What is Daily Light Integral and How Does it Affect Your Crop?

Not all light is created equal, at least according to plants. As we’ve covered on our blog, plants are only able to absorb certain wavelengths of light to photosynthesize. Understanding what type of light wavelengths plants respond to best and how to measure this light is a necessary aspect of ensuring the success of your crop.

PAR wavelengths are between 400 and 700nm (visible light is between 380 and 740nm). It’s measured in PPFD, the number of photons delivered to a square meter per second. Growers use PAR to determine how much usable light is successfully reaching their plants. If you’re familiar with PAR, chances are you’ve also heard the phrase daily light integral (DLI). DLI and PAR are interrelated and both valuable measurements for understanding light efficacy.

Putting DLI into Context

DLI refers to the amount of PAR delivered to a single square meter over the course of a day. It is measured in mol·m−2·d−1: moles per square meter per day. A mole is a large constant called Avogadro’s number (6.02 x 1023 photons per mole). That might seem like a huge, unimaginable number, but plants generally need anywhere from 10-50 mol·m−2·d−1 to thrive. On a cloudless day in Arizona at the height of summer, the average daily light integral is often close to 50 – 60 mol·m−2·d−1. This means a desert plant, such as a saguaro cactus, receives trillions of photons over the course of a day in June.

Generally speaking, different plants exhibit varying DLI requirements. This is due to the individual species’ evolution, as DLI changes according to latitude, environment, season and weather conditions.

Understanding DLI for Cannabis

When growing cannabis indoors, DLI is much easier to control than outdoors. When growing outdoor crops, there are countless variables that can affect DLI — anything from cloud cover to shady trees might cause a dip in these numbers. Consider the following:

  • Cannabis is considered an especially high DLI crop, but short-day flower crop; it needs very high DLI throughout its lifecycle and a shorter day to trigger flowering.
  • As DLI increases, so does the plant’s ability to photosynthesize, to an extent, reaching an apex around 40 mol·m−2·d−1 It often plays an important role in ensuring stem and root health and might boost the number of flowers.
  • Cannabis plants start to flower when they detect a shorter day. In the northern hemisphere, this occurs naturally in the autumn; many short-day plants flower during this time of the year.

Calculating DLI:

Use the formula below to easily calculate DLI. The 3600 number is a quick reference, as it comes from another formula that simply adds the dimension of time. In reality, it’s simply multiplying 60 seconds by 60 minutes, since our measurement is in a single second. So we multiply 3600 by the number of hours the lights are on for. This number is multiplied by the actual PAR readings from a quantum meter. Then, divide by 1,000,000 to calculate the DLI for the crop. The equation looks like the following;

(PPFD) μmol m-2s-1 x (3600 x photoperiod) / 1,000,000 = DLI

Let’s try an example that can better foster this understanding:

We walked into a flower room with a handheld meter and our quantum sensor showed 400 μmol m-2s-1. Because we flower with 12/12 cycles, we have all the info to figure out our DLI:

(3600 x 12) = 43,200

400 x 43,200 = 17, 280,000

17, 280,00/ 1,000,000 = 17.28 mol·m−2·d−1

Because cannabis plants need such intensity, growers should invest in the lighting fixtures that can deliver enough light, at the right intensity, without skyrocketing electrical costs. LEDs are able to provide superior light quality for a high DLI:

ProGrowTech LED lights are ideal for growing cannabis at high DLI. Explore your options.