Garden & Greenhouse


Gaining Control with Control Modules

Posted June 19th, 2014 by Eric Hopper in

There are many variables that contribute to a successful greenhouse or indoor garden. Environmental conditions, plant nutrition and, in the case of indoor horticulture, artificial lighting are just a few of the components that make up an operational garden. Many inexperienced gardeners think that greenhouse or indoor gardening is as simple as placing a few plants in some soil and letting them grow. This is fine for gardeners who are satisfied with mediocre results from their hobby. More experienced horticulturists and hobbyists who want the most from their gardens have a deeper appreciation for each of the complex variables that make a garden flourish. Experienced gardeners also understand that a consistent environment along with a specialized feeding regiment will maximize production in the garden. In order to create consistent environmental conditions and to ease some of the work associated with maintaining a greenhouse or indoor garden, some horticulturists are using devices known as control modules. Control modules, commonly known as controllers, are electronic devices that aid in the automation of ventilation, CO2 supplementation, fertilization and artificial lighting.

Atmospheric Controllers

Atmospheric controllers are probably the most common controllers found in greenhouses and indoor gardens. These controllers are devices that help to automate individual components of the garden’s ventilation and atmospheric systems. Essentially, the temperature, humidity and air exchange of the garden space are all automated by these types of controllers. A basic thermostat controller that turns on a heater or a fan when the garden space reaches a predetermined temperature can be considered an atmospheric controller. However, some atmospheric controllers are much more sophisticated than a basic thermostat. In fact, some atmospheric controllers control multiple devices at once, including air conditioners, dehumidifiers, intake and exhaust fans and CO2 enrichment equipment. Some of the latest high-tech atmospheric controllers allow the user to continuously monitor the garden’s environment via PC or portable electronic device. This allows the grower to determine exactly when a particular change is occurring within the garden’s environment and how to make adjustments to create a consistent environment.

The most impressive atmospheric controllers contain special microprocessors and customized algorithms. These particular controllers can actually “learn and react” to the garden’s conditions. For example, let’s say every day at 5:00 p.m. the greenhouse or indoor garden’s temperature suddenly rises beyond the desired parameters. The controller turns on the air conditioner to correct the problem. Over an extended period of time, if the same issue is occurring at the same time each day, the microprocessor will recognize the problem and act intuitively to correct it. In other words, at 4:45 p.m. the atmospheric controller will automatically turn on the air conditioner to react before the temperature rises above the desired parameter. These “smart” controllers offer two huge advantages to horticulturists. First, they keep the room’s atmospheric conditions within the desired parameters almost continuously creating a consistent environment in which plants thrive. Second, the intuitive control of devices, such as an air conditioner, will create heightened efficiency for the device and help to extend its life.

CO2 Monitor/Controller

CO2 enrichment is a way that greenhouse and indoor horticulturists can boost their plants’ growth rates. Plants “breath” CO2 and expel oxygen. CO2 is actually just one component in a complex chemical reaction known as photosynthesis. As a grower increases the amount of available CO2, the rate at which photosynthesis occurs also increases (assuming other parameters, including environmental conditions and nutrition, are in check). The two most common devices used to enrich a garden’s atmosphere with CO2 are a steel or aluminum tank which holds compressed CO2 or a CO2 “burner” which uses propane or natural gas as a fuel to create CO2. Unfortunately, a gardener cannot just open up a compressed tank of CO2 and expect the plants to flourish. Plants capitalize on CO2 enrichment most effectively when there is a level of consistency. A CO2 monitor/controller should be used to maintain the most consistent CO2 levels in the garden’s atmosphere. These specialized controllers are designed to measure the amount of CO2 in the garden’s atmosphere and control the CO2 enrichment device (CO2 tank or CO2 burner). Most CO2 controllers allow the user to set the desired concentration of CO2 for the atmosphere. The high-tech CO2 controllers contain microprocessors and algorithm computers that are similar to the high end atmospheric controllers. These controllers also “learn” from the ongoing measurement of CO2 concentration and will intuitively make corrections to maintain consistent levels of CO2 at all times.

Just about every CO2 controller is also equipped with a photosensor and a ventilation override. The photosensor allows the controller to be functional only when the sun is shining or the artificial lights are on. Plants only use CO2 during the light cycle so it is wasteful to have the CO2 device operating during the dark cycle. The ventilation override is useful for greenhouses or indoor gardens where there is an exhaust fan system. The purpose of the ventilation override is to turn off the CO2 device when the fan turns on. This prevents the CO2 from being exhausted from the room and wasted. Many CO2 controllers are compatible with a variety of atmospheric controllers and can be linked (chained) together in order to make all the components work to the highest efficiency.

Nutrient and pH Auto-dosers (Fertilization Control)

Auto-doser systems are usually reserved for large greenhouse and commercial hydroponic growers. These systems can be used to adjust the nutrient concentration of the nutrient solution or the pH of the nutrient solution. The most practical application for these devices is within large scale, recirculating systems. In a recirculating system, after the plants are fed the nutrient solution the pH and nutrient concentration will change. After the solution returns to the reservoir, an auto-doser controller will make the necessary adjustments to the solution to maintain the user’s set parameters.

Lighting Controller

For indoor horticulturists no component is as important to the garden’s success as the artificial lighting. There have also been an increasing number of greenhouse hobbyists who are reaping the benefits of supplemental lighting within the greenhouse. A lighting controller is a controller that automates a horticultural lighting system. The most basic lighting controller, and still the most commonly used among indoor horticulturists, is nothing more than a souped up light timer. The main difference between these controllers and a standard timer is the amount of electrical load they can handle. A series of horticultural lights can amount to a large electrical load and lighting controllers are designed to safely manage the electrical current.

More sophisticated lighting controllers, typically used by greenhouse gardeners, can be equipped with photosensors or other light measuring systems. These specialized controllers are designed to increase the efficiency of the supplemental horticultural lighting system. Some lighting controllers will only activate the lights when there is not sufficient natural light, while other high-tech controllers will continuously monitor the amount of light in the greenhouse and control the supplemental lighting to maintain a certain level of light at all times. The high-tech controllers allow the horticulturist to customize lighting needs for particular crops and get the most out of the supplemental lighting system.


Multi-controllers, also known in the indoor horticulture community as a “brain”, are controllers that combine the functions of multiple controllers in one unit. In other words, a multi-controller can be an atmospheric controller, a CO2 controller, an auto-doser and a lighting controller all at the same time. It is also common for multi-controllers to include other options that individual controllers do not have. For example, many multi-controllers will have special outlets for hydroponic pumps which can be set up much like a cycle timer. A multi-controller can be a very useful tool but can also be very expensive especially for a gardener who may only be using a few of the available options.

After making an investment in the multiple pieces of equipment needed to operate a functional greenhouse or indoor garden, it only makes sense to get the most out of that hardware. Controlling one or multiple pieces of equipment in a garden can offer many advantages to a horticulturist. Automating the devices found within a greenhouse or indoor garden not only gives the gardener peace of mind, it also frees up some of his or her time. Controllers will extend the longevity of air conditioners, fans, CO2 equipment, horticultural lighting and other hardware because the devices are triggered when needed and, in some cases, are intuitive thereby creating a lessened work load for the device. Whether it is to save time, increase efficiency, or to maximize the performance of equipment, greenhouse and indoor horticulturists stand to gain multiple benefits by the use of controllers. The most important benefit of controllers is the consistency they help to create within the garden’s environment which, in turn, results in healthier plant growth.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at

Want more information? Read these articles:

Environmental Consistency is the Pivotal Factor for Successful Propagation

Environmental Greenhouse Controls

Gaining Control with Control Modules

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