Dehumidification in indoor grow rooms and greenhouses is a moving target

When choosing dehumidification equipment, there are a number of variables that have to be considered to ensure that it operates effectively with the crop and production facility.

By David Kuack

One of the major challenges facing indoor growers is managing relative humidity. Unlike temperature control, humidity control is not as straightforward.
“Indoor growing, including production of cannabis in warehouses and leafy greens in vertical farms, is being done in well-sealed buildings,” said Dr. Nadia Sabeh, president of Dr. Greenhouse Inc. in Sacramento, Calif. “Plants are being irrigated and plants are transpiring and generating water. Many of these indoor growing facilities have nowhere for that water to go because they are using recirculating air conditioning systems.
“Growers are trying to figure out how to remove the moisture. They need to dehumidify the space in addition to cooling it, which is temperature control. There is a need for combined temperature and humidity control in a sealed environment, which is specific to growing indoors.”

Transitioning to greenhouses
Sabeh said some indoor growers are making the transition from closed buildings to greenhouses.
“Growers are trying to produce cannabis as well as other crops, including leafy greens and berries, to take advantage of natural sunlight available in greenhouses,” she said. “They have been moving from these indoor facilities where they are used to being able to better control humidity. Now they are trying to grow in Florida, Texas, Hawaii, or other locations where it is very humid outside.
“They don’t want to use conventional pad-and-fan cooling and ventilation because that will introduce moisture into the space which will increase their humidity control challenges. In some cases this is about controlling powdery mildew and other disease pathogens and vapor pressure deficit (VPD).”

Dr. Nadia Sabeh said one of the advantages ornamental and vegetable greenhouse growers have over greenhouse cannabis growers is that they have learned how to grow under constantly changing outdoor climates.
Photos courtesy of Dr. Nadia Sabeh, Dr. Greenhouse Inc.

“For greenhouse growers in Florida it is much harder to control relative humidity because they are constantly fighting the humidity outside and inside,” she said. “In Arizona, greenhouse growers are having a harder time controlling too low of a humidity. However, they can use evaporative cooling to lower the VPD and the temperature simultaneously.

“In areas with a temperate climate where it’s not too humid, it is easier to control the humidity. But growers are still subject to the outdoor conditions where the temperature and humidity fluctuate all of the time. Theoretically growers can control the humidity better in warehouses or vertical farms than in greenhouses because they are not dealing with the outside climate.”

Sabeh said one of the advantages greenhouse growers of ornamental and vegetable crops have over greenhouse cannabis growers is that they have learned how to grow under constantly changing outdoor climates.

“Growers have been producing ornamentals along with tomatoes, lettuce, cucumbers and peppers in greenhouses for a long time, living with the variability and using crop management techniques,” she said. “That’s what is happening in the cannabis industry now. Many cannabis growers don’t yet have the experience of producing in greenhouses. Cannabis warehouses and grow rooms are equipped with air conditioning and dehumidifiers and some growers don’t understand why this equipment can’t cost-effectively be installed and operated in greenhouses.

“These growers aren’t aware of other variables that can be changed to adapt to the conditions in greenhouses. Some production techniques that could help growers through really humid or dry periods include changing irrigation frequencies or shading the greenhouses to reduce the solar radiation to reduce the heat stress and the water stress on the plants.”

Sabeh said growers also need to be aware that greenhouses aren’t technically a closed structure like a warehouse.

“There is infiltration in a greenhouse,” she said. “There is usually one air change per hour even if a grower isn’t actively ventilating. There is cold or warm air that naturally trickles into a greenhouse that also has to be controlled.”

Choosing a dehumidification system

Sabeh said most indoor cannabis growers are using dehumidifiers to remove the water in the air.

“An increasing number of cannabis growers are using a combined system where the air conditioning system can do both the cooling and dehumidification all in one system,” she said. “Our preferred way of designing most facilities is to have everything all in one place. This makes it easier to control and there are fewer control points. There is less disparity or separation of the different systems that aren’t communicating with each other. This can also help to reduce the price and the risk involved with equipment that goes down.”

Growers can use portable dehumidifiers to temporarily boost dehumidification capacity in grow rooms or greenhouse bays where humidity levels are rising.

Sabeh points out that all dehumidification equipment is not created equal.

“Many of these dehumidifiers are rated for the same temperature and relative humidity and that’s what growers will see on the manufacturers’ websites,” she said. “They’ll indicate how much water their dehumidifiers will remove in a day, for example 100 pints per day when growing at 80ºF and 60 percent relative humidity. What a lot of growers don’t realize is when they are growing at 70ºF and 50 percent relative humidity the dehumidification ability of that equipment drops very quickly. It could be half of what it is indicated on a website. Growers need to be aware of the climate conditions under which they are growing their crops in order to determine how many dehumidification units that they need.”

Impact of plants on dehumidification

Sabeh said relative humidity control is a constantly moving target with plants.

“Plants are dynamic so this is another challenge with dehumidification,” she said. “The transpiration rate of small seedlings is much different than full grown plants. That is something we are trying to figure out. Models have been developed for tomato and lettuce crops, but transpiration rates are variable from crop to crop. We don’t yet have all the data or the research done on these different crops. That is one of the biggest challenges trying to calculate or estimate what the moisture removal needs are for the different crops grown in warehouses and vertical farms.”

A new indoor grow room equipped with two large dehumidifiers hung from the ceiling with a fan coil unit for cooling to the far right.

Crop development stage can also impact how much dehumidification is required.

“With cannabis we estimate the dehumidification needs to be less for a vegetative room than for a flowering room and practically nothing for a clone room,” Sabeh said. “In a clone room we want 100 percent humidity. We don’t want dehumidification at this crop cycle stage. 

“In a cannabis grow room there might be one plant per square foot. In a vertical farm growing lettuce there may be four plants per square foot. And then multiply that by nine levels. The actual amount of biomass in that vertical farm is much higher. With a lot of vertical farms the plants are grown hydroponically so the water is constantly moving.”

Sabeh said irrigation frequency is also going to impact the relative humidity.

“Cannabis plants may be watered once or twice a day or maybe once every other day,” she said. “We were shocked to see how much moisture is in some of these grow rooms. Looking at the data collected, we can actually identify irrigation events just by looking at the humidity levels in these rooms.”

Controlling diseases, pests

Sabeh said one of the paramount reasons for controlling relative humidity with cannabis is disease control.

“Being able to control humidity for cannabis growers is essential to being able to sell their product,” she said. “Cannabis growers have to control humidity to prevent Botrytis and powdery mildew because they’re limited in regards to using chemical controls on their plants.

“There are some fungicides and pesticides that can be used on cannabis, but only through certain maturity phases of the crops. There can’t be any residues on the flowers when they go to the lab for testing before they go to market. They have to meet regulations and be able to pass the lab tests.”

Unlike cannabis growers, ornamental growers don’t have to deal with the same strict restrictions on the use of chemical controls for pests and diseases.

“If there is going to be a trickledown effect from all of the cannabis production, it’s that horticultural producers are going to become more regulated,” Sabeh said. “The cannabis industry is educating regulators on how plants are grown in greenhouses. That effect will eventually trickle down to other horticultural crops.”

For more: Dr. Greenhouse Inc., (916) 476-6078;;

David Kuack is a freelance technical writer in Fort Worth, Texas;

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