Humidity control is one of the most critical factors in greenhouse farming. It directly affects plant health and growth, disease prevention, and overall crop quality.

Excessive humidity can create a breeding ground for mold and mildew, while insufficient humidity can stunt plant development and reduce yields. Striking the right balance is essential to maximize productivity and protect your investment.

In this guide, we’ll explore how to optimize greenhouse humidity to achieve higher crop yields. From understanding the optimal humidity for crops, to leveraging advanced tools like dehumidifiers and automated climate control systems, we’ll cover everything you need to know to create a thriving growing environment.

Let’s dive into the science and strategies behind effective greenhouse humidity management.

In order to properly optimize your greenhouse humidity levels, it’s important to understand how and why humidity has such a big impact on your crops.

Besides impacting growth, humidity is also one of the leading factors in disease development.

Some of the most common molds and mildews, such as botrytis and powdery mildew, are a direct result of high humidity. This is due to the nature of these fungal pathogens, which require moist conditions, or even free water in order to develop.

Maintaining ideal humidity levels can completely prevent the development and outbreak of greenhouse diseases. Growers around the globe limit greenhouse humidity levels in order to create a mold and disease-free growing environment, without any sprays or chemical treatments.

So, humidity has a major impact on plants, making humidity control a critical part of greenhouse climate optimization.

Whether building a new facility, or retrofitting an existing one, there are a few factors to consider when incorporating humidity control into a greenhouse design.

First, you must determine the right relative humidity level for your crops.

Every plant has its own optimal humidity range, in which it maximizes nutrient uptake and processing. This leads to optimal growth and development.

Some crops are also susceptible to specific diseases or mildews. These can often be prevented by maintaining a relative humidity level that doesn’t allow the pathogens to develop.

So, keeping your greenhouse humidity within the ideal range for your crops will lead to larger yields and higher quality.

Another critical factor to consider when it comes to humidity control is temperature. In fact, the two are so directly related, that you can’t effectively discuss or control greenhouse humidity without temperature control.

Humidity is, most commonly, measured as relative humidity (RH). But what is it relative to?

Relative humidity measures the amount of moisture in the air, as a percentage of the maximum moisture the air can contain.

Warm air can “hold” more water vapor, or moisture, than cooler air. So, as the air warms up, it can contain more moisture, and vice versa. That means relative humidity is directly impacted by the temperature.

So, even without changing the actual amount of moisture in the greenhouse, relative humidity can still shift drastically. When the temperature drops, relative humidity rises, and when temperatures rise, relative humidity goes down. This is known as the temperature and humidity balance.

This is important to understand, because your humidity control system must take the temperatures in your greenhouse into account.

The third key factor for moisture management is airflow, air circulation and ventilation. While closely related, these are separate elements. Ventilation refers to air exchanges with the outdoors, while airflow, or air circulation, refers to the movement of air inside the greenhouse.

Ventilation is one of the most basic greenhouse operations. Growers ventilate in order to refresh the air in the space and to regulate temperature and humidity. It’s an effective method, when the weather outside is comfortable and allows it.

Airflow is achieved through fans. Greenhouses utilize a wide range of fans, including horizontal fans, vertical fans, and air circulation fans. Greenhouse airflow management is important, as it breaks up any microclimates that may form between plants or inside dense foliage.

Constant air movement also stimulates the leaves, promoting optimal transpiration and nutrient uptake.

Humidity control for greenhouse farming is more complex than other climate control factors, such as temperature, lighting, etc.

This is mainly due to the traditional method of reducing humidity – using greenhouse ventilation systems, combined with heating. As mentioned above, ventilation is an effective way to regulate your greenhouse climate when conditions allow it.

However, weather isn’t always favorable. When conditions aren’t right, you won’t be able to achieve a humidity reduction. This includes cold, damp, or rainy days, as well as during the night, in most climate zones.

Even when it is possible to reduce moisture using ventilation, it’s far from an accurate control method. Weather changes minute-to-minute. So, you’ll never be able to precisely maintain an accurate relative humidity setpoint, ultimately reducing control over the greenhouse climate.

Ventilation can also be a wasteful practice. As you exchange air with the outdoors, you release treated air and introduce untreated air. In most cases, this means you release heat, requiring you to compensate with additional heating.

So, without precise humidity control systems, it’s nearly impossible to maintain optimal relative humidity (RH) levels in the greenhouse. This leads to fluctuations between high and low humidity, which stresses the plants.

When humidity is too high, plants struggle to transpire. There isn’t enough “free space” in the air to release the moisture, causing the process to slow down, or even stop completely.

So, high humidity limits transpiration, and therefore nutrient uptake. It slows growth, limits development, and has a negative impact on flowering and fruit set rates.

Additionally, as mentioned above, high humidity can lead to disease and mold outbreaks.

Low humidity can also harm plant development. When it’s too dry, most plants will react by closing the stomata on the leaves, limiting transpiration in order to preserve water.

This ultimately has similar results – reducing transpiration, inhibiting nutrient uptake, and limiting growth and development. Consistently low humidity will cause plants to dry out, shrivel, and ultimately die.

An ideal climate control protocol should combine different methods in order to create ideal growing conditions, alternating for best results. This is ideal for greenhouse energy efficiency as well, allowing growers to constantly optimize for cost-effectiveness.

Active dehumidifiers for greenhouses provide full coverage, being able to control humidity at all times, without relying on outdoor conditions.

Condensation-based dehumidification systems, such as DryGair, actively extract moisture from the air. They maintain a desired relative humidity setpoint, at a high energy efficiency rate. The ability to control humidity throughout the night, as well as during periods when weather isn’t beneficial for ventilation, enables effective year-round cultivation.

So, using dehumidifiers is a highly effective, cost-efficient method to create the ideal conditions for your crops.

Another important element for your greenhouse operation strategy is automated greenhouse climate control. In fact, modern precision agriculture relies on automation to optimize most greenhouse operations.

Automating your climate control technology allows you to not only optimize, but to do so constantly. As conditions shift throughout the day, or throughout the year, automation allows your greenhouse to adapt in real time.

When it comes to humidity control, there is a variety of smart greenhouse solutions. These range from automated humidity monitoring, using advanced humidity sensors and data logging, to smart dehumidification systems that react in real time to maximize effectiveness, as well as efficiency.

Humidity management is, in essence, water management. The water you use for irrigation is the same water that will eventually become moisture.

So, balancing the amount of water you use for irrigation, with the amount of water you extract through dehumidification, is critical. If your irrigation water greatly exceeds the amount of moisture you can handle, you’ll struggle with high humidity.

Using active dehumidifiers with water meters allows you to calculate the exact amount of water you remove from your greenhouse. This lets you track your water balance precisely.

It’s worth noting that dehumidifiers collect the water they extract from the air. You can then use this water to mix with irrigation or drainage water or use for other purposes. So, besides controlling humidity, dehumidifiers also improve water recycling in greenhouses.

One of the main objectives of humidity control is disease and pest management. It allows greenhouse growers to manage diseases and molds without using traditional toxic sprays.

Preventing plant diseases in greenhouses is a challenge. Plants create moisture, so in a closed, untreated greenhouse, high humidity is inevitable. This humidity is the leading cause for mold development. Moist or wet conditions are necessary for fungal spores to turn into full-blown mold outbreaks.

Traditionally, growers would use fungicides and other toxic treatments in order to manage mold outbreaks. However, in recent years it’s become apparent that greenhouse mold prevention is far more effective and efficient than combating diseases after they break out.

Most mold prevention boils down to one simple concept – condensation prevention. By reducing humidity, you can avoid dew point condensation, meaning moisture won’t turn into free water in the greenhouse. This can almost completely prevent greenhouse mold and mildew outbreaks.

Bylinky S.R.O., the largest fresh herb producer in the Czech Republic, used to struggle with mildew outbreaks.

“We used to have a big problem with downy mildew on basil, especially during the summer”, says technical manager, Václav Bajko, in an interview. “The problem is most pressing when it rains outside and the temperatures are high, those are ideal conditions for the development of downy mildew on basil leaves.”

“Our goal is to achieve a maximum of 80% relative humidity in the greenhouse. During the winter, as soon as humidity rises above 80%, we heat and ventilate. But this isn’t possible during the summer months, from spring to autumn” he adds.

The conditions during the Czech summer don’t allow for humidity control through ventilation. It rains often, leading to high outdoor humidity levels, and it’s warm as well, so heating wouldn’t help solve the issue. These conditions make ventilation ineffective, as it doesn’t reduce humidity inside the greenhouse.

In 2020, Bylinky invested in their first two DG-12 units, out of 6 they would eventually purchase. The dehumidifiers provide Bajko with the tools needed to control humidity, at all times.

Václav Bajko sums things up “Since using DryGair we’ve had no problem with downy mildew on basil, which is our main and best-selling product.”

When it comes to climate control systems, such as humidity management, one of the most important factors to consider is energy efficiency.

Energy efficiency ultimately determines the cost of running your humidity control, whether relying on ventilation or active dehumidification. Choosing sustainable greenhouse practices may make the difference between a profitable greenhouse operation, and a failing one.

The energy you use to control humidity is mostly determined by the method you choose, based on your greenhouses’ specific needs and situation.

The difference between relying fully on ventilation and heating, as opposed to integrating active dehumidification, is significant. In fact, greenhouses in most climate zones can save 50% on heating costs by adding dehumidifiers to their toolbox.

These savings are realized by retaining heat in the greenhouse. As discussed above, ventilation releases a lot of heat, which in most cases, needs to be compensated for. So, by keeping the greenhouse sealed and insulated, and using dehumidifiers to reduce humidity, you can drastically cut energy costs.

It’s also worth noting that condensation-based dehumidification systems, which are most commonly used in horticulture, run on electricity, while heating mostly relies on fossil fuels. This opens the door for more savings, as renewable energy sources become available, and fossil fuel taxes rise.

Choosing the right equipment for your greenhouse has a large impact on profitability. It’s important to choose the most efficient equipment, as well as building the right operation protocol.

If you decide to invest in greenhouse dehumidification, you should compare energy efficiency rates, to ensure you choose the most energy-efficient dehumidifiers. Efficiency can be measured as water extraction per kWh.

However, it’s critical to ensure the extraction rates you compare refer to the same temperature and relative humidity, as these determine the efficiency rate. It’s always ‘easier’ to extract water vapor in higher temperatures and high humidity.

Dehumidifiers that are designed for horticulture use, rather than general industrial purposes, are designed to operate at lower temperatures, around 18°C (64°F) and 80% RH. These conditions are common in greenhouses and therefore provide realistic efficiency rates.

It’s also important to compare additional features, such as air circulation fans, compatibility with climate control interfaces, and smart systems.

Before diving into your dehumidification market research, our experts have answered some of the most frequently asked questions about greenhouse humidity and humidity control.

Different crops have different optimal relative humidity ranges, in which they maximize growth and quality. For example, the ideal humidity for tomatoes is generally around 60-85% relative humidity.

The specific relative humidity level can differ according to variety. It’s also common practice to maintain slightly higher humidity during the day, and lower overnight.

These are some common crop-specific humidity requirements for greenhouse vegetable and flower crops:

• Vine crops (tomatoes, cucumbers, peppers) – 60-85%
• Leafy greens (lettuce, bok choy, mustard greens etc.) – 60-80%
• Basil – 70-85%
• Tulips – 70-75%
• Gerbera daisies – 50-70%
• Roses – 70-80%

Optimizing your systems and protocols is necessary for effective and efficient humidity control.

Using humidity monitoring tools, such as relative humidity sensors, lets you introduce more precision to your protocol. Monitoring provides valuable information, determining your total necessary dehumidification capacity, as well as minute-to-minute optimization.

Modern-day facilities use smart greenhouse technology, including advanced sensors, climate control computers, and real-time automation, to consistently create the ideal growing environment, while minimizing operation costs.

Humidity control is a critical aspect of closed environment horticulture. Like temperature, radiation, and irrigation, you can use humidity control as a tool to optimize crop yields, improve quality, and prevent common greenhouse diseases.

Additionally, utilizing dehumidifiers in your greenhouse can offer more than yield improvements. It’s an effective way to modernize your greenhouse, reduce energy consumption, and improve the sustainability of your business, among other things.

Interested in hearing real-life experiences with greenhouse humidity control? Check out Miljøgartneriet’s story – How dehumidification plays a crucial role in the largest tomato producing greenhouse in Norway.