Daily Light Integral

If you are new to growing plants indoors or trying your hand at taking the outdoor garden inside to gain more control and harvest more of your favorite foods…then you too are probably confused about how to use grow lights correctly?

This blog isn't going to teach you everything there is to know about lights, but it will give you some useful tools to get you passed the decision-making process and kickstart you into gardening mode.

To know how to properly light plants there are three main things to consider.

  1. Light Quality,
  2. Light Quantity,
  3. The Lighting Needs Of The Plant.

For starters, ‘PAR’ will be our basis for light quality. PAR stands for Photosynthetically Active Radiation. It's the usable wavelength of light plants can utilize to properly photosynthesize. Ranging from 400 to 700 nanometers, true full spectrum LED grow lights provide plants with light across that entire PAR spectrum.

Next, we need to measure light quantity or intensity. This is done with both measurements PPFD and DLI.

PPFD stands for Photosynthetic Photon Flux Density. It might sound fancy or complicated, but it's simple…and important. This is a measurement of how much light is hitting the plant leaves every second. Also not just any light, but only the light in the power spectrum. [Remember PAR?] A heat map makes it easy to visualize light quantity. The brighter parts of plants are getting more light than the rest.

Further than PPFD, ‘DLI’ takes the instantaneous measurement of PPFD and converts it into something that we can actually use. DLI stands for ‘Daily Light Integral’, and as the name suggests, it's a calculation of how much light a surface receives during an entire 24 hour period day. We now know how we're measuring light quality and quantity. Now it's time to cover the lighting needs of the plants themselves.

Many handy tables and charts are available online that shows you the lighting needs of many different plant varieties.

Let’s as an example say I need to know the lighting needs of fresh seedlings. Looking at a chart, new seedlings need a DLI of 6 to 10. Remember, DLI is how much light the plant needs or receives during an entire day. Now that we understand quality, quantity, and the needs of the plants, it's time to learn how to measure PPFD and DLI so we can set up the perfect lighting situation for our plants. To do that, you can use mobile apps or use an industry standard Quantum (PAR) Sensor / Meter.

Move your phone with app or proper quantum meter around the whole growing area and write down the readings in the center and each of the four corners. This will give you a good idea of the overall light intensity of the whole growing area. Alternatively if you don't want to use an app, another method exists. It's not as accurate, but it will get you in the ballpark. Most reputable grow light manufacturers put out accurate PPFD maps. You can take these PPFD readings and put them into a DLI calculator online along with how many hours the light will be on each day. To calculate a DLI output based on a PPFD map, take all the readings for a specific height and average them. For instance, the average PPFD for a small LED light at 18 inches is ±468. Using the DLI calculator online, you can enter 468 for the PPFD and a 12 hour photoperiod [for short day flowering].

So, if this light runs for 12 hours a day at 18 inches from the plant canopy, it should deliver a DLI close to 20. You can reduce the light cycle to 10 hours and drop the DLI to 17. Alternatively, you can increase the light cycle to 14 hours and raise the DLI to 24.

Use the light height and photoperiod to dial in the DLI for your plants. If your DLI is off, don't worry, it's easy to adjust. If you need to increase the DLI, you can lower the light, or leave it on longer, or a combination of the two. If you need to lower DLI, you can raise your light height, turn it off sooner, or a combination of the two.

Use a PAR Map while you adjust the height of the grow light so you know when you hit that perfect DLI output for your particular cultivar. So that's my system for simple plant lighting. It's not perfect and it doesn't cover every single lighting detail, but hopefully it makes you more confident with purchasing and setting up your grow lights. I can recommend starting off with a good full spectrum LED Grow light. They're easy to use, energy efficient, and they don't generate a lot of heat.

Getting hold of your new grow light is half of the task completed. Setting it up in an efficient way that will boost your production will seal the deal for an epic harvest return.

About the Author

Christoff Boltman is the Seeds for Africa / H2GRO Hydroponic specialist. Christoff has been growing hydroponically for the last 15 years and welcomes the opportunity to share his experience and knowledge with our customers. Feel free to pop in at our showroom and Christoff will be delighted to show you around and advise you on what would best suit your needs.