What is DLI?

DLI is one of those lesser known metrics, at least in the non-commercial cannabis sector. Daily Light Integral (Which uses a scale of 0-70) is how many moles of Photosynthetically Active Radiation (The spectrum used by your plants) are received by each square meter each day. In other words, how much PPFD your plant canopy receives throughout each 24hr period.

Different plants have different DLI needs, for example the Moth Orchid (Phalaenopsis) only needs a DLI of 2 and shouldn’t be given above 10, while roses need a DLI of 10 to meet their minimum requirements and can happily exceed 30.
Cannabis needs a minimum of 35-50 DLI (During flower) in order to grow to it’s genetic potential, but can use up to 65 if other environmental conditions are optimal.

How do we measure DLI?

To expand on the above, we measure DLI as PPFD per square meter at canopy level per day (mol/m²/day). To accurately measure this, you would need a PAR meter that is designed to measure the light range your lighting puts out, HPS and LED have different total spectrums and thus slightly different meters to measure them.

If you don’t have one of those handy Apogee meters (My preferred PAR meter source), you can reference PPFD charts for your particular light, either from the manufacturer or more reliably from independent testers. If you are using this method make sure you know what the PPFD is at a given distance for the light and then realize that the DLI you come up with will be at that distance, moving further away will lower the DLI received.

“But wait, what is PPFD?”

Well, PPFD stands for Photosynthetic Photon Flux Density, and is a measurement of the amount of Photosynthetically Active Radiation at a given distance from the light source and is measured in μmol (Micro-moles).

“What is this Photosynthetically Active Radiation?”

Photosynthetically Active Radiation, otherwise known as PAR is a measurement of the light spectrum in the 400-700nm range, which contains most of what is needed for photosynthesis.

OK, let’s calculate your DLI

Now that you have measured or estimated your PPFD, we can calculate it into DLI.

First, take your PPFD and multiply it by 3600 in order to get HLI (Yeah I just made that up, hourly light integral). The reason we multiply by 3600 is that PPFD is measured in seconds, so you have to multiply it by 60 to get per minute, then another 60 to get per hour.

Now that we have the hourly light integral, multiply that by how many hours you are running your light to get μmols per day. You will end up with a fairly large number and will need to reduce that by dividing it by 1,000,000 in order to convert μmols into moles.

I’ll lay it out here in a more math friendly format:

1. PPFD * 3600 = μmol/hr
2. umol/hr * hrs of light = μmol/day
3. μmol/day / 1,000,000 = DLI

Lets do an example exercise:
Say we have a light that is putting out 500PPFD at canopy level and you’re in flower running a 12/12 cycle.

500PPFD * 3600 = 1,800,000 μmol/hr
1,800,000 μmol/hr * 12 = 21,600,000 μmol/day
21,600,000 / 1,000,000 = 21.6 DLI is what your light and hours are providing.

Just for fun, let’s take your target DLI and turn it into PPFD

So, now you know how to calculate the DLI you are providing your plants with your current light and schedule, but you want to know how much light you need to hit “X” number of DLI. Well we can indeed do that as well!

1. Take your target DLI number
2. Multiply that by a million to get μmol/day
3. Divide the DLI by hours of light (Ex: 12 hours if in 12/12, 18 if in 18/6, etc.)
4. Divide that number by 3600 to get the PPFD you need to reach your target!

Example Time:
Lets say you want to shoot right for 50 DLI during flowering on a 12/12 schedule (A worthy goal)

50 * 1,000,000 = 50,000,000 μmol/day
50,000,000 / 12 = 4,166,667 μmol/hr
4,166,667 / 3600 = 1157PPFD in order to reach 50 DLI.

Lets talk about DLI requirements for different stages

OK so now we know how to calculate DLI, and how to calculate PPFD requirements from a target DLI. Now we need to touch a bit on the lighting requirements of various stages of Cannabis growth.

Seedling Stage:
During the seedling stage, cannabis needs a DLI of about 13-15. This can be achieved on an 18/6 schedule with about 200-230 PPFD OR if you run the light 24hrs (Yep, I just went there) you would only need 150-175PPFD. (See how knowing the light per hour comes in handy when calculating your schedule?)

Vegetative Stage:
For the vegetative stage cannabis ideally should receive a DLI of 20-35, although this is somewhat contested by some growers who like to pack on as much DLI as they can. To use 20-35 though, and running an 18/6 schedule you would need 308-540 PPFD, or on a 24hr schedule it would only require 231-405 PPFD.

Flowering:
Ok, here is where you need to break out the big guns, or turn up your big guns if you have them dimmed during the vegetative stage.

For flowering the ideal DLI range is 35-65, and since photoperiod plants are limited in how many hours per day you can run the lights, it requires much more powerful lighting than you can get away with in the vegetative stage.

To hit that target range in 12 hours per day you need… 810 PPFD to 1500 PPFD. Yep you read that right, to maximize photosynthesis of the cannabis plant you can use up to 1500PPFD for a 12hr light cycle, and yes this is without adding Co2.

But! don’t worry if you can’t hit a DLI of 65 for your plants, because not even the sun breaks 65 for outdoor plants.

Wrapping it up

Ok so we have learned what DLI is, how it’s used, how it’s calculated, and how it’s applied to cannabis. Yes, most recommendations by both fellow growers and even lighting companies actually recommend lights that produce a lower DLI than is ideal for cannabis, having lower DLI does not mean the plant will not grow or produce flower but it will mean that the plant will not grow to the maximum size and yield that it’s genetically capable of since the “power source” for all that growing comes from… Light! More of it = happier, and bigger plants as long as you maintain the right atmosphere and nutrient levels for said lighting conditions.

I’d also like to point out here another little known factor about DLI and how it relates to cannabis. As you can see above, with the limited hours you can run during flowering for a photoperiod plant, you need quite powerful lights to get high into those DLI numbers… but with autoflowers you can give them the same DLI with as low as half the light output because you can run them longer per day even during flower. Just a little nugget of info for your brain to kick around.

There is indeed much more information relevant to this subject, but I hope this short overview will give at least 1 person a little more knowledge.