Set the light to the limiting factor
When you increase the light, the room must supply more CO₂, water, airflow and feed, and must remove more heat. This paper shows how to calculate the PPFD limit of each supply system. The lowest limit is the PPFD limit of your room. The paper also shows the equipment that you can add to get a higher limit.
Purpose and scope
Limit of the data: The EC and runoff values for the rows with high PPFD are for advanced steering. With ambient CO₂ at high PPFD, the plant uses the added light with less efficiency. No test shows that 800 µmol at the canopy is a PPFD limit for all rooms. Calculate the size of the HVAC and of the water supply from the readings of your meters. Do not use the value in one cell of a table.
The smallest supply controls the rate of photosynthesis. Light can control the rate only until a different supply is not sufficient. When a different supply is not sufficient, more light makes only heat and causes stress to the plant.
Two laws show this effect. Liebig's law of the minimum shows that the smallest supply controls the growth rate of a crop. This law is correct also when all the other supplies are very large. Blackman's law of limiting factors gives the same information for the rate of photosynthesis at one time. When you increase the supply of the limiting factor, the rate increases. When you increase a different supply, the rate does not change.
Rodriguez-Morrison and the other authors measured a yield that increases in a linear relation to 1,800 µmol at ambient CO₂[1]. Measurements on leaves show that CO₂, light and temperature have an effect on each other. But they do not show a PPFD limit of 800 µmol at the canopy[2]. A room at 1,500 µmol with ambient CO₂ can have low efficiency, or can cause stress to a cultivar that has a low tolerance for light. Thus use the crop response and the yield for each kWh to examine the room.
Calculate which system has the lowest PPFD limit. Then increase that limit, or set the PPFD to a value less than the limit. The paper Grow-room systems shows that the room is one system. This paper gives numbers for the system and shows how to find its weakest part.
The light causes the plant to use more CO₂, water and feed. The light also causes more heat. The room must supply CO₂, water, feed and airflow, and must remove the heat. The yield increases with the light only when all these supplies are sufficient. The first supply that is not sufficient gives the PPFD limit of your room.
Definitions
This paper uses six terms. If you know these terms, go to the next section.
Evidence and limitations
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.
- When you increase the PPFD, the plant uses more water and nutrients, and the heat load increases. Frequently the plant also uses more CO₂.
- The yield of the canopy can continue to increase at PPFD values higher than the saturation point of one leaf.
- Ladder tables as engineering guides to the size of the equipment, in rooms with strong control.
- Ladder tables with a very high runoff EC and feed EC as start values for a new grower.
- Formulas that multiply the PPFD by a constant to find the quantity of water, with no data on VPD, LAI or CO₂.
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.
Sequence of light steps
Two tables show the full system, row by row. Find your PPFD in the first column and read across the row. All the values in the row must be correct at the same time. If they are not correct, the PPFD in that row is not possible. The first row is for a room with ambient CO₂ (600 µmol). The last row is for a sealed room with all supply systems at full capacity (1500 µmol).
The first table shows the air and gas values of each row: the gas that the plant uses and the climate around the plant.
| Light (PPFD) | DLI | CO₂ setpoint | Air temperature in the day | VPD (RH) | Air speed in the canopy | Result |
|---|---|---|---|---|---|---|
| 600 | 26 | 400–450 ppm | 25 °C | 1.2 kPa (62%) | 0.3–0.5 m/s | Ambient air is sufficient |
| 800 | 35 | 600–800 ppm | 26 °C | 1.3 kPa (62%) | 0.4–0.6 m/s | CO₂ enrichment starts to help the yield |
| 1000 | 43 | 800–1000 ppm | 27 °C | 1.3 kPa (63%) | 0.5–0.7 m/s | CO₂ is necessary |
| 1200 | 52 | 1000–1200 ppm | 28 °C | 1.4 kPa (63%) | 0.6–0.8 m/s | Large supply systems |
| 1500 | 65 | 1200–1500 ppm | 29–30 °C | 1.4 kPa (65%) | 0.7–1.0 m/s | All systems at full capacity |
| The VPD changes only by a small quantity. It is a setpoint for the condition of the plant, and it does not change with the light. The quantity of water that you add and remove to keep the VPD changes with the light, because the transpiration increases (Table 2). The air temperature increases by a small quantity, because CO₂ and heat together increase the light saturation point. | ||||||
The second table shows the water, feed and heat values of each row: the quantities that you must supply and remove to keep the PPFD. The cost of a high PPFD is in this table.
| Light (PPFD) | Transpiration (water out of the plant) | Irrigation (water to the substrate) | Feed EC | Heat from the light | Cooling for sensible heat | Dehumidification |
|---|---|---|---|---|---|---|
| 600 | 2.2 L/m²/d | 3.0 L/m²/d | 2.0–2.4 | 222 W/m² | 2.1 kW /10 m² (0.6 ton) | 2.2 L/m²/d (4.7 pt) |
| 800 | 3.0 L/m²/d | 4.0 L/m²/d | 2.4–2.8 | 296 W/m² | 2.8 kW /10 m² (0.8 ton) | 3.0 L/m²/d (6.3 pt) |
| 1000 | 3.7 L/m²/d | 4.9 L/m²/d | 2.8–3.2 | 370 W/m² | 3.9 kW /10 m² (1.1 ton) | 3.7 L/m²/d (7.8 pt) |
| 1200 | 4.4 L/m²/d | 5.9 L/m²/d | 3.2–3.6 | 444 W/m² | 4.6 kW /10 m² (1.3 ton) | 4.4 L/m²/d (9.4 pt) |
| 1500 | 5.6 L/m²/d | 7.4 L/m²/d | 2.4–3.2 (advanced: a maximum of approximately 3.6) | 556 W/m² | 5.6 kW /10 m² (1.6 ton) | 5.6 L/m²/d (11.7 pt) |
| Formulas used: transpiration ≈ PPFD × 0.0037 L/m²/d. Irrigation = transpiration ÷ 0.75. Heat from the light = PPFD ÷ 2.7. Dehumidification load = transpiration (1 L ≈ 2.1 US pints)[4]. At the highest PPFD, a high CO₂ concentration decreases the transpiration by a small quantity. For HPS or for LED with 2.0 µmol/J, add approximately 35% to each value of heat, cooling and airflow. | ||||||
Read the two tables together. The problem is then easy to see. From 600 to 1500 µmol, the light increases 2.5 times. The transpiration, the feed and the heat also increase 2.5 times.
You increase the feed EC and the irrigation volume together. Thus the quantity of nutrient that moves through the plant each day increases approximately five times, and not 2.5 times. Plants in high light do not use only a small quantity more nutrient. They use much more nutrient and much more water, and they make much more heat, all at the same time.
Example of the light steps: a 50 m² flowering room
The tables give values for each m². This section uses one room as an example. The room has a flowering canopy of 50 m². The room is 10 m × 5 m. It has approximately 35 × 650 W fixtures and approximately 150 m³ of air. Use the values in the tables to calculate the list of equipment for the room.
| Light (PPFD) | Fixture load | Cooling for sensible heat | Airflow of the air handler | Dehumidification | Irrigation | CO₂ concentration to keep |
|---|---|---|---|---|---|---|
| 600 | 11.1 kW | 10.9 kW (3.1 ton) | approximately 2,100 m³/h (1,250 CFM) | 111 L/day (235 pt) | 148 L/day | ambient CO₂ |
| 800 | 14.8 kW | 14.8 kW (4.2 ton) | approximately 2,850 m³/h (1,680 CFM) | 148 L/day (313 pt) | 197 L/day | approximately 700 ppm |
| 1000 | 18.5 kW | 18.6 kW (5.3 ton) | approximately 3,600 m³/h (2,120 CFM) | 185 L/day (391 pt) | 247 L/day | approximately 1000 ppm |
| 1200 | 22.2 kW | 22.2 kW (6.3 ton) | approximately 4,280 m³/h (2,520 CFM) | 222 L/day (469 pt) | 296 L/day | approximately 1200 ppm |
| 1500 | 27.8 kW | 27.8 kW (7.9 ton) | approximately 5,370 m³/h (3,160 CFM) | 278 L/day (587 pt) | 370 L/day | approximately 1400 ppm |
| The cooling for sensible heat is for the fixtures only. In a sealed room, add the heat from the dehumidifiers and the pumps. The airflow of the air handler is approximately 680 m³/h for each cooling ton (400 CFM/ton, with 1 cooling ton = 3.5 kW). This airflow is different from the airflow of the fans in the canopy, which keep 0.5–1.0 m/s through the leaves. The quantity of CO₂ gas for the first fill of a sealed room of 150 m³ to 1000 ppm is only approximately 90 L. The seal of the room has the largest effect on the quantity of CO₂ that you use each day. | ||||||
Examine the values for cooling and dehumidification from 1000 to 1500 µmol. The load of the fixtures increases by 50%. The dehumidification increases from 185 L/day to 278 L/day (391 to 587 pints), and the number of dehumidifiers increases from two to three. The cooling increases from approximately 18.6 kW to 27.8 kW (5.3 to 7.9 tons).
The photons are the part with the lowest cost. The cooling load and the removal of water cause the large costs and the problems. In almost all rooms, the limiting factor is one of these two systems, and it is not the lights.
Control of EC at high PPFD
Examine the feed EC again. You must increase the EC when the light increases. Growers most frequently do not do this step. As a result, the yield cannot increase, and no clear sign shows the cause.
The cause is mass flow. When the light is brighter, the growth is faster, and the plant absorbs more nutrient each day. The plant gets more nutrient because of two effects, and the two effects increase with the light. First, more water moves through the plant (the transpiration column of Table 2)[4]. Second, each milliliter of this water contains more salt (a higher EC).
If the feed is not sufficient for a canopy in strong light, the fade starts at the bottom and moves to the top. The light is available, but the nutrient is not sufficient.
| Light (PPFD) | Feed EC (mS/cm) | Target runoff EC | Feed for each m² each day | Method for the shots | If the EC is incorrect |
|---|---|---|---|---|---|
| 600 | 2.0–2.4 | 3–4 | approximately 3.0 L | Apply a smaller number of shots, with a larger size for each shot. Use a wider dryback. | Low EC: the new growth is slow and light green. |
| 800 | 2.4–2.8 | 4–5 | approximately 4.0 L | When the canopy becomes larger, increase the frequency of the shots. | A balanced feed shows at this PPFD. |
| 1000 | 2.8–3.2 | 5–6 | approximately 4.9 L | Apply many shots. Use a smaller range. | Low EC: fade in the bottom part of the canopy. |
| 1200 | 3.2–3.6 | 6–7 | approximately 5.9 L | Apply frequent shots. Monitor the runoff EC. | High EC: tipburn and dry edges on the leaves. |
| 1500 | 2.4–3.2 (advanced: a maximum of approximately 3.6) | Advanced, when the runoff EC is much more than the feed EC | approximately 7.4 L | Use a high frequency and a large volume. Do a check of the EC each day. | An EC that is too high or too low causes damage in a short time. |
| The table is for clean source water (EC less than 0.4), a balanced high-ratio nutrient and an inert substrate. Coir has a buffer for cations. Thus use the lower half of each range. See Coco and crop steering and Nutrient deficiencies. Increase the EC as a lever only after the irrigation volume is correct. The EC is not an alternative to the irrigation volume. | |||||
The EC and the light also increase together because of a second cause: the EC is a lever for steering. When the concentration of salt in the solution around the roots increases, the solution causes a resistance to the flow of water into the roots. The name of this resistance is osmotic pressure.
A higher EC in the root zone increases the osmotic pressure and decreases the uptake of water by a small quantity. As a result, the vegetative growth is slower and the plant changes to generative growth. This effect helps in the flowering stage. Thus, at high PPFD, you increase the EC for two tasks at the same time. The first task is to supply feed for the faster growth. The second task is to keep the generative balance when the irrigation volume is large.
When you increase the EC for steering, also increase the irrigation volume. If the volume is not sufficient, the concentration of the salts increases and causes damage to the plant.
Find the limiting factor
Each supply system has a PPFD limit. The limit is the maximum PPFD for which the system is sufficient. Calculate the limit of each system. The lowest number is the PPFD limit of your room. Light that is more than this limit does not increase the yield. The table shows how to change each installed system into a PPFD number for each m² of canopy.
| System | In your room | PPFD limit of the system |
|---|---|---|
| CO₂ | The setpoint and the measured crop response | You cannot calculate one PPFD limit from the setpoint only. With ambient CO₂, the yield can increase at a PPFD of more than 800 µmol. CO₂ enrichment can increase the efficiency with which the plant uses the added light at high PPFD. |
| Cooling | Installed capacity for sensible heat (kW) | PPFD ≤ 2,700 × kW ÷ m² (or 9,500 × ton ÷ m²) |
| Dehumidification | Nameplate capacity (L/day) | PPFD ≤ 270 × L/day ÷ m² (or 128 × pints/day ÷ m²) |
| Irrigation | Maximum L/day that the system can supply | PPFD ≤ 200 × L/day ÷ m² |
| Feed and EC | The highest EC that you can use | Find the row of Table 4 that agrees with the EC |
| Airflow | Movement of air in the canopy | A condition that is correct or incorrect, and not a value that you adjust. See below. |
| All values are for LED at 2.7 µmol/J. For fixtures with less efficiency, decrease the constant for cooling. Airflow does not give one clear number, because it is a condition that must be correct first. If you cannot keep 0.3–1.0 m/s through all the canopy, the gas exchange decreases and all the other PPFD limits decrease to approximately 900–1000 µmol. | ||
Airflow is different from the other systems, because it is a condition and not a value. A thin film of air that does not move is on each leaf. The CO₂ in the room can go into the pores of the leaf only through this film, and the movement is diffusion. The name of this film is the boundary layer. When the airflow makes the boundary layer thin, the CO₂ moves easily into the stomata. When the boundary layer is thick, the plant gets approximately the CO₂ concentration of ambient air, also in a room with 1500 ppm.
A room can have 1500 ppm of CO₂. But the CO₂ for the leaf is not sufficient if the airflow does not remove the boundary layer. In a canopy with a high density, air that does not move is a limit for the CO₂. The sensor in the room does not show this limit. Make sure that the airflow is correct before you read the other PPFD limits.
Calculate the six numbers and find the lowest number. This number is the PPFD limit of the room. The diagram shows an example.
Examples of limiting factors
The section gives four examples of the method, for four frequent types of room. In each room, one supply is not sufficient and all the other supplies are sufficient. This one supply controls the yield. Do not increase the light to correct the problem.
The room: 50 m², 21 kW (6 ton) of cooling, CO₂ to 1500 ppm, good fans and two dehumidifiers of 97 L/day each (410 pints/day, 194 L/day in total). The numbers: the PPFD limit of the cooling is 2,700×21.1÷50 ≈ 1140. The PPFD limit of the dehumidifiers is 270×194÷50 ≈ 1050. The PPFD limit of the CO₂ is 1500.
The limiting factor: the dehumidification, at approximately 1050 µmol. The correction: operate the lights at 1050 µmol, or add a third dehumidifier. The PPFD limit then becomes 1140 µmol, the limit of the cooling. After that, the cooling is the next limiting factor.
The room: large cooling and dehumidification capacity, but no CO₂ enrichment. The room has ambient CO₂ at 420 ppm. The data: the efficiency of the leaf decreases when the PPFD increases[2]. But Rodriguez-Morrison and the other authors measured a linear canopy yield to 1,800 µmol at ambient CO₂, in one cultivar and one room[1].
The limit that you use: there is no PPFD limit of 800 µmol for all rooms. Monitor the canopy temperature, the bleaching, the DLI and the yield for each kWh. Decrease the light when the crop has damage. Also decrease the light when the added yield is not sufficient for the cost of power and climate control. Before you add CO₂, make sure that the room has sufficient capacity for the added heat, water and safety load.
The room: CO₂ to 1200 ppm, and strong cooling and dehumidification. But the canopy has a high density, and the laminar air in the bottom half does not move. The numbers: there is no clear number. The airflow does not remove the boundary layer, and thus the CO₂ cannot go into the stomata in the canopy. The PPFD limit decreases to approximately 900–1000, but the room has 1200 ppm.
The limiting factor: the condition of the airflow. The sign: the outer buds are in good condition, but the inner part of the canopy is moist and has larf. The correction: defoliate the canopy and add airflow below the canopy before you change the lights or the CO₂.
The room: the climate and the gas are sufficient for 1300 µmol. But the irrigation is only a small number of short shots, and the feed EC stays at 2.4. The numbers: the canopy must have a minimum of 4.5 L/m²/day and an EC of 3.4. It gets approximately 3 L and an EC of 2.4. The PPFD limit for the water is approximately 900 (200×(the L that the system can supply)÷m²). The low EC gives approximately the same PPFD limit.
The limiting factor: the irrigation volume and the EC. The sign: wilt in the middle of the day, and light green leaves with fade in the bottom canopy. The correction: make the shot volume and the shot frequency correct first. Then increase the EC with Table 4. Do not do these steps in the other sequence.
Set the light intensity to the limiting factor
When you know your lowest PPFD limit, you have two correct alternatives.
Alternative one: set the light to the limit. If your PPFD limit is 1050 µmol, operate the lights at 1050 µmol. The photons that make the PPFD more than this limit do not change into yield. They change into heat, humidity and stress.
When you decrease the light to the limit, the growth does not decrease. You also use less power, you have more cooling headroom, and the room is more stable. For fixtures that you can adjust to a lower intensity, the change has no cost, and the effect occurs immediately.
Alternative two: increase the PPFD limit, then examine the room again. Add equipment only to the system that is the limiting factor. If you add CO₂ to the room in Example B, the effect is very large. If you add CO₂ to the room in Example A, there is no effect, because the dehumidifier and not the CO₂ is the limiting factor.
Persons frequently do not know that when you increase one PPFD limit, a different system becomes the limiting factor. If you correct the dehumidifier in Example A, the cooling gives a limit of 1140 µmol. If you increase the limits in the incorrect sequence, you add equipment that does not change the result.
Do not operate the lights at a PPFD that is more than the PPFD limit. The added light is not neutral. It causes bleaching and foxtails on the top buds, and it increases the leaf temperature and the VPD. When the dehumidifier is the limiting factor, the humidity can increase to a value at which bud rot occurs. The added electricity has a cost, and the quality decreases. When you decrease the light to the limit, there is no cost.
There is also a cost limit, and it is lower than the limit of the plants. The yield continues to increase to 1500–1800 µmol[1]. But the cooling load and the removal of water for the highest PPFD increase more quickly than the yield. For the last 300 µmol, it can be necessary to add a third dehumidifier and a larger cooling system (AC). The yield then increases by only one to nine percent.
Find your cost limit. At the cost limit, the added yield of the next 100 µmol is not sufficient for the cost of its climate equipment. This cost limit is frequently one step less than the PPFD that the plants can use.
Troubleshooting
Each row of the table shows one limiting factor. The symptom shows which limiting factor your room has.
| Symptom | The limiting factor | Correction |
|---|---|---|
| Bleaching and foxtails on the top buds at high PPFD | The PPFD, the temperature or the DLI is more than the tolerance of the cultivar or the canopy. | Measure the canopy temperature and the distribution of the light. Decrease the PPFD. Do a test of the CO₂ as a different variable. |
| The RH does not decrease. The VPD becomes very low in the middle of the day. | The transpiration is more than the capacity of the dehumidification. | Add dehumidifier capacity or decrease the light. Dehumidification is usually the limiting factor. |
| High PPFD and a yield that does not increase | The CO₂, the water or the feed did not increase with the light. | Find the lowest PPFD limit. Increase it, or set the PPFD to the limit. |
| Wilt in the middle of the day at peak light | The irrigation volume is less than the transpiration. | Apply larger shots or more shots. Correct the volume before you change the EC. |
| Light green leaves and fade in the bottom canopy in the last part of the flowering stage | The feed EC is too low for the PPFD. | Increase the EC by one step in Table 4. Do a check of the runoff EC. |
| Tipburn and dry edges on the leaves | The feed EC is too high for the PPFD (more than the value in the row). | Decrease the EC by one step. Or increase the PPFD and the irrigation volume until they agree with the EC. |
| The outer buds are in good condition. The inner canopy has larf and is moist. | The condition of the airflow is not correct. The airflow does not remove the boundary layer in the canopy. | Defoliate the canopy and add airflow below the canopy. The CO₂ cannot have an effect in air that does not move. |
Expected results and limitations
The yield of cannabis has an almost linear relation with the light, to approximately 1500–1800 µmol[1]. But this result has a condition, and the tests give the condition. The result is correct only when you increase the CO₂, the temperature, the water and the feed to agree with the light. If these supplies are not sufficient, the same lights cause bleaching on the top buds and a heat problem. The linear relation is correct only when all the supplies increase with the light.
Your first task is the limiting factor of your room, and it is not the lights. In approximate sequence, the most frequent limiting factors are the dehumidification, the cooling, the CO₂ and the supply to the root zone. The correction of the current limiting factor usually gives the yield with the lowest cost. One more kilowatt of light does not give the yield with the lowest cost.
Measure the values that show the limiting factor. These values are the leaf temperature, the runoff EC, the RH and the air speed in the canopy, and the CO₂ below the canopy. A sensor at the edge of the room does not show the air in the canopy that does not move and has a high humidity.
- When you increase the light, the room must supply more CO₂, water, feed and airflow, and must remove more heat. Calculate the size of each supply system for the light (Tables 1 and 2).
- Calculate the PPFD limit of each installed system. The lowest limit is the limit of the room (Table 5).
- Set the light to that limit. Light that is more than the limit uses power but does not increase the yield, and it decreases the quality.
- To use a higher PPFD, increase the limit of the system that is the limiting factor. Then examine the room again. The limiting factor changes.
This paper gives information on one lever of the room. Read it with Grow-room systems, CO₂ enrichment and Airflow design.
References
- 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/
- 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/
- 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
- 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
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.