How Much Light (PPF) Do You Need for Indoor Cannabis?

How much light do you need for cannabis

To set up an efficient home cannabis grow, it is important to determine the optimal size of your grow light. With High Intensity Discharge (HID) lights, such as High-Pressure Sodium (HPS) and Ceramic Metal Halide (CMH) it is possible to use wattage to determine how much light you need for your grow tent. However, LED grow lights can now be significantly more efficient than HID lights, so the wattage rules no longer apply. In this article, we discuss why it is important to match the size of the grow light with the size of the tent, we explore the science of cannabis photosynthesis to determine how much light cannabis plants can use, and we explain how to use Photosynthetic Photon Flux (PPF) to determine the optimal grow light size for your grow tent. We include our Grow Space Calculator to help you to determine the optimal amount of light for your space and estimate the harvest potential.

This article is part 3 of the Coco for Cannabis Grow Set-up Guide.

The Coco for Cannabis Grow Set-up Guide
  1. Grow Tents and Harvest Sizes
  2. How to Evaluate and Compare Grow Lights for Cannabis
  3. How Much Light (PPF) Do You Need for Indoor Cannabis?
  4. Grow Light Calculator (Coming Very Soon)
  5. Airflow, Ventilation and Exhaust Systems for Grow Tents (Coming Soon)
  6. Managing Heat and Humidity in your Grow Tent (Coming Soon)

Matching the Grow Light to the Grow Space

When setting up your indoor cannabis grow, we recommend that you start by thinking about the yield that you would like to be able to harvest each cycle. As we explain in our guide, "Grow Tents and Harvest Sizes", the yield of each grow is limited by the space, so your yield goals should determine the size of your grow tent. The size of your grow tent then determines the amount of light that you need.

You can grow cannabis plants under small lights or large lights. Many growers use less light than they could and still produce decent harvests. However, the efficiency of the grow and the quality of the harvested cannabis is best when the grow lights are matched to the grow space.

Not Enough Light Produces Larf

When the light is insufficient for the space, it can result in lower quality cannabis and more work trimming. Large plants that receive inadequate light will produce a lot of low-quality buds that we call "larf". Many growers mistakenly think that larf is the result of budding sites not receiving light. In reality, larf is the result of a plant that, in total, has more budding sites than energy to develop them. If the plant receives less than optimal light and has a large number of budding sites, it will produce larf.

Too Much Light Is Damaging or Wasteful

It is even more important to avoid giving the plants too much light. As we explain below, there is a limit to the amount of light a plant can use, and excessive light will cause damage. If you have too much light, you could avoid damage by raising or dimming the light. Raising the light wastes energy and reduces efficiency. If you must dim the light, then you are not taking full advantage of your investment. In either case, you would save money and be more efficient if you had lights that were properly matched to the space.

How Much Light Do You Need?

There are various recommendations for how to determine the correct amount of light, however many of them are outdated, not applicable, or based more on marketing than science. We advise you to ignore the manufacturer's claims about coverage area and instead focus on scientific measurements about the amount of light.

Using Wattage to Determine the Amount of Light

With HID lighting (HPS and CMH) it is possible to use wattage to estimate the amount of light. This is because all HID lights are similar in terms of photon efficiency: they convert electricity into usable light at about the same rate. With HID lighting, the rule is about 40 watts per square foot (430 watts per square meter). This means that a 600-watt HID light is adequate for a 4' x 4' tent and a 1000-watt HID light is perfect for a 5' x 5' tent.

LED and HPS Equivalency

High Pressure Sodium (HPS) have traditionally been the preferred HID light for flowering cannabis plants. When LED grow lights came into the market, many manufacturers tried to sell their lights by claiming a "HPS equivalency". Since HPS lights were traditionally measured in terms of watts, LED lights came to be marketed with an "equivalent wattage". However, there has never been a standardized way to make these "equivalencies". Each manufacturer comes up with their own metrics to establish them and most are gross exaggerations. We advise you to completely disregard the manufacturer's claims about "HPS equivalency" or "equivalent wattage" and focus instead on the PPF that each fixture produces.

Using PPF to Determine the Amount of Light

Photosynthetic Photon Flux (PPF) measures the amount of Photosynthetically Active Radiation (PAR) that a fixture can produce. As we explain in our article, “How to Evaluate and Compare Grow Lights for Cannabis” there is a difference between “Total PPF” and “Usable PPF”. “Total PPF” describes all the light that a fixture produces. “Usable PPF” describes the portion of that light which arrives to the plants and can be used for photosynthesis. Usable PPF is the proper measurement to determine the total amount of light needed for a grow space. Please see our article, “How to Evaluate and Compare Grow Lights for Cannabis” for a complete discussion of grow light metrics.

The PPF rule is 65µmol (Usable PPF) per square foot or 700µmol (Usable PPF) per square meter. To understand why this is the optimal target for the amount of light, it is useful to review cannabis photosynthesis and consider how much light cannabis plants can use.

How Much Light Can Cannabis Plants Use?

It is common to hear that "more light is better" and since many home growers use insufficient lighting for their space, it is often true. However, there is a limit to the intensity of light that cannabis plants can use. If you expose plants to more light intensity than they can use in photosynthesis, it will not increase yield. In fact, too much light can reduce both the yield and the quality of the harvested cannabis.

The rate of photosynthesis and photosynthetic efficiency can be limited by several factors including carbon dioxide, light intensity, temperature, oxygen, water, minerals, age, leaf anatomy and more. In many grow tents, light is the limiting factor. However, as you increase the intensity of the light, other factors like carbon dioxide will become the limiting factor. When photosynthesis is limited by any factor other than light, the leaves reach their light saturation point.

Light intensity that is beyond the saturation point dictated by photosynthesis can damage plant tissue. Therefore, when leaves reach their saturation point, the plant will attempt to protect itself with photoprotection responses. These include things such as chlorophyll or leaf movement, anatomical changes, non-photochemical quenching and thermal dissipation. All these photoprotection efforts by the plant waste energy and can lower yield.

If the plant cannot adequately protect itself from excessive light using photoprotection responses, it will begin photoinhibition. Photoinhibition decreases the rate of photosynthesis and reduces growth and harvest potential. However, symptoms of light stress do not become apparent if the plant is able to cope with the excessive light. Symptoms such as chlorosis occur only when photoinhibition can no longer effectively protect the plant.

Cannabis Photosynthesis: Carbon Dioxide and Light Limits

There are many areas of cannabis science that have not yet had adequate research. Fortunately, photosynthesis is one of the exceptions. In 2008, Chandra et al. published extensive research into cannabis photosynthesis. The data they provide offer the most accurate measurement of how much light cannabis plants can use.

The data from Chandra et al. show that cannabis plants are like many other terrestrial plants. In ambient concentrations of carbon dioxide (CO2), cannabis leaves begin to saturate when the light intensity is 500µmol (PPFD). The limiting factor is CO2. This shows up in the data as the concentration of CO2 within the leaves drops when the intensity of light is above 500µmol. Additional light at this point produces diminishing returns, but it will lead to more photosynthetic activity. However, there is a limit. Cannabis plants begin photoinhibition when the intensity of the light reaches 1000µmol (PPFD). Additional intensity, beyond 1000µmol (PPFD), will lower the rate of photosynthesis and can damage plant tissue.

These limits are largely dictated by the concentration of CO2. Ambient CO2 levels are around 370µmol mol. When CO2 levels are higher, cannabis plants can process more light energy before they become limited. The data from Chandra et al. show that when CO2 concentrations are 750µmol mol, cannabis plants can perform well at a light intensity of 1500µmol (PPFD) without inducing photoinhibition. This allows larger harvests from the same amount of space. However, successfully increasing the concentration of CO2 in the grow space requires sealing the space. The costs of setting up and running a sealed grow space are considerable. Most home growers are better served by using a ventilated grow space and accepting the limits imposed by the ambient levels of CO2. See our guides, "Airflow, Ventilation and Exhaust Systems for Grow Tents" and "Managing Heat and Humidity in your Grow Tent" (coming soon).

Optimal Grow Light Size for Cannabis

The data from Chandra et al. confirm that the optimal intensity of light for peak cannabis photosynthesis is between 500 and 700µmol (PPFD). It also shows that we should avoid going over 1000µmol (PPFD) in intensity which could lead to damage. With artificial lighting, the distribution of light is never perfect. Therefore, we want to ensure that all areas of the canopy get at least 500µmol (PPFD) and that no spot receives more than 1000µmol (PPFD). A target average of 700µmol (PPFD) is ideal.

The Optimal PPF for Cannabis:

Since the optimal average intensity is 700µmol PPFD, the optimal amount of light is 700µmol Usable PPF per square meter. This converts to 65µmol Usable PPF per Square Foot. To calculate the total amount of light that you need for your grow tent in Usable PPF, simply multiply the square footage by 65 (Sq. ft x 65 = µmol Usable PPF).

Usable PPF, Total PPF and Calculated PPF

There are three ways that PPF is measured or calculated. PPF figures that are reported may be a “Calculated PPF”, or they may be measurements of “Total PPF” or “Usable PPF”. As we explain in our article, “How to Evaluate and Compare Grow Lights for Cannabis”, there are significant differences between the different types of PPF data. To make accurate measurements and comparisons, it is important to understand what type of PPF values you are working with.

Total PPF

Total PPF describes the total amount of light emitted by a fixture. Total PPF measurements are taken in a device called an “integrating sphere” which measures all the photons produced by the fixture. However, even in ideal grow set-ups, 10-15% of these photons will be lost to radiance or reflection.

Usable PPF

Usable PPF describes the amount of PAR photons that are produced by a fixture and arrive to the canopy of the plants. It is the number that we care about because it describes the amount of light that will be available for photosynthesis. Usable PPF is measured in a field setting that simulates a grow tent. Accurate measurements depend on careful testing with specific protocols.

Calculated PPF

Manufacturers often don’t report PPF data at all. When they do report it, it is usually a “Calculated Value” and not an actual measurement. Calculated values are determined based on the diodes in the fixture and assume 100% efficiency. As a result, calculated values are typically significantly higher than Total or Usable PPF.

Estimating Usable PPF

Often the only PPF data that are available are calculated values. This creates a need both for more independent testing and for some way to estimate the Usable PPF from the calculated values provided by manufacturers. We have analyzed test data from dozens of fixtures to create formulas to make these estimates. In our Grow Space Calculator below, you can see the Optimal Usable PPF along with our estimates for Total PPF and Calculated PPF for any size grow space. You can then match these to the type of data that is available for different fixtures.

Grow Space Calculator

Enter the size of your grow tent or space in our Grow Space Calculator below. It will estimate the PPF needs and harvest potential of your grow. To learn more about how we estimate harvest sizes, see our Grow Light Calculator (coming soon).

CocoforCannabis.com

Grow Space Calculator

Grow Space

Optimal Light

Calculated PPF: µmol
Total PPF: µmol
Usable PPF: µmol

Estimated Harvest

Low Benchmark
grams grams
oz oz

Common Grow Tent Sizes

CocoforCannabis.com
Dimensions Sq.Feet Calculated PPF Usable PPF: Low Harvest 0.55g/µmol Benchmark 0.75g/µmol
2' X 4' 8 709µmol 520µmol 286g 390g
3' X 3' 9 798µmol 585µmol 322g 439g
4' X 4' 16 1419µmol 1040µmol 572g 780g
5' X 5' 25 2217µmol 1625µmol 894g 1219g

Common Grow Tent Sizes

CocoforCannabis.com

Tent Size

2' X 4' 3' X 3' 4' X 4' 5' X 5'

Square Feet

8 9 16 25

Calculated PPF(µmol)

709 798 1419 2217

Usable PPF(µmol)

520 585 1040 1625

Low Harvest

286g 322g 572g 894g

Benchmark Harvest

390g 439g 780g 1219g

The Next Steps

Once you determine the amount of light you need, it is time to choose your light fixtures. Be sure to read our guide, "How to Evaluate and Compare Grow Lights for Cannabis".

See the Grow lights that we recommend in our Equipment and Product Guide.

The Coco for Cannabis Grow Set-up Guide
  1. Grow Tents and Harvest Sizes
  2. How to Evaluate and Compare Grow Lights for Cannabis
  3. How Much Light (PPF) Do You Need for Indoor Cannabis?
  4. Grow Light Calculator (Coming Very Soon)
  5. Airflow, Ventilation and Exhaust Systems for Grow Tents (Coming Soon)
  6. Managing Heat and Humidity in your Grow Tent (Coming Soon)

Get Support from our Community!

We have a wonderful community of growers and you are welcome to join! If you are looking for advice and support throughout your grow, we encourage you to start a Grow Journal in our Grower's Forum. It is a great way to share your experience and ask questions along the way.

We also have a live chat room, which is a great place to ask questions, get quick answers, and chat with fellow growers. We welcome all growers who want to learn, share and grow together!

Need Grow Equipment?

Check out our Product and Equipment Guide for our Recommended Grow Lights, Grow Set-up Equipment, DIY Projects, Germination Supplies, Plant Training Supplies and everything else you will need during the grow!

Read All Our Articles!

Citation

 

Author: Dr Photon and Dr Coco

About Our Product Recommendations

At Coco for Cannabis, our mission is to help growers maximize the success of their cannabis crops by providing scientifically accurate information and sharing proven growing practices. The products that we recommend are the actual products that we chose to purchase and use ourselves. We do not accept sponsorship or advertising and will not recommend any product that we would not buy for ourselves. Please see our Product and Equipment Guide.

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Disclaimer:

The information on cocoforcannabis.com should not be considered as financial, legal, or medical advice.
You are responsible for knowing and following the local laws that pertain to cannabis cultivation, possession, and use. Decisions to grow cannabis should be made in consultation with a lawyer or qualified legal advisor. Decisions to use cannabis should be made in consultation with your doctor or medical professional.