Modern cultivation is characterized by a constant drive to improve production. This is achieved, for the most part, by controlling and optimizing the climatic conditions in the growing space in order to provide the best conditions for plant growth.
The most basic and intuitive factors - temperature and lighting - have long been dealt with effectively. But an often-misunderstood climate aspect is often overlooked: humidity. This has led to the use of inefficient treatment methods, resulting in unresolved crop loss and reduced product quality.
How Humidity Affects Cultivation
Humidity plays a major part in plants’ well-being.
Plants constantly perform transpiration as part of their metabolism. In cases where humidity is too high, and the air is saturated with water vapor, transpiration ceases, causing nutrient uptake to come to a halt. This restricts the plant’s physiological functions, impeding growth.
High moisture levels have another effect. Many fungal spores and bacteria, commonly found in greenhouses, require humid conditions or free water in order to appear in the form of molds or diseases. Mold outbreaks may occur even during short humidity spikes. But lowering humidity after the fact fails to inhibit their spread. It is therefore crucial to maintain an optimal humidity range at all times.
As previously stated, plants transpire as part of their metabolism. This alone is enough to cause humidity to rapidly build up in an enclosed space. So, although the outdoor climate has a direct effect on greenhouse conditions, high levels of humidity are always a peril in a growing space.
Traditional Means of Humidity Reduction in the Greenhouse
Although it's often misunderstood, humidity hasn’t been completely neglected by growers.
Humidity is most often measured in RH (relative humidity). This takes into consideration both the amount of water vapor in the air and the air’s temperature. As warmer air can physically carry more water vapor, heating effectively reduces the relative humidity, allowing plants to transpire freely and minimizing the risk of condensation. This method is often unviable, especially in warm climates or during hot seasons, when heating will increase temperatures beyond optimal range.
Another basic method of humidity reduction is ventilation. Growers open the greenhouse in order to release the humid air and let in dryer air from outdoors. This method, although often very effective, has strict limitations. Greenhouse air temperatures are always kept within optimal limits. So, when this air is released, and new air introduced, it must be reheated or cooled. This is an inefficient use of energy - it's literally throwing expensive heat out the window!
In addition to its often-inefficient energy use, there are situations in which ventilation does not provide a solution to humidity at all, such as during nighttime, rain or humid outdoor conditions. Additionally, some crops require periods of complete darkness, making it nearly impossible to ventilate naturally. Introducing exterior air in this form may also cause fluctuations in temperature and humidity, resulting in dangerous conditions in which diseases and molds may break out.
For these reasons, growers using traditional heating and venting methods still encounter high relative humidity levels, and even condensation, in the greenhouse. This is most common during dusk and dawn, when conditions change rapidly, leading to suboptimal growth and disease outbreaks.
Using Technology to Battle Humidity
With a better understanding of humidity and its effects, today’s growers have turned to technology to assist them.
Some operations use air conditioning and other temperature control systems, which usually require a large initial investment to achieve a small reduction in humidity. Air conditioning, involving cooling processes, causes water to condense, as the warm humid air passes through the chilled metal. The water is collected and removed, resulting in slightly dryer air being released to the space.
Heating and cooling systems are designed for temperature control. Due to the relationship between temperature and relative humidity, they do achieve a slight reduction in humidity, but at a rather high energy cost. Additionally, this method fails in disconnecting the two factors, meaning humidity cannot be reduced without being forced to affect temperature as well. This both inhibits their use as a means of humidity reduction and undermines energy efficiency.
The next obvious step is to introduce dehumidifiers to the growing space. Dehumidification technology is far from new. It is used in manufacturing processes, libraries and more. Allowing cultivators to disconnect humidity control from temperature control finally allows for dehumidification efforts to be performed when necessary, regardless of the current temperature.
Dedicated Dehumidification in Modern Agriculture
Although many growers still use dehumidifiers built for various purposes, some modern-day agricultural dehumidifiers are designed and optimized specifically for cultivation in the greenhouse environment based on an understanding of the optimal growing conditions required by plants.
This new generation of dehumidifiers is created to operate best under the conditions most commonly found in growing environments, usually meaning moderate temperatures and mid to high relative humidity. While most industrial solutions are designed for around 80oF (27oC), agricultural dehumidifiers are optimized to operate at around 64oF (18oC), making their water removal much more efficient in common growing conditions.
Agriculture-dedicated dehumidifiers remove water from the air using controlled condensation. This is achieved by funneling the air through a cooling mechanism located inside the dehumidifier. Although this is similar to the drying effect of cooling systems, dehumidifiers cause condensation in a controlled unit and are designed to maximize water removal, rather than temperature control. This makes them able to achieve a significantly greater reduction at a much lower energy input.
That said, effective and efficient dehumidification requires much more than a good dehumidifier to achieve.
Optimizing Humidity Reduction Efforts in the Greenhouse
A standard greenhouse environment does not contain one climate. Rather, it is comprised of numerous microclimates, each with drastically different conditions.
The most critical microclimates are found immediately surrounding the plants. This is known as the boundary layer. As plants transpire, the air surrounding them tends to be cooler and more humid than the rest of the space. When foliage is thick and plant placement is dense, boundary layers overlap, creating a dire situation in which condensation may occur on the plants themselves, even if dehumidification efforts are in place.
Microclimates may undermine all efforts to control humidity, or any climate aspect, and can be very destructive. This is why the agricultural discussion has seen a recent shift towards emphasizing climate uniformity. Homogeneous conditions may be achieved with the use of proper air circulation that reaches the entire space, including among the plants.
The Importance of Proper Greenhouse Management
Providing the best conditions for growth is about more than purchasing the right equipment. Greenhouses are extremely dynamic environments, requiring a fair share of know-how and attention to operate optimally.
For efficient humidity control, attention should be paid to the conditions outdoors. During moderate periods, ventilation may suffice throughout the day, but it is important to note that during dusk and dawn, humidity tends to build up rapidly. Additionally, it is recommended to close the greenhouse and use thermal screens while dehumidifying overnight.
Humidity is, in essence, a leading climate factor in greenhouses. Its interconnectivity with other aspects of climate control is huge, and its effect on plant health and growth, as well as the economics of a greenhouse, is immense. Using the right equipment, designed for agriculture, coupled with informed greenhouse operation protocols can produce amazing results in both crop quality and quantity.