How to Find the Perfect Photoperiod for Cannabis with Ideal DLI ?

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In the previous articles, we have covered how growers could utilize the light intensity factor in a grow lighting system to maximize crops’ growth and how to use the inverse square law to determine the optimal lighting area as well as the mounting height of the fixture to the canopy in a system.

There are many factors to be considered in a successful and efficient growing system. An efficient lighting system is a complex system that every factor needs to be maximized to its full potential and works with others coherently. Have you wonder if your plants are getting enough lights? Or am I giving them too much light? Your crops will tell you about their overall health by giving visual signs.

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With too much light, for example, cannabis plants will get yellow leaves while the veins and stems are still green. This is especially true for the leaves on top of the plant.

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Too much light will cause white buds. Heavily light bleached buds might even lose all its cannabinoid potency.

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Signs of not enough light with Cannabis are slow growth and stretching. Plants do this to expand itself to get closer to a light source, this will increase the spacing between nodes thus taking more space and low yields.

When plants are supplied with the right amount of photo energy they need, node space will be reduced, overall growth is stimulated, flower size is increased, as well as reduces the chance of plants to pest outbreak, the list goes on. Professional cultivators will not wait until negative signs start showing to address the problems in a lighting system because with proper design and calculation of the grow system, most damages and inefficiencies could be easily avoided. (Recap + How professional cultivator avoid insufficient grow)

In this article, we will cover what is DLI, why it is essential to growers and how to utilize it along with light intensity to achieve a sufficient and successful lighting system. As we mentioned that many new growers might not know how much light is ideal for their plants’ growth. DLI (Daily Light Integral) is the factor that will help them to determine the optimal photoperiod needed to expose their crops under. DLI describes the number of photosynthetically active photons (photons in the 400 – 700 nm range of wavelength) that are landed on a specific area over a period of 24 hour. Given from last article, light intensity is measured in PPFD ( ). Plants are just like us; they need energy to thrive and have various stages of developments. Plants get their energy through the process called photosynthesis. During the process of photosynthesis, plants capture photons and convert it to energy which they could utilize for growth. Each plant has a maximum DLI in its respective developmental stage. DLI is a measurement that helps cultivators to decide on the “Just Right” amount of light a crop needs to flourish. By using the equation in Figure 1. We could calculate and predicate photoperiod in most general cases.

Figure 1. DLI in Relation to Light Intensity and Photoperiod (DLI Calculation)

In this article, we will focus on the developmental cycles of cannabis as well as the respective DLI to each growing stage.

Cannabis has four developmental stages:

  1. Seedling
  2. Vegetative
  3. Flowering
  4. Maturing.
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Luckily the ideal DLI for each stage of development has been researched and studied by many scientists. Growers could use this information to determine the optimal photoperiod of cannabis at different stage of the growth. Given the ideal DLI in Figure 2. at different grow stages of cannabis

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Figure 2. Ideal DLI w/ Respect to Grow Stage of Cannabis.

DLI equation from Figure1. to be further defined with units is shown as below,

Equation 1. DLI in Relation to PPFD and Photoperiod

For instance, with our FLUTTERBY 800W, at a mounting height of 18 inch, the average PPFD is 565.81 across a 4’ X 4’ grow area.

Figure 3. Color-Coded Lighting Intensity and Its corresponding Mounting Height

Plugging in the ideal DLI value (Figure 2.)  at week 6 which is 45 during the cannabis vegetative stage into the DLI equation (Equation 1.). By plugging in all the known factors into the DLI equation, we get the ideal Photoperiod = = = 22.00 hours.

From the calculation, we could conclude that 22.00 hours is the best level of photoperiod for cannabis at its vegetative stage in week six; with the parameters of our FLUTTERBY 800W, PPFD of 565.81 at 18-inch mounting height, as well as 4’ X 4’ grow area.

The ideal DLI values is a key factor to growers; at different developmental stages of the crops, they require different DLIs. We can easily determine the photoperiod of the plants in each lighting system by using the DLI equation as well as the given parameters of the lighting system.  (How DLI is calculated based on PPFD)