Control Yourself: Understanding the Basics of Indoor Climate Control

By Lynette Morgan
Published: December 27, 2016 | Last updated: April 21, 2021 05:46:15
Key Takeaways

Whether you choose to manually monitor and control your growing environment, or you install the most sophisticated automatic controllers and climate adjustment equipment, indoor gardens need some degree of assistance to maintain good growth rates and healthy plants.

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Protected cropping, whether it’s an indoor garden or backyard greenhouse, allows gardeners to create an oasis of plant life, nestled within a cozy and productive environment. While that may seem relatively straightforward, climate control within a hydroponic garden is a little more complex than simply providing four walls and a roof.


The factors that make up an optimal growing environment are all interrelated, tend to differ somewhat between plant species and not only affect growth, but factors such as pests and diseases.

Providing a Protected Environment for Plants

A plant’s environment consists of four main factors that affect growth and development: temperature, light, gasses (carbon dioxide and oxygen), and humidity or moisture levels in the root zone and air surrounding the foliage.


To complicate things further, these factors all interact with each other. For example, if CO2 enrichment is provided to boost photosynthesis, optimal light levels are somewhat higher than if CO2 was just run at ambient levels, and it’s this combination of increased light and CO2 that gives the greatest boost in growth.

Another example is the link between humidity and air temperature. Cold air holds much less water vapor than hot air. When growing plants, this means it’s not just a simple case of aiming for one ideal relative humidity value.

Temperature and light interaction is another commonly misunderstood component of environmental control. If light levels are low, but the environment is warm, plants will elongate with weak, stretched growth and thin leaf cuticles, making them prone to disease.


On the other hand, high light and cool growing conditions can stunt growth and produce overly compact plants. High light levels and high temperatures may result in scorched leaf tips, small, thickened foliage and other physiological problems in plants not adapted to these conditions.

What Can Go Wrong?

Many common plant problems are related to climate control issues, although these are not always obvious to inexperienced growers. Pests such as mites are more prolific under warm, dry conditions and increasing humidity can be used to help slow population growth. Many fungal and bacterial pathogens such as botrytis (gray mold) need high humidity or even the presence of moisture on leaves to infect plant tissue.


Some common physiological conditions seen in hydroponic crops also have environmental causes. Blossom end rot of tomatoes and peppers and tip burn of lettuce and other greens are sometimes induced by high humidity and warm growing conditions, which slows the rate of transpiration from the leaves, hence restricting calcium transportation up through the plant and into new developing tissue.

Other lesser-known problems related to environmental conditions include oedema, glassiness of leaves, leaf roll, scorch of leaves and fruit, blotch, catface, russeting, cracking and crazing of tomato fruit, flower desiccation, lack of pollination and fruit set, fruitlet drop, lack of flower development and many others.

Controlling the Climate of a Growroom

Within a protected growing environment, we have many tools to control the range of factors influencing the climate. At the simplest level this may consist of manually operated vents or windows to give some control over temperatures and humidity while making use of natural light and ambient CO2.

At the more advanced level there are totally enclosed growing facilities that finely control all aspects of climate with heaters, chillers, air-conditioning units, humidifiers, dehumidifiers, air movement fans, full-spectrum artificial lighting, CO2 enrichment, root zone electronic moisture meters and computer-integrated controllers and sensors that continually monitor and adjust the environment automatically.

Most growers use a level of climate control somewhere in between these two extremes based on their budget, size of the growing area and crop requirements.

Heating and Cooling

Hydroponic systems have been set up in some of the most extreme climates on the planet and a major key to the success of these has been temperature control. Most plants we grow hydroponically have an optimal temperature range between 60 and 79°F, while outside temperatures can be drastically different from this.

In a well-insulated indoor garden using HID lighting, temperature buildup is a common problem when the lights are on. Heaters with air mixer fans are relatively simple to install and use to warm the air.

Heat removal, particularly in warm climates and under hot summers, needs additional equipment. Simply venting out the warm air and drawing in cooler air is not an option when it’s sweltering outside, even worse if that heat is combined with high humidity.

In this situation growers have two options. If the outside air being drawn in is of sufficiently low humidity, evaporative cooling can be used to lower temperatures. As water evaporates into the air, energy is lost, which causes the air temperature to fall.

Common methods of evaporative cooling are fan and pad systems, or evaporative cooling walls in greenhouses where air is drawn through or over wet absorbent pads. The cooled air is then circulated through the plants, lowering temperatures in the environment until it is vented out the other side.

For fan and pad cooling in an indoor garden to work at maximum efficiency, the growing area needs to be as airtight as possible so air being drawn in from the outside is forced over the wet pads.

In climates where outdoor humidity is high, evaporative cooling cannot be used. In this situation, air-conditioning units are a good alternative and can also give a higher degree of temperature reduction when required. Air conditioning, however, produces relatively dry air so it may need to be re-humidified with a humidifier, wet pads or open pans of water before being circulated over plants.


Light levels are a vital component of the indoor growing environment, particularly in indoor gardens where no natural light is available. The most common problems many growers face is under-powered lighting systems that don’t provide sufficient light energy for maximum photosynthesis, or overly dense plantations that create a high level of shading within the canopy once plants approach maturity.

Indoor gardens benefit from a wide spectrum of artificial lighting wavelengths. While blue (450 to 495 nm) and red light (629 to 759 nm) power photosynthesis, shorter wavelengths such as ultraviolet have been shown to produce a much greater accumulation of plant flavonoids and other beneficial compounds, as well as playing a role in increased resistance to pest and disease attack, enhancing red coloration and producing more compact plants.

Humidity and Vapor Pressure Deficit

Relative humidity (RH) is a commonly used measure of water vapor held in the air and a term most indoor gardeners are familiar with. However, RH is related to the temperature of the air with cool air holding much less water vapor than warm air.

For example, air at 50°F can hold 0.33 oz. of water per 35 cu. ft., while air at 86°F can hold three times as much. When using RH, it’s hard to set one optimal RH value, particularly when temperatures vary throughout the day and night. For this reason, it’s more accurate to use vapor pressure deficit (VPD) as a measure of the water vapor content of the air and how this affects plant growth.

VPD is a measure of the difference (or deficit) between the pressure exerted by the moisture currently in the air and the pressure at saturation. Saturated air will condense to form dew or condensation and leaf wetness that can lead to fungal diseases.

Plants don’t want an overly dry atmosphere (high VPD) that sucks moisture out of the foliage, but they also don’t want a wet environment (low VPD), which slows transpiration and promotes disease.

VPD is typically expressed in the units kPa (kilo-Pascal) with the range for most plants being 0.45 to 1.25 kPa with the optimum being around 0.85 kPa. Most indoor gardens are best run at 0.8 to 0.9 kPa for healthy mature plants with cuttings needing a more humid environment with lower VPD values.

Technically, VPD more closely matches what the plant experiences in relation to temperature and humidity effects on growth and transpiration. It combines the effects of both humidity and temperature into one value, so it’s easier to use when setting environmental controls.

The VPD can be adjusted with the use of household humidifier and dehumidifier units. Simpler options are to vent out overly moist air from around the plants and draw in drier air from outside when possible, or use open pans of water or wet walls when humidification is required.

Carbon Dioxide

The gaseous component of climate control is often one that is overlooked. Ambient air CO2 levels are around 340 ppm (or 0.034% by volume) outside. However, in an enclosed garden environment, if fresh air is not being continually brought in to replenish CO2, these levels can be completely depleted through the process of photosynthesis.

In the confined air volume of an indoor garden, and even in large greenhouses with restricted ventilation, CO2 depletion is a relatively common problem, particularly where there are many sizable plants, plenty of light and rapid growth rates.

Those who rely on ambient CO2 levels have to ensure there is always sufficient fresh air coming in from outside to meet the CO2 requirements of the indoor garden, even if that means venting out warm air and bringing in cold air from outside which must then be heated.

Using CO2 monitors is one way of checking to see if depletion is occurring. Even a drop of 100 to 150 ppm from ambient levels will start to slow plant growth. Through enrichment, CO2 levels can be as high as 1,500 ppm.

Levels more than 2,000 ppm start to become toxic to many plants, and 5,000 ppm is the threshold for human safety. While there is much debate over which level of enrichment is ideal for each crop, plants will use more CO2 under warm, high light conditions than under dull, cool conditions.

Climate Controllers

Electronic climate controllers are widely used in commercial greenhouses to precisely control the growing environment 24 hours a day. These days there are some excellent scaled-down models of controllers for use in indoor gardening situations.

Provided they are set up and run correctly, they can take much of the worry out of climate control, particularly when you can’t be there to check that all is well several times a day.

Controllers range from the basic models, which simply monitor and control one aspect of the environment, such as heating when temperature falls, through to fully-integrated models that monitor temperature, humidity/VPD and CO2 while also running fans, evaporative coolers, air conditioners, heaters, humidifiers, dehumidifiers, ventilation systems and CO2 enrichment equipment.

Having a good understanding of climate control means understanding what a plant’s requirements are, how these climate factors interact and how they are best adjusted for maximum productivity in your protected growing environment.


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Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants

Profile Picture of Lynette Morgan

Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.

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