Increase the PPFD in steps to prevent bleaching
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Increase the PPFD in steps to prevent bleaching

This paper shows how plants adapt when the light intensity increases. It shows how to make a list of PPFD values for each week and how the CO2 sets the safe PPFD limit. It also shows how to read the first signs of a problem. The paper includes new data (2024-2026) on the effect of high light on quality, and on far-red light and UV.

Basic10 diagrams12 sources~11 min to read
In this paper
01 Purpose and scope02 Definitions03 Evidence and limitations04 Acclimation of the plant to a higher PPFD05 Light and CO2 together06 PPFD values for acclimation in each growth stage07 Set the PPFD limit08 Fixture height and dimmer09 Troubleshooting10 New tests11 Expected results and limitations12 References
Start here

Purpose and scope

Two errors of new growers cause most of the light damage in a grow room. The first error is to apply full-power light to weak clones that have new roots. The second error is the opposite. The grower applies light that is too low to plants in the flowering stage, because the grower thinks that the light can burn the plants[1].

The two errors have the same correction. The plant adapts to the light intensity in a period of some weeks. When the plant has time at each value of the PPFD, it changes the structure of the parts of the leaf that absorb light. Thus its capacity agrees with the light.

If you increase the intensity too quickly, or to a value that is too high for the CO2, the plant has too much energy. This energy does not increase the growth. It causes damage: white tips with bleaching, and the growth stops.

The quantity of light that a plant can receive changes very much in one full cycle. A weak clone can receive approximately 80 µmol/m²/s. A canopy at peak flower with CO2 enrichment can receive a maximum of approximately 1500 µmol/m²/s[3]. This paper gives information on the acclimation of plants and on a list of PPFD values for each week. It also gives information on the highest PPFD that is safe and on how to read the signs of a problem.

Light is a ramp, not one stepThe correct method: the intensity increases in steps that are sufficiently small.02755508251100cloneVegetative 1Vegetative 2Vegetative 3Photoperiod changeflowerpeakPPFD µmol/m²/s
Figure 1. A ramp in small steps lets the plant make capacity before each new step of light. A fast change to full power, in one step from low to high, is too fast for the plant. The plant cannot use all the photons.[7]
The persons for this paper

This paper is for each person who caused heat damage to a clone. It is also for each person who does not increase the light because the light can cause damage. Read it with the papers crop steering and plant state dashboard.

Glossary

Definitions

This paper uses five terms. If you know these terms, the paper is easy to read. Each term occurs again in the next sections.

PPFD (Photosynthetic Photon Flux Density)The intensity of the light at the canopy that the plant can use. The unit is µmol/m²/s (micromoles of photons for each square meter in each second). PPFD is the intensity at one time.
DLI (Daily Light Integral)The total quantity of light that a plant can use in one day, in mol/m²/day. When the PPFD increases or the number of hours of light increases, the DLI increases. DLI is the total dose of light for the day.
PhotoperiodThe hours with light and the hours without light in each day. A photoperiod of 18/6 (18 hours of light) is usual for vegetative growth. A photoperiod of 12/12 causes flowering.
Photoinhibition and bleachingDamage that occurs when a leaf absorbs more light energy than it can use. The leaf has too much light. The light causes a chemical reaction that damages the leaf. The molecules that this chemical reaction makes are reactive oxygen species (ROS). ROS cause damage to the leaf tissue, and the tips of the top leaves become white.
AcclimationIn a period of some weeks, a plant changes the structure of the parts of the leaf that use light. The plant makes more chloroplasts, makes the surfaces of the leaf thicker, and makes protective enzymes. Thus the plant has the capacity for each new value of the light. The plant adapts to each step before the next step.
PPFD: intensity at one time. DLI: total for the day1PPFDthe intensity atthe leaf at onetime→2hours of lighttime with thelights on→3DLIPPFD x hours =dose for the dayThe same PPFD gives much less DLI with 12/12 than with 18/6.
Figure 2. PPFD is the intensity at one time. DLI is the total of the light for the day. When you change the photoperiod, the DLI changes, also when the PPFD stays the same.
Terms in the facility Nano Banana 2
Photoinhibition and bleaching
Photoinhibition and bleaching
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. Increase the PPFD (and thus the DLI) in small steps. Thus the plant has time to adapt, and bleaching does not occur.
Photoperiod: the light cycle changes the growth stagehours of light and dark in 24hVegetative 18/618h light6h darkFlower 12/1212h light12h dark12 hours of continuous darkness cause flowering and keep it in progress. A light leak can cause stress or vegetative growth.
Diagram. The light cycle for each stage.
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
  • A large and sudden increase of PPFD causes photoinhibition and bleaching. A ramp of PPFD decreases this risk.
  • At equal PPFD, a photoperiod of 12/12 gives a DLI that is one-third (approximately 33%) less than a photoperiod of 18/6.
Grower method
Many growers use this method in their rooms. Start with it, then change it for your room.
  • Ramps of more than one day, with a dimmer or with the height of the fixture, in commercial rooms.
  • The maximum light intensities that growers can use in a room with no CO₂ enrichment. They give information about stress and ROI.
Weak
The view of one grower, a small number of sources, one test, or a method that works in one room
  • One percentage of yield increase from far-red light for all cultivars.
  • Schedules with the instruction that the PPFD must be X µmol on day Y, with no data on the leaf temperature and the VPD.

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.

How the plant adapts to more light

Acclimation of the plant to a higher PPFD

The plant makes new parts for the higher PPFD. In each week, the plant makes more chloroplasts (the small green parts of a cell that absorb light) and thicker protective surfaces on the leaf. It also makes a higher density of the enzymes that change the absorbed energy into sugar[8]. Thus each new step of light has parts that are prepared to use it.

A plant that has the parts for a moderate PPFD cannot use a sudden large quantity of photons. The parts that absorb light continue to absorb energy, but the leaf cannot use all of it. The energy that the leaf cannot use makes ROS faster than the protective enzymes of the leaf can remove them[5]. You see tips with bleaching, and the growth stops[6].

Thus a ramp in small steps is the correct method. When you add light in steps that are sufficiently small for the plant, the capacity increases with the intensity. As a result, each photon makes sugar and does not cause damage[7].

Acclimation in a safe loop1Small light stepadd one step ofPPFD→2Make new partsmorechloroplasts andenzymes→3Higher capacityprepared formore light→4Next stepagain, in safestepsEach step is sufficiently small for the plant to make the parts before the next step.
Figure 3. The safe ramp is a loop. First, the PPFD increases by a small step. Then the plant makes capacity. Then the next step occurs. [8]
Fast change and steps: light that the plant usesSame PPFD at the end: with the fast change, most light becomes damage and not growth.028558211035%Fast change to full95%Ramp in steps
Figure 4. If a plant receives a large PPFD before acclimation, it changes much of the light into damage and not into sugar. A plant with a ramp uses almost all the light.[5]
The plant makes the damage of bleaching

Decrease the PPFD, or give the plant more time for acclimation. Do not add more light, because more light increases the damage. White tips on the top leaves show that the leaf makes ROS faster than it can remove them. The ROS cause damage to the tissue of the leaf.

How the CO2 sets your PPFD limit

Light and CO2 together

In each green cell, the plant changes light energy and CO2 into sugar. Sugar is the material for all growth. The name for this change is photosynthesis, and it has two connected stages. The light reactions absorb energy from the photons. Then the Calvin cycle uses CO2 from the air to change this energy into sugar. The two stages must increase together[2].

If you increase the light but the CO2 stays low, the result is the same as when you increase the PPFD too quickly. The light reactions continue to absorb energy, but the Calvin cycle does not have sufficient CO2 to use this energy for sugar. The energy collects in the leaf and causes the same bleaching from oxidation. When you examine the leaf, you cannot find the difference between the two errors[6].

A system with high PPFD must have a CO2 concentration that agrees with the PPFD. With ambient air (approximately 400–600 ppm CO2), a PPFD of much more than approximately 850 µmol/m²/s uses electricity, but most of the light does not make sugar[2]. For 1200 µmol/m²/s, the CO2 must be approximately 1000–1200 ppm. For 1500, the CO2 must be approximately 1200–1500 ppm[1].

Low CO2: the energy causes damage1Light reactions fullphotons absorbedquickly→2Calvin cycle: low CO2no CO2 to makesugar→3Too much energythe leaf cannotuse it→4Bleachingoxidationdamage, whitetipsHigh light with low CO2 causes the same damage as a PPFD that increases too fast.
Figure 5. When the CO2 is the limiting factor, more light only increases the damage.[5]
The PPFD limit increases with the CO2For a higher PPFD range, the CO2 must agree with the range. If not, the result is only bleaching.ambient air, approximately 950with CO2 to 1200full system 150040010001600
Figure 6. The CO2 sets the highest PPFD that the plant can use. When the PPFD is more than the limit of your CO2, more light does not help and can cause damage.[2]
Keep this fact

Light increases the growth only when the CO2 is sufficient. When the CO2 is not sufficient, more light does not increase the growth. The growth stops and the leaf becomes too hot.

The values to use

PPFD values for acclimation in each growth stage

A cycle of approximately 14 weeks (approximately 98 days) in a grow room is an example. In this cycle, the light increases stage by stage[4]. The PPFD for clones is low. The PPFD in the vegetative stage increases in regular steps. After the change to the flowering photoperiod, the PPFD increases again to its peak, and then it decreases by a small quantity at the end.

The change to 12/12 decreases the daily light integral (DLI) by approximately one third (approximately 33%) at the same PPFD. The cause is that the lights are on for a smaller number of hours[1]. This effect is usual and correct. Do not increase the dimmer to a value that is too high because of it.

Daily light integral in the cycleDLI in mol/m2/day. The DLI decreases at 12/12, but the PPFD increases.0142944586Clone16Vegetative 126Vegetative 236Vegetative 326To 12/1240Flower52Peak46Ripening
Figure 7. The DLI increases in the vegetative stage, decreases at the change of the photoperiod (a smaller number of hours of light), and then increases again when the PPFD for flowering increases.[4]
A ramp for each stage. Use these ranges as start values. You can change the values for your room. The peak PPFD that you keep changes if your CO2 or your climate changes.[1]
StagePhotoperiodPPFD range (µmol/m²/s)Information
Clone18/680 → 300Use a low PPFD while the clone makes roots and the parts that use light.
Vegetative bulking18/6300 → 650Increase the PPFD by approximately 100 each week.
Flower acclimation12/12600 → 950The plant adapts again after the DLI decreases at the change of the photoperiod.
Peak flower12/12950, 1200 or 1500Keep the PPFD at the limit of your tier.
Maturation12/12950 → 850Decrease the PPFD by a small quantity during ripening.
The change of the photoperiod

The DLI decreases at 12/12. This effect is usual and correct. At the start of flowering, let the plant adapt again from approximately 600 to 950. Do not increase the lights to the peak PPFD on the day of the change.

Set your PPFD limit

Set the PPFD limit

The environment of your room sets the safe peak PPFD, and the PPFD that you want does not set it. The PPFD that you can keep is the PPFD that your CO2, your climate and your cooling can supply at this time. To increase the limit, you must increase the capacity of the full system, and not only the dimmer.

With ambient air, the correct PPFD limit is approximately 950 µmol/m²/s. When the PPFD is more than approximately 850, each added µmol gives less yield, because the CO2 is not sufficient to use the added light[2]. A middle system with CO2 that agrees with the PPFD gives a limit of 1200. The 1500 tier is only for advanced growers. The full system for the environment is necessary for this tier[3].

Select the tier that your environment can supply. If the CO2 is less than the value in the table, a higher PPFD causes only bleaching.[1]
Tier 1, BasicTier 2, MiddleTier 3, Advanced
Peak PPFDapproximately 95012001500
Necessary CO2400–600 ppm (ambient air)1000–1200 ppm1200–1500 ppm
Necessary conditionsNone. A PPFD of more than approximately 850 does not help the yield.Good control of the VPD, and CO2 enrichmentControl of the leaf temperature, a method for the substrate, and a cultivar that can use the high light
Peak PPFD that each tier can useEach bar is light that the plant can use only if the CO2 agrees with it.041282512381650950Basic (ambient air)1200Middle (+CO2)1500Advanced (full system)
Figure 8. For the limit of each tier, the full system is necessary, and not only a higher setting of the light.[3]
Do not add more light than your CO2 can use

Do not operate Tier 3 light with Tier 1 air. This has the highest cost and causes bleaching of the plants. First, use the full PPFD limit that you can supply with CO2. Then try a higher limit.

Adjust the height and the output

Fixture height and dimmer

Two levers set the PPFD at the canopy: the dimmer of the fixture and its hanging height above the plants. The two levers change the quantity of light on the leaves, but the effect of each lever is different.

The dimmer is the lever with the best precision for small steps that are the same each time. It changes the intensity, but it does not change the distribution of the light or the radiant heat at the canopy. When you change the height of the fixture, the distribution and the heat also change. Thus a change of the height is a larger adjustment.

When you use one of the levers, make sure that the PPFD value is correct. Measure the PPFD at the canopy with a meter, or read it from the distance chart of the fixture. Do not use the watts of the fixture or the position of the dimmer as the PPFD. The same fixture gives very different values at different heights. Also measure the PPFD again when the distance between the canopy and the light becomes shorter. As a result of the stretch of the plants, the PPFD increases also if you make no change.

Two levers and one correct number1Dimmersmall steps,same each time→2Hanging heightlarger steps anda change of heat→3Measure at the canopymeter ordistance chart→4Measure againPPFD increaseswith stretchDo not use the dimmer position or watts as data. Measure the PPFD at the leaves.
Figure 9. Use the dimmer to set the intensity and the height to set the distribution and heat. Always measure the PPFD at the canopy. Do not use an estimate.
The distance to the light changes

A plant can have a stretch of 15 cm (6 in) in the direction of the light in one week. The distance between the plant and the light then becomes shorter. As a result, the PPFD on the plant increases by a large quantity, also if you do not touch the lights. Measure the PPFD again after each period of fast growth.

Read the plant

Troubleshooting

Too much light causes white tips with bleaching at the top of the canopy, on the leaves near the source. The growth also stops[6]. But two different errors cause the same symptom, and you cannot find which error caused it from the leaf only.

The first error is to increase the intensity too quickly. The second error is high light with low CO2. The two errors cause too much energy in the leaf and the same damage from oxidation. Thus the symptoms are the same[5]. Do not try to find the error from the leaf only. Prevent the two errors: increase the intensity in small steps and keep the CO2 correct for your intensity.

The damage is the same, but the causes are different. Prevent the two causes before the damage occurs. Do not try to find the cause after the damage.
SymptomPossible causeCorrection
White tips with bleaching at the top of the canopyThe PPFD increases too quickly, or the light is high and the CO2 is lowDecrease the PPFD by one step. Make sure that the CO2 agrees with your intensity.
Bleaching at a PPFD that you think is safeThe leaf temperature or the VPD is not in the correct rangeFirst correct the climate. Heat and dry air cause bleaching also at a safe PPFD.
The growth stops at high lightThe capacity of the plant is not sufficient, or the CO2 is the limiting factorKeep the intensity the same. Give the plant time to adapt. Do a check of the CO2.
Light green leaves, much stretch and flowers with low densityThe light is too low for a long time, because the grower thinks that more light causes damageIncrease the PPFD in steps. A light that is too low also decreases the yield.
Do not decrease the light too much after bleaching

After bleaching, decrease the light by one step only. Then increase the light again in steps. After bleaching occurs one time, growers frequently decrease the light for flowering to a value that is much too low. As a result, the yield decreases. A light that is too low for a long time causes damage to a crop. The damage is as large as the damage that bleaching causes to plants[1].

New data · 2024-2026

New tests

Acclimation and the correct CO2 for the PPFD are the primary information in this paper. New tests (2024-2026) do not change this information. They give more information on three effects. The first effect is the effect of a high PPFD limit. The other two effects are from the spectrum levers far-red and UV. The data show that the effects of these two levers are smaller than many persons think.

A high PPFD limit with sufficient CO2 increases the quality, and not only the weight. In a test in 2024, the PPFD increased from 600 to 1200 µmol/m²/s. The cannabinoid content increased by approximately 60%, and the terpenoid content increased by approximately 40%. These two increases came from two causes: a heavier inflorescence and higher concentrations. The light-use efficiency was almost constant[9].

A previous test shows that the yield of dry flower has an almost linear relation with the PPFD, to approximately 1800 µmol[1]. Thus the data agree: a correct ramp to a high PPFD limit gives a better grade and more mass. This result is correct if the CO2 and the climate agree with the PPFD. Without sufficient CO2, the added light causes only bleaching (see the section above).

PPFD from 600 to 1200 (with correct CO2): more metabolitesFrom a test in 2024: more inflorescence mass and higher concentrations.01835527060%Cannabinoid content40%Terpenoid content
Figure 10. More light with sufficient CO2 increases the concentration and also the weight. It is a lever for quality and not only for yield.[9]

Far-red light can increase the cannabinoid yield in some cultivars, but it can decrease the potency in other cultivars. Far-red light at the end of the day lets you decrease the photoperiod from 12 to 10 hours, with approximately 5.5% less energy. In some cultivars, far-red light at the end of the day increases the cannabinoid yield. One cultivar showed a total cannabinoid yield that was approximately 70% higher[10].

But when you add far-red light to all the spectrum (a lower ratio of red to far-red), the inflorescence mass can increase, and the concentration of cannabinoids and terpenes can decrease[11]. The buds are higher and larger, have a lower density, and are weaker. Far-red light also causes stretch. Use far-red light as a tool for one cultivar at a time. Do not use it as a standard part of the spectrum because you think that more is better.

UV light does not usually increase the potency in new cultivars. A previous test found that added UV-B light did not increase the yield or the cannabinoid content[3]. A test of UV spectra in 2024 also showed that the cannabinoids did not increase. High UV-B light decreased the THC and burned the leaves. Only the lowest dose of UV-A light changed the terpene profile by a small quantity (linalool +29%, limonene +25%, myrcene +22%), and the yield stayed the same[12].

Cultivars with high THC are near their maximum potency. Thus do not think that UV light increases the potency. A careful low dose of UV-A light can change the aroma by a small quantity. This effect is the maximum. Added UV light usually decreases the efficiency.

First the intensity, then the spectrum

First make the ramp and the CO2 correct. Then change the spectrum. Far-red light and UV light give small effects, and each has an unwanted effect. You use them with an intensity that is correct. They are not an alternative to it.

If a plant does not have full acclimation and the CO2 is too low, a change of the spectrum does not help. A correct ramp to your PPFD limit, with sufficient CO2, gives a better result.

Results and limits

Expected results and limitations

The light intensity is only one input. Each PPFD target in this paper is for an environment in which all the other values are in range. The leaf temperature is approximately 26–28°C (79–82°F). The VPD is 1.2–1.5 kPa. The root zone has sufficient capacity. The cultivar has a tolerance for the light load[2].

If you increase the light and the CO2 without these conditions, the plants become hot and have stress. The yield does not increase.

These conditions are necessary for each tier. The light gives a good result only when the temperature, the humidity and the root zone are also in range.
Necessary for all tiersTarget
Leaf temperatureapproximately 26–28°C (79–82°F)
VPD (drying power of the air)1.2–1.5 kPa
Root-zone capacitySufficient water and oxygen for the quantity that the plant uses
CultivarA cultivar with a tolerance for the light that you use
Use the correct tier for your room

A cycle without errors at the correct PPFD limit gives a better result than a cycle with errors at a higher limit. For most new growers, the best procedure is to use the ambient air limit of approximately 950 without errors. First make the acclimation and the CO2 correct. After that, you can try 1200 or 1500.

The light is only one input. It gives a good result only when all the other values of the environment are correct. The paper plant state dashboard shows how to read all the data of the plant. The paper signal and noise shows how to use correct signals and not noise.

Related papers
Coco and crop steering
Use water to control the plant
Signal and noise
Find the difference between plant changes and sensor noise
Plant state dashboard
From sensor data to a decision

References

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  2. Chandra, S., Lata, H., Khan, I. A., & ElSohly, M. A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants, 14(4), 299-306. https://doi.org/10.1007/s12298-008-0027-x https://pubmed.ncbi.nlm.nih.gov/23572895/
  3. Llewellyn, D., Golem, S., Foley, E., Dinka, S., Jones, A. M. P., & Zheng, Y. (2022). Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Frontiers in Plant Science, 13, 974018. https://doi.org/10.3389/fpls.2022.974018 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.974018/full
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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.