most basic, it lets growers harness sunlight, while still isolating their plants from the environment.
But modern horticulture takes things a step further. In a closed growing space, growers can begin to optimize their growing conditions, including temperature, radiation, day length, and more, all contributing to larger, healthier plants.
One of the least discussed yet most critical climate factors in cultivation is humidity. High humidity leads to disease and mold outbreaks and inhibits plant growth and development.
By introducing humidity control measures, namely dehumidification, growers can avoid high humidity — preventing diseases, increasing yields, improving produce quality, and even saving on energy.
Plant Transpiration Makes Greenhouses Extremely Humid
Plants continuously transpire water vapor. Contrary to common belief, they do so even over-night, when they’re seemingly less active. This means that growing plants, in any closed space, without humidity control, will result in constant high humidity.
Transpiration is a crucial part of a plant’s metabolism.
Simply put, plants are made up of pipes, known as xylem. These pipes carry water throughout the plant, from the roots to the leaves, where water is eventually evaporated. They do so in order to transport nutrients throughout the different parts of the plant, supplying the necessary compounds where they’re needed.
This process seemingly defies gravity. So how does it work?
Water evaporation from the leaves creates a pulling effect, called the xylem pull. This force causes water to travel up the plant, which, in turn, leads the roots to suck up water from the ground. Similar to drinking from a straw.
This fascinating process is one of the building blocks of plant life. But it also means the plants must constantly transpire in order to stay active.
The Problem with Greenhouse Humidity
So, in a closed growing environment, humidity buildup is inevitable. But why is that even a problem?
High humidity slows metabolism
There are two main ways in which humidity negatively affects cultivation. The first, is the impact on plants’ ability to transport nutrients and effectively grow, develop, and bear fruit.
Plant metabolism relies on transpiration. But in order for water to evaporate, the air surrounding the plant must be able to absorb more water. If the air is already saturated, meaning 100% relative humidity, water won’t be able to evaporate.
So as relative humidity rises, plants are able to transpire less and less water, inhibiting their ability to take in and process the nutrients they need to develop.
Humidity promotes mold & disease development
The second way in which high humidity harms plants is by creating an environment ripe for molds, mildews, and diseases. Many of the most common greenhouse diseases may only develop in humid conditions, or in the presence of water.
This is a serious issue for most growers, as when relative humidity reaches 100% (often referred to as the dew point), water begins to condense on various surfaces, including pipes, rafters, and even on the leaves themselves. This creates a hotbed for disease development. Once molds develop, it’s extremely hard to eliminate them, as simply reducing humidity after the fact doesn’t stop the spread.
Humidity related diseases vary. Some may cause plants to shrivel, rot and die, while others simply slow them down and hinder their development. In both cases, these diseases are costly to manage, reduce yields and decrease the value of the produce.
Why Traditional Greenhouse Humidity Control Methods are Insufficient
Most growers attempt to avoid high humidity to prevent these issues. But the most common traditional solution is inefficient and often ineffective.
How heating & venting worksControl Methods are Insufficient
The most widespread method growers use to deal with greenhouse humidity is to combine heating and venting. These can work simultaneously to reduce relative humidity through two separate mechanisms.
Relative humidity (RH) is a tricky concept. It represents how saturated the air is with water vapor, as a percentage. So, 100% RH means the air is completely saturated and can’t physically carry any more water molecules.
But air and water have a unique relationship. As the air gets warmer, its ability to hold water increases. So, heating the air in fact reduces relative humidity, without actually removing any water vapor.
The problem is that reducing relative humidity by heating has strict limitations. Plants require specific temperatures to maximize their growth potential. So, if conditions in the greenhouse are already too warm, heating will only cause more harm.
On the other hand, ventilation, meaning releasing moisture from inside the greenhouse, reduces relative humidity by physically removing water vapor. When the conditions outside are right, this method is very effective.
Heating & venting is energetically inefficient
Growers combine these two methods in a way that often requires them to work simultaneously. Ventilation may be effective at reducing humidity, but it also introduces new air from outside. Air that isn’t acclimated and is often too cold for the plants. This creates a vicious cycle, in which growers heat the greenhouse air, only to release it, and heat new air.
Needless to say, this method requires large amounts of energy, and is very wasteful. Not to mention expensive, and environmentally harmful.
Furthermore, in order to effectively release humid air, weather conditions outside must allow it. If it’s rainy, humid, or too cold outside, ventilation may fail to reduce humidity, leaving growers with no practical solution. This is often the case during the night, when temperatures drop, causing relative humidity to spike.
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Controlling Humidity
with Dehumidifiers
Luckily, over the past couple of decades, technology has caught up with growers’ needs, providing a viable and efficient solution for all situations: dehumidification.
Using dehumidifiers to prevent disease
Most dehumidifiers designed for horticulture use refrigeration to remove water vapor from the air. This is done by passing the humid air through cold coils, in order to force condensation. Contrary to condensation elsewhere in the greenhouse, the water collected in the dehumidifier is safely drained.
Thus, growers can reliably reduce humidity from inside the greenhouse, without requiring ventilation or heating. They can even collect the water they extract and reuse it.
This means growers can effectively avoid dew point condensation, preventing the formation of various molds and diseases.
Stimulating growth with dehumidification
But controlling humidity isn’t just about avoiding very high humidity and condensation.
Just like plants have a preferred temperature range, they also have a preferred relative humidity range. For most plants, this range is between 60% to 80%. When RH is within optimal range, plants develop at an optimal pace — they grow larger, quicker, healthier, and provide more fruit.
Saving energy by reducing
heating & ventilation
By utilizing dehumidifiers at times when heating and venting are ineffective, or when they require large amounts of energy to constantly reheat, growers can drastically cut their energy costs.
When taking energy savings into account, along with larger yields and higher product quality, it’s no wonder greenhouse and indoor growers around the world are turning to dehumidification to increase their profits.
Rom Meir is a writer and researcher, specializing in horticulture, ag-tech and sustainability. His work with DryGair deals in the intricate connection between greenhouse and grow room physics, agronomy, and energy. Prior to working with DryGair, he gained experience in environmental regulation consultancy, hydroponic cultivation, and solar energy. DryGair manufactures one of the leading dehumidification solutions for greenhouses and grow rooms in the cannabis industry. Learn more at drygair.com.