How Understanding DLI Will Make You a Better Grower

January 17, 2023 steven

There’s a lot of buzz in the grower community around the term Daily Light Integral these days, but few growers truly understand this incredibly useful plant-lighting concept. DLI is a measure of photosynthetic light intensity over a given span of time (24 hours) to determine the total amount of energy a plant receives throughout the course of a day. If you’re already confused, don’t fret. We’re going to break down what DLI really means and how it’s used by modern food producers as well as hemp growers to maximize yield and quality.

DLI and PAR

To understand DLI it’s important to have a grasp on the more common lighting metrics currently being used in horticulture. For decades, amateur and professional growers alike measured light intensity in their gardens using the lumen or footcandle standards. While these units are great for determining proper lighting levels for a factory or office space, they fall short when it comes to measuring light for photosynthesis. That’s because lumens and footcandles were developed to measure how light is perceived by humans. Since our eyes are most sensitive to green light at about 555nm, the lumen and footcandle standards are also biased in this way.

Plants, on the other hand, utilize all the light in the visible spectrum almost equally, so it’s important to use a measuring standard that takes this reality into account. That’s where photosynthetic photon flux density (PPFD) comes into play. The PPFD standard gives equal value to every photon (particle of light) within the visible range (400-700nm). Readings on a PPFD meter, also known as a PAR meter, are shown in μmol/m²/sec, which simply means the number of photons which fall on one square meter every second. Hemp growers utilizing a PAR meter will typically dial their lighting system to about 200-400 μmol/m²/sec for vegetative growth or around 800-1,000 μmol/m²/sec for flowering plants. More on that later. You may be asking yourself right now, “what in the heck is a ‘μmol’”? To answer that, let’s take a quick detour back to high school chemistry class. Don’t worry, we’ll keep it brief.

Photons are small, infinitely small, and there are a lot of them. To put it into perspective, a typical LED grow light releases more photons in one second than the total number of grains of sand on Earth. To help deal with these mind-bogglingly large numbers we borrow a scientific unit normally reserved for counting atoms, the Mole. The Mole (abbreviated mol) is based on the total number of atoms in 12 grams of carbon-12, which is generally understood to be 6.02 x 10²³. This number won’t be on the test, but it’s important to remember that the mol is a shorthand unit for measuring things that are incredibly numerous. The μmol adds the Greek prefix mu, represented by the symbol μ-, to divide by one million. In other words, one mol is equivalent to one million μmol.

What does any of this have to do with DLI? Since DLI is a measure of the total photons falling on a surface during a 24-hour period, a very large number, we need to use the mol unit to avoid having to write out all those zeroes. Let’s go back to the PAR meter example with a reading of 400. If your plants are receiving 400 μmol/m² photons every second, how many photons do they receive throughout the course of a day? Assuming 18 hours of light, that’s 64,800 seconds, so roughly 25,920,000 μmol photons each day. That’s still a pretty large number to deal with, so we can simplify again by going from the μmol unit back to the mol unit by dividing by one million. This tells us that a PPFD reading of 400 for 18 hours would give us a DLI of about 26, which is a much easier number to remember.

Crop Requirements

With this in mind, let’s take a look at the DLI requirements of some common crops as observed through university research:

DLI requirements of some common crops

Species DLI – Minimum DLI – Typical DLI – Optimized

In this chart, target DLI numbers are split into three ranges: minimum levels needed for maintenance growth (indicated in yellow); typical levels for highest efficiency (indicated in green); and optimized levels for maximum growth and yield (indicated in red). As you can see there is a high degree of variability between different species when it comes to DLI requirements. Some cultivars can benefit from incredibly intense light levels, but it’s important to note plants exposed to these conditions will typically need additional water and fertilizer to accommodate increased photosynthesis. In closed environments the addition of supplemental CO₂ may be necessary.

In the Real World

Here’s an example of how understanding DLI can help you custom tailor your lighting system to specific crop requirements. Let’s assume you’re growing a crop that is photoperiod sensitive; this species needs at least 12 hours of darkness to maintain flowering growth. The target DLI for this particular variety is 34, which must be achieved within 12 hours so as not to disrupt the reproductive cycle of the plant. The desired PPFD can be found by dividing the DLI requirement by the number of hours, then multiplying by 277.8. So, for this example 34/12*277.8 = 787.1, or just under 800 PPFD. Use a PAR meter and raise or lower your grow light (or its dimmer) to achieve the appropriate intensity.

Daily Light Integral becomes especially important when supplementing sunlight in a greenhouse environment. DLI maps that show the average monthly sunlight intensity for specific states or geographic areas are readily available online. Once you know the average sunlight intensity in your area and your specific crop’s DLI requirements, it’s a simple matter of plugging in the numbers to see how much supplemental lighting you’ll need. Say you have a greenhouse in central Michigan and you plan to grow a winter crop with a DLI requirement of 24. Based on a DLI map of Michigan you can expect about 14 mol of sunlight per day during the month of January. You’ll need your supplemental fixtures to provide the additional 10 mol of light to achieve the target intensity. If your lighting array produces a PPFD of 200 at the canopy you can use some basic math to figure out how long to leave the lights on each day. Divide the additional DLI needed by the PPFD of the supplemental lighting array and then multiply by 277.8. For our greenhouse example this would be 10/200*277.8 = 13.89, so you would need to keep the lights on for around 14 hours to achieve the necessary DLI for your crop.

hours of light chart

The above chart should be used as a reference for comparing PPFD, DLI, and hours of light exposure. The numbers 100-1,000 on the left represent PPFD, while the numbers 6-24 at the top represent hours of light, and the numbers in the center show the resulting DLI achieved.

Utilizing DLI calculations will give you more flexibility in the design and implementation of plant lighting systems. Understanding daily light integral will give you another important tool for optimizing plant health and production in outdoor, indoor, and greenhouse environments. Mastering these concepts will take your growing skills to the next level.

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