Cannabis grow room systems
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Basic · Grow-room systems

Cannabis grow room systems

A grow room is one connected system, and not a list of devices. Light, heat, humidity, air, and water have an effect on each other. This paper shows these connections. Then you can install and adjust the systems in the correct sequence and prevent problems before one problem causes the next.

Basic4 diagrams10 sources~18 min to read
In this paper
01 Purpose and scope02 Definitions03 Evidence and limitations04 Connections between the systems of a grow room05 Quantity of light06 Climate: temperature, humidity and VPD07 Airflow in the connected system08 Root-zone supply09 System setup sequence10 Disease risk in connected systems11 Troubleshooting12 Expected results and limitations13 References
01 · Read this first

Purpose and scope

Light, heat, humidity, and water are one problem and not four different tasks. A new grower gets a light, a fan, a humidifier, and a nutrient bottle, and thinks that each is a different task. When you increase the light, the room becomes hotter and the plants use more water. Then the humidity of the air increases, and the dehumidifier must remove more water. Each part has a connection to the other parts.

This paper shows these connections. Thus you can install and adjust the systems in the correct sequence. Then one problem does not cause the next problem. It is not necessary to know about cultivation before you read this paper.

02 · Terms

Definitions

PPFDThe quantity of light that falls on the canopy at this time, and that the plant can use. The unit is µmol/m²/s.
DLI (daily light integral)The total quantity of light that the plant gets in one day (mol/m²/day). DLI = PPFD × the hours that the light is on. This value has the primary effect on growth.[2]
TranspirationThe plant moves water up from the roots and releases it as vapor through small pores in the leaves (stomata). The water that moves up also moves the nutrients that are in the water, from the roots to the leaves. The evaporation of the water decreases the temperature of the leaf. With transpiration, the plant keeps a stable temperature and gets its nutrients.
VPD (vapor pressure deficit)VPD shows how strongly the air removes water from the plant. The temperature and the humidity together set VPD, and not only the humidity. Dry, warm air removes water strongly. Cool, humid air is almost full of water, and it removes water weakly. VPD is one number that gives the strength of this removal, and it shows the rate of transpiration of the plant.
CO2Carbon dioxide. The plant uses it with light to make sugar. More CO2 can increase growth if the light is sufficiently high.
StomataSmall pores on leaves that you can see only with a microscope. The plant can open and close them. They open to absorb CO2 and to release water. They close when the plant has stress.
SubstrateThe material that contains the roots (coco, rockwool, peat). The substrate has an effect on the water, the air, and the salt at the root.[7]
Stomata: the adjustable pores of the leafturgor in the guard cells opens and closes the pore↑Open (turgor high)CO2 in, water vapor out(transpiration)Closed (turgor low)gas exchange stops, plantkeeps water
Diagram. Stomata open to absorb CO2 and to release water vapor. They close to keep water in the plant. The climate controls when they open and close.
The PPFD during the day gives the DLIThe PPFD multiplied by the hours gives the DLI in mol/m2/day. The DLI is the area below this curve.02505007501000offmiddleoffPPFD umol/m2/s
Diagram. The light intensity during the day gives the DLI. The DLI is the total quantity of light that the crop receives.
VPD: the correct climate bandVPD is one value for the temperature and the humidity together. Keep it in the band for the stage.wet airclone / vegetativefloweringdrystress0.2 kPa1 kPa1.8 kPa
Diagram. The temperature and the humidity together give the VPD. The VPD is the one value to control for the climate.
Quality of the information

Evidence and limitations

Three types of information

We want each paper to be correct. Thus we examine each paper for information that is not solid. This information comes from the view of one grower, from a small number of sources, or from grower methods that have no controlled test. We identify this information, and we do not show it as solid.

Frequently there is no paper for the decision that you must make. Then we use the reports of other growers and the methods that work in the rooms that we know. This information can help you, but it is not the result of a laboratory test. Use the methods that work for your plants, for your room and for your meters. If a table does not agree with your crop, use the data from your crop and record the difference.

Solid
Tests of plants, specifications or many sources that agree show that the information is correct
  • Light, heat, humidity, CO₂, water and airflow have an effect on each other. When you change one of them, all the other items change.
  • VPD shows the effect of the temperature and the RH together on the plant, in one value.
Grower method
Many growers use this method in their rooms. Start with it, then change it for your room.
  • VPD ranges of the type 0.8 to 1.5 kPa for each stage, as start values in horticulture.
  • The decision to use a sealed room or a room with vents.
Weak
The view of one grower, a small number of sources, one test, or a method that works in one room
  • Accurate VPD values that are best for crops other than cannabis as mandatory values for cannabis.
  • VPD as the only cause of tacoing (leaf edges that bend up) in all conditions. Frequently, light, leaf heat and VPD together are the cause.

If you find an error, make a report. We will correct it. Make the report on GitHub, with the name of the paper and the information that is incorrect. If you have a source, include it: Make a report of an accuracy problem. The regulations, labels and licenses of your area always override each procedure in these papers. The label weak in a paper shows the points where the risk of an error is highest.

03 · How the parts connect

Connections between the systems of a grow room

One sequence of parts operates your room. When you change the first part, each part after it changes:

A change of the light changes all the next parts1Lightenergy goes tothe leaf→2Leaf heatleaf hotter,morephotosynthesis→3Transpirationplant absorbsand releaseswater→4Room humiditythat water vaporthen fills theroom→5HVAC loadAC anddehumidifiermust remove itMore light gives more yield, but only if the other parts operate at the same rate.
Figure 1. The sequence of a coupled system. Thus the step ‘add more light’ does not give a good result if your climate and airflow cannot remove the larger quantity of water from the plants.[3]
One procedure that prevents most errors

When you increase one input, find the other inputs that must change with it. More light → more transpiration → more humidity → more dehumidification and more water and feed. The inputs change in groups, and not only one at a time.

04 · Light: the primary lever

Quantity of light

Light is the first lever: it sets the quantity of all the other inputs that the plant uses. In cannabis, the yield of flowers has an approximately linear increase with the light, up to very high intensities (approximately 1800 µmol/m²/s in one test, a yield increase of approximately 4.5×).[1] Bright light gives good results. But the other parts of the room must control the heat, the humidity, and the water use that the light causes.

More light, more flower (approximately linear)PPFD (µmol/m²/s) and relative flower yield. The yield increases at more than the saturation PPFD of one leaf.02855821103040055800781200961600
Figure 2. The yield of the whole plant continues to increase at light intensities much higher than the saturation of one leaf. The canopy uses light that the top leaves cannot use.[1] The yield increases more slowly at very high intensities. At these intensities, the CO2, the climate, and the water must agree with the light.
Leaf and canopy: a frequent error

Do not set the room light with values from one leaf. When you measure one leaf, photosynthesis has its maximum at a moderate light intensity. Then you can stop to increase the light at this point. The whole canopy continues to use more light to make more flower at much higher intensities.[1]

05 · Climate: temperature, humidity, VPD

Climate: temperature, humidity and VPD

Temperature and humidity have one effect together: VPD. VPD has a large effect on the rate of transpiration of the plant. There is a middle range in which the conditions are satisfactory. If VPD is too low, the plant becomes slower. If VPD is too high, the plant closes its stomata to keep water. As a result, the CO2 uptake and the growth stop.[5]

VPD: use the middle rangeApproximate. The best range changes with stage and cultivar. A stable VPD is better than a VPD that changes.humid / slowbest rangegenerativestress: stomata close0.4 kPa1.2 kPa2 kPa
Figure 3. A middle range that you can use is approximately 0.8–1.2 kPa (the range is different for each cultivar, and in the last stage of flowering it is frequently approximately 1.2–1.5). Drier air can cause generative growth. But at more than approximately 1.5 kPa, the plant closes its stomata and its growth stops.[5]

CO2 is the other lever for the climate. When you add CO2, photosynthesis and water-use efficiency increase. But the plant also transpires less. It closes part of its stomata, and it does not open them.[4] Growth increases only when the light is high.

06 · Movement

Airflow in the connected system

Air that moves has two important tasks. It removes the thin film of air with high humidity that does not move and stays on each leaf. It also decreases the temperature of the leaf by convection.

Faster air makes this film thinner. As a result, the CO2 uptake and the water-use efficiency increase (more carbon fixation for each unit of water). But the total water use frequently increases in bright rooms.[6] Air that moves also keeps all the plants in the room in the same climate.

More information on airflow

The paper Airflow design for indoor cultivation gives the full information on boundary layers, fan positions, velocity targets, and dead zones. In this paper, it is sufficient to know one fact. Without air movement, your light, climate, and CO2 settings do not have the same effect on each leaf.

07 · Root zone and water supply

Root-zone supply

All the parts before the root zone cause the plant to use much water, and the root zone must supply this water. When the room has more light and drier air, the plant transpires more and uses more water and nutrients at the roots. Your substrate and your irrigation are the supply part of the same system.[7]

The root zone is also a lever for crop steering: a controlled dryback in the root zone causes generative growth and can increase potency.[8] The paper on coco and crop steering gives the full information on the mechanisms. The irrigation must agree with the water use that the other parts of the room cause. The irrigation must not operate with a timer that does not change.

08 · Setup sequence

System setup sequence

The inputs have connections to each other. Thus the sequence in which you set them is important. Do the steps in this sequence:

  1. 1
    1. Select your light target
    Select the DLI and the PPFD that your room can supply. This sets the necessary quantity for all the other systems.
  2. 2
    2. Set the climate for the light
    Set the temperature and the humidity to a correct VPD for that light intensity. If the light is high, add CO2.
  3. 3
    3. Supply the airflow
    Use sufficient air movement to mix the air in the room and to move air to each leaf. Refer to the airflow paper.
  4. 4
    4. Apply the correct feed
    Set the irrigation and the feed EC for the transpiration that the previous steps cause. Then use dryback for crop steering.
  5. 5
    5. Crop steering and adjustment
    Only at this time, cause more generative growth or more vegetative growth. Change one lever at a time, and monitor all the parts of the sequence.
09 · Disease risk

Disease risk in connected systems

A room that is warm and humid, with a high density of plants, gives large plants, and it also gives mold. Bud rot (Botrytis) starts and increases quickly at a humidity of more than approximately 70% and at moderate temperatures. A thick canopy keeps humid air that does not move in its inner part, and your room sensor does not measure this air.[10]

A room sensor can show an incorrect value

Do not use only the average value of the room. A sensor in open air can show a good value of 60%. At the same time, the inner part of a large cola can be at 85% humidity, and the cola rots. Airflow through the canopy and correct spacing between plants prevent the disease.[9] The paper on mold risk gives the full information.

10 · When a problem occurs

Troubleshooting

SymptomCause in the systemProcedure
The room humidity does not decreaseThe light and the transpiration cause more water than your dehumidifier can removeAdd dehumidification capacity, or decrease the light. Also increase the air exchange.
Leaf curl at the edges, at high lightThe cause is frequently light, leaf heat, and VPD together, and not only VPDFirst, examine the PPFD and the leaf temperature. Then examine the VPD (use a lower temperature and more humidity). Add CO2. Make sure that the airflow is correct.
A high light intensity and a low yieldThe climate, the CO2, and the water did not increase with the lightSet the CO2, the VPD, and the feed for the light intensity
Hot canopy, slow growthThe airflow is too weak, and the leaf cannot release heatIncrease the air movement at the canopy
Bud rot in week 6 and the weeks after itA canopy with a high density and humidity that stays in the canopyDefoliate the plants and increase the space between them. Increase the airflow through the canopy. Decrease the RH.
11 · Facts and limits

Expected results and limitations

Three procedures that keep the system in balance
  • Think in groups: when you change one input, find the other inputs that must change with it.
  • Balance the inputs. Do not use the maximum. A balanced room at moderate light gives a better result than a bright room with a climate that cannot agree with the light.[3]
  • The substrate and the cultivar change the correct value. Use the numbers from the references as start points only.

Next, read the papers on crop steering, airflow, and mold. They are parts of this system.

Related papers
Coco and crop steering
Use water to control the plant
Airflow and fans
How much air to move, and where to put the fans
Mold risk
Find and stop bud rot

References

  1. Rodriguez-Morrison V, Llewellyn D, Zheng Y (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Front. Plant Sci. 12:646020. https://pmc.ncbi.nlm.nih.gov/articles/PMC8144505/
  2. Faust JE, Logan J (2018). Daily light integral: a research review and high-resolution maps of the United States. HortScience 53(9):1250-1257. https://doi.org/10.21273/HORTSCI13144-18
  3. Collado CE, Hernandez R (2025). Vegetative and reproductive stage lighting interactions on flower yield, water-use efficiency, terpenes and cannabinoids of Cannabis sativa. Scientific Reports 15:s41598-025-27437-4. https://www.nature.com/articles/s41598-025-27437-4
  4. Chandra S, Lata H, Khan IA, ElSohly MA (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiol. Mol. Biol. Plants 14(4):299-306. https://pmc.ncbi.nlm.nih.gov/articles/PMC3550641/
  5. Inoue T, et al. (2021). Minimizing VPD fluctuations maintains higher stomatal conductance and photosynthesis, improving plant growth in lettuce. Front. Plant Sci. 12:646144. https://pmc.ncbi.nlm.nih.gov/articles/PMC8049605/
  6. Schymanski SJ, Or D (2016). Wind increases leaf water use efficiency. Plant, Cell & Environment 39(7):1448-1459. https://onlinelibrary.wiley.com/doi/10.1111/pce.12700
  7. Malik M, Tlustoš P (2025). Soilless growing media for cannabis cultivation. Agriculture 15(18):1955. https://www.mdpi.com/2077-0472/15/18/1955
  8. Caplan D, Dixon M, Zheng Y (2019). Increasing inflorescence dry weight and cannabinoid content in medical cannabis using controlled drought stress. HortScience 54(5):964-969. https://doi.org/10.21273/HORTSCI13510-18
  9. Punja ZK, Collyer D, Scott C, Lung S, Holmes J, Sutton D (2019). Pathogens and molds affecting production and quality of Cannabis sativa L. Front. Plant Sci. 10:1120. https://pmc.ncbi.nlm.nih.gov/articles/PMC6811654/
  10. Mahmoud M, BenRejeb I, Punja ZK, Buirs L, Jabaji S (2023). Understanding bud rot development, caused by Botrytis cinerea, on cannabis grown under greenhouse conditions. Botany / Can. J. Bot. 101(8). https://doi.org/10.1139/cjb-2022-0139

Each citation in the paper, for example [1], refers to an item in this list. Each item is a primary source or an instruction from an authority, unless the item shows a different type. The results of cannabis tissue culture change much with the genotype. Before you use the dilutions, hormone doses and local regulations, make sure that they agree with the primary sources.