Precision Environmental Control for Grow Rooms

May 31, 2017 / by Dave Meadows

A logical starting place to discuss the grow process, and therefore the conditioning requirements, is with the mother plant in the mother room. To ensure consistency in the final product, the production plants are cloned from an existing mother plant. For this reason the health of the mother plants is critical to a successful grow operation.  The mother room may have redundancy requirements for the HVAC equipment that the other grow rooms don’t.

Though each grower often has his or her own preferred environmental conditions for each individual grow room, common set points for this room are 75°F and 50% RH.


Cuttings are taken from the mother plant and moved to the clone room (often referred to as the nursery).  Conditions in this space are also precisely maintained to ensure that the young plants have the greatest opportunity to develop a strong root system. The clone room is often kept slightly warmer than the other grow spaces and the clones prefer temperatures between 75°F - 80°F with moisture content between 40% RH and 50% RH.  The clones are sensitive to temperature and lighting changes. For this reason the lights are often left on 24 hours a day during this part of the plants lifecycle.  Small pests such a thrips and spider mites are always harmful to the cannabis plants, but especially so for immature plants.  In some states regulations prohibit the use of chemical pesticides and therefore other approaches such as UV-C lighting and oscillating fans are used to kill these micro pests. These eradication strategies are used in all stages of the grow cycle.   

Once the plants reach a certain height they are moved into the vegetative (Veg) room.  The purpose of this room is to manipulate the plants to grow tall, strong, and develop a good root bulb. Several environmental control features are manipulated to achieve this “growth spurt.”  Oscillating fans are positioned throughout the various grow rooms to “exercise” the plants and make it more difficult for micro pests to land on them.  Lights are turned on for 18 hours a day to imitate the long growing hours of summer. Temperature swings during the lights on/lights off cycle can stress the plants, which can result in low-yield hermaphrodites, so tight environment control during this cycle is required. Precision environmental control units such as computer room air conditioners (CRAC) are well suited to providing the tight control needed to limit these temperature swings.  Environmental conditions in a vegetative room typically range from 60°F - 75°F and 40 - 50% RH.  

When the plants reach their optimal height they are moved to the flowering room (often referred to as the “bloom room”). The plants can be physically moved or the Veg room can become the flowering room by altering certain environmental parameters. Flowering buds are the part of the plant where most of the useful cannabinoids are found. The goal of flowering room operations is to produce the largest flowering buds possible, as the higher the yield at this stage the more profitable an individual room will be.   In the flowering room the lighting sequence is changed to 12 hours on and 12 off.  This stimulates the plants to devote their resources to producing larger flowering buds.  During the lights on cycle carbon dioxide (CO2) is added to the room to stimulate the plants growth rate.  The typical ambient CO2 level may be around 300 PPM (parts per million), but by raising the CO2 level to 1,200-1,500 PPM the growth rate of the plants can be almost doubled. The oscillating fans play a part in the CO2 enrichment strategy as they help keep the slightly heavier CO2 molecules from stratifying.  This CO2 enrichment strategy can be applied in other phases of the plants life cycle but many growers only use it in the flowering rooms.  In the flowering room 75° to 78°F seems to be the target of most growers, with a relative humidity of 40% - 50%.  Once optimal leaf growth is achieved the plants are harvested and sent either to the extractor or to a curing room.  The extractor removes the cannabinoids for use in edibles or oral medications; the curing room prepares product that is intended to be smoked.     

The curing room is used for drying the cannabis in a controlled fashion to ensure the optimal final properties of the smokable end product.  Curing is critically important in maintaining the quality of the plants and typically takes a week, but the duration is dependent on many variables. Curing breaks down chlorophyll and dramatically improves the flavor and smoothness of the smoke, reduces the chance of mold or bacteria forming on the plants, and reduces unpleasant odors associated with freshly cut plants.

There’s much debate about the rate and conditions at which the plants should be dried.  Specially designed air conditioners should be used that can maintain both the low temperature and low relative humidity that are the optimal conditions during this final phase.  The environmental conditions in the Curing room range from 60°F - 75°F and 30 - 50% RH.

Download the full white paper to learn:

  • Detailed description of the grow process with environmental requirements

  • Methods of moisture control

  • Control of micro pests (thrips and spider mites)

  • How Computer Room Air Conditioners (CRACs) are suited to grow room applications

Download the Full White Paper Here

Topics: STULZ USA, Energy Efficiency, Grow Room

Dave Meadows

Written by Dave Meadows

Dave Meadows is the Director of Industry, Standards and Technology at STULZ USA. He has a BS in Mechanical Engineering from the University of Maryland, Baltimore County, and is an active participant on ASHRAE committees.

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