To set up an efficient home cannabis grow, it is important to determine the optimal size of your grow light. 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 (below) to help you to determine the optimal amount of light for your grow space.
This article is part of the Coco for Cannabis Grow Light Guide.
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 is receiving 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 Estimate the Optimal Amount of Light
With High Intensity Discharge (HID) lights, such as High-Pressure Sodium (HPS) and Ceramic Metal Halide (CMH) it became common to use wattage to determine how much light you need for your grow tent. 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.
Wattage was never the correct metric to measure the amount of light. It was a reasonable proxy for HID lights only because all HID lights convert electricity into usable light at about the same rate. However, LED grow lights can now be significantly more efficient than HID lights. The wattage rules no longer apply to most home growers.
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 an “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”.
Measuring the “Amount of Light” (Number of Photons)
Rather than measuring the electricity, we can directly measure the amount of light. But this brings us to an important point, in indoor horticulture, we are not actually interested in “light”, we are interested in Photosynthetically Active Radiation (PAR). The photons that are in the PAR wavelengths provide the energy that powers photosynthesis. When we ask, “how much light” what we really need to know is “how many PAR photons”.
Using PAR to Determine the Optimal “Amount of Light”
There are two ways that PAR photons are measured. Photosynthetic Photon Flux (PPF) is a count of all the PAR photons. Photosynthetic Photon Flux Density (PPFD) describes the density of PAR photons at some specific spot (the place it is measured). As we explain below, there is a limit to the density of photons that plants can use. The maximum density of photons (PPFD) ultimately determines the maximum quantity of photons (PPF) that we can provide in a grow space. To learn more about grow light metrics like PAR, PPF and PPFD, be sure to read our Grow Light Metrics Primer.
The PPF rule for optimal lighting is 65µmol (Usable PPF) per square foot or 700µmol (Usable PPF) per square meter. To understand why this is the optimal target for grow light size, it is useful to review cannabis photosynthesis and consider how plants respond under different densities of PAR photons.
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 density of photons (PPFD) that cannabis plants can use. If plants are exposed to a higher density of photons than they can use in photosynthesis, it will not increase yield. In fact, when PPFD is too high, it 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, photon density, temperature, oxygen, water, minerals, age, leaf anatomy and more. In many grow tents, photon density is the limiting factor. However, as you increase the density of photons, 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.
Photon density (PPFD) 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 energy by 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 photon density is 500 µmol (PPFD). The limiting factor is CO2. This shows up in the data as the concentration of CO2 within the leaves drops when the photon density is above 500 µmol/m2. Increasing photon density 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 photon density reaches 1000 µmol/m2 (PPFD). Additional photon density, beyond 1000 µmol/m2 (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 370 µmol mol. When CO2 levels are higher, cannabis plants can process more photon energy before they become limited. The data from Chandra et al. show that when CO2 concentrations are 750 µmol mol, cannabis plants can perform well at a photon density of 1500 µmol/m2 (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.
Optimal Grow Light Size for Cannabis
To determine the optimal grow light size, it is important to consider the optimal PPFD (density of light) for growing cannabis and how that Optimal PPFD converts to an Optimal PPF (amount of light).
The Optimal PPFD for Cannabis:
The data from Chandra et al. confirm that the optimal photon density for peak cannabis photosynthesis is between 500 and 700 µmol/m2 (PPFD). It also shows that we should avoid going over 1000 µmol/m2 (PPFD) 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 500 µmol/m2 (PPFD) and that no spot receives more than 1000 µmol/m2 (PPFD). We recommend an average of 700 µmol/m2 (PPFD). With most grow lights, an average of 700 will ensure that you stay within the optimal range for peak photosynthesis in all regions of the canopy.
The Optimal PPF for Cannabis:
PPFD is a density measurement which is expressed as micromoles per square meter. To convert PPFD into a quantity measurement, we multiply it by the area in square meters. Since the optimal average photon density is 700 µmol/m2 (PPFD), the optimal number of photons is 700 µmol Usable PPF per square meter. This converts to 65 µmol Usable PPF per Square Foot. To calculate the total amount of light that you need for your grow space in Usable PPF, simply multiply the square footage by 65 (Sq. ft x 65 = µmol Usable PPF).
Usable PPF, Total PPF and “Calculated Values”
There are three ways that PPF values are measured or calculated. PPF values may be a “Calculated PPF”, or they may be measurements of “Total PPF” or “Usable PPF”. As we explain in our Grow Light Metrics Primer, 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.
Usable PPF
Usable PPF is the measurement that matters to us as growers. Usable PPF describes the number of PAR photons that arrive to the canopy of the plants. It is the value that we care about because it describes the number of photons that are 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. To learn how we measure Usable PPF in our grow light tests, read our Grow Light Testing Protocol.
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. As a result, Total PPF is always higher than Usable PPF.
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
Use our Grow space PPF calculator to estimate the PPF needs and harvest potential of your grow. To learn more about how we estimate harvest sizes, see our article, “Calculating Grow Light Efficiency Coverage and Harvest Potential”.
The Grow Light Calculator
Be sure to check out our Grow Light Calculator! It estimates the efficiency, coverage and harvest potential of grow light fixtures. You can enter your own data or browse the preloaded fixtures.
