How to choose LED grow light for your plants?

This article discuss why it is important to match the size of the grow light with the size of the tent, it explore the science of cannabis photosynthesis to determine how much light cannabis plants can use. 

First of all, we need to understand an important parameter：Photosynthetic Photon Flux (PPF) and Photosynthetic Photon Flux Density (PPFD).

The PPF is the total amount of light in the PAR zone that is produced by a light source each second. So PPF measures the “photosynthetically active photons emitted by a lighting system per second”.

The PPFD is the measurement of PAR delivered to a specific area. It is expressed as micromole-per-meter squared-per second (μmol/m2/s). This is the only measurement that informs us of the amount of light being delivered to a crop which enables photosynthesis.

In the following content, we will use the two parameter as a measurement standard to explain how to choose a suitable LED grow light.

To set up an efficient home cannabis grow, it is important to determine the optimal size and right PPF and PPFD of your grow light. 

     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. 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. At the end of the article, I will estimate the PPF needs and harvest potential of several common sizes of tents.

How Much Light Can Cannabis Plants Use?

      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 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.

About The Right Light

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.

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 LED 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).

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