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Daily Light Integral (DLI) : How Much Light Do Your Plants Need?

Daily Light Integral (DLI) : How Much Light Do Your Plants Need?

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Daily Light Integral (DLI)

At this point in your growing experience, you probably consider yourself more advanced than a rookie grower. This likely means you are ready to take the next step in plant care and learn how to most efficiently grow your plants. A factor that most new growers, as well as some experienced growers, do not consider is daily light integral (DLI). A quick google search will yield many different results from college papers to Wikipedia articles and everything in between. But how are you going to be able to understand this complex scientific plant process with so much information out there? We are going to do our best to help you understand DLI and how this knowledge will make you a better grower.

Daily Light Integral (DLI)

Photo by Callie England

What is Daily Light Integral?

Daily light integral, also known as DLI, refers to the amount of photosynthetically active radiation (PAR) that your plant receives over a period of time (usually a 24-hour period). PAR may seem like a bunch of super complicated science talk, but it is actually quite simple. As we talk about in our “Choosing the Best Grow Light for your Indoor Gardenthere are invisible waves all around us, from microwaves to radio waves. Some of these waves are at a certain wavelength that they activate the chloroplasts in plants to begin the process of photosynthesis. These waves are usually within the visible light spectrum, meaning that the unaided human eye is capable of seeing it. Once a plant performs photosynthesis, it can then use the sugar it produces to grow, which is exactly what you want when cultivating plants or a garden. The more PAR your plants get the more they will grow. A good friend of ours, Leslie Halleck, is a well renowned certified professional horticulturist (ASHS) who has over 25 years of experience in hybridizing science with home gardening. In her book, Gardening Under Lights, she talks about DLI and how it affects your plants. We were able to learn a lot about DLI and more from her book, so we definitely recommend checking her out for more information on plants.

How does DLI affect my garden?

Plants, just like humans, need basic essentials to survive. From nutritious soil and water to sunlight, most growers already understand how to cultivate and care for their plants. The part that you may not think about is how much of each of these you need. You have to find the balance between not drowning your plants while also not letting them dry out. This is the same with light, if you have a grow light for your plants, you cannot leave it turned on 24 hours a day as you would burn the leaves and dry it out. On the other hand, you also cannot leave your plant in the dark all day as it would not grow. This is where DLI comes in handy. By knowing how much DLI your plant needs, you can more efficiently grow them.

Why is DLI helpful to my plants?

Plants need light to perform photosynthesis. Without dragging out too many memories of high school biology class, that is the process by when plants convert light, water and carbon dioxide into sugar and oxygen. This is how plants grow, they use the sugar they make to build more mass. By optimizing the number of light photons that hit your plant, you are optimizing the growth rate of your plant.

How can I calculate DLI?

Daily Light Integral (DLI)

Equation to determine DLI

There is the equation and as you probably expected it has a lot going on. However, it’s easy to break down. PPFD is the photosynthetic photon flux density which is fancy science talk for the amount of photosynthetically active photons that fall on a surface of a plant in a certain amount of time. Photoperiod is the amount of time a plant needs in sunlight to properly grow. The rest is just math conversions to get the rates for the equation. The most important part of this equation is photoperiod. Without knowing the photoperiod, you are just guessing at the optimal DLI for your plant. If you know that your carrots are long day plants or your soybeans are short day plants, you will be able to properly calculate your DLI for each plant and thus give your plants the best light. DLI is like a proportion. A high intensity of light (PPFD) with a short-time frame will have the same DLI as a low intensity light with a long-time frame. The secret is finding the perfect balance for your specific plant.

What is the difference between PPF and PPFD?

As you may have seen in the specs portion of our grow lights, we have the PPF of our lights listed. PPF is the total number of light photons released by a light source. PPF and PPFD are basically the same, the only difference is that PPFD refers to the number of light photons that hits a specific area (i.e.: a plant). The height of the light also factors into this. As seen in the photo below, light will go down from the source in a triangular shape and dissipate the further it gets away from the light source. This results in two things, that the center of the light will produce the highest PPFD and that the closer the light is the higher the PPFD will be. Daily Light Integral (DLI)

The PPFD of this light fixture will be higher for the dark green leaf than the light green leaf. This is because the dark green leaf is located centrally in this beam of light and closer to the fixture. An increased PPFD will result in a higher DLI for the dark green leaf over the same time frame than the light green leaf. Image created on

Is there a perfect DLI?

The short answer is no. Plants are very diverse and thrive in different circumstances. Nature is the best testament to this as there are very few ecosystems where plants do not exist. Even in the harshest environments like deserts, plants such as cacti thrive. So luckily for you, you do not need to produce perfect conditions for your plants to thrive in.

What factors affect DLI?

  • Latitude
  • Time of year
  • Length of day
  • Window tint
DLI varies widely so it is important to do your research to find out how these factors affect you and your plants. Outdoor DLI can range from 5 to 60 mol*m*d and Greenhouse DLI rarely goes above 25 mol*m*d. Daily Light Integral (DLI)

Plants growing in a greenhouse will rarely have a DLI over 25 due to the opaque windows. This is important to keep in mind if you grow in a greenhouse, you may need to add light fixtures if you need a higher DLI. Sourced via

All you need to do is get it in a good range. If your DLI is too low, you can have slow rooting, delayed flowering, weak branching or low flower number. Your plant will be weak as it is unable to produce lots of plant matter. If the DLI is too high it can result in leaf burn or drying out, which will be detrimental as your plant will lose its color while also losing the capability to produce more plant mass. The Aspect and Highland Plant lights come equipped with timers for you to plan out the proper DLI for your precious plants, to make sure they are getting the light the do (and don't) need.