The most common two questions people ask me are:
“What is the best grow light for my garden?”
“How high should my grow light be above my plants?”
Understanding daily light integral (DLI) will help you answer those questions. By understanding some simplified concepts you’ll be able to buy and place grow lights in your garden with confidence. Before we get started, there are three terms you should be familiar with.
(PAR) Photosynthetically Active Radiation
Light with a wavelength roughly between 400 to 700 nanometers. This is the wavelength range that plants use for photosynthesis. When you hear people talking about “far reds” or “blues”, they’re referring to different PAR wavelengths. Quality full-spectrum LED lights provide lighting in the full PAR wavelength spectrum. We won’t be talking much about PAR in this article, because it doesn’t actually have anything to do with intensity or duration, but it does impact the quality of light your plants receive.
Think of PAR as the wavelength range of “plant usable” light.
(PPFD) Photosynthetic Photon Flux Density
PPFD is a measurement of how many photons of “plant usable” light are reaching a given surface area every second. The surface area we’re interested in is the surface of our plants and their leaves. Think of PPFD as light intensity hitting the plant right this second. This info is useful, but only covers a brief moment in time. DLI will help us turn this instantaneous measurement into something more useful.
PPFD measures how much light is hitting the plant in one second.
(DLI) Daily Light Integral
DLI is a measure of the total amount of photons that a specific surface area receives during a 24 hour period. In other words, DLI measures how much light a plant has received during a day. This measurement is what you use to make sure your plants are getting the right amount of light.
DLI measures how much light the plant receives all day.
What is DLI used for?
Inches, centimeters, meters, and so on are measurements. Carpenters use measuring devices to make sure dimensions are correct. Doors, windows, sinks, and other items have common measurements. This standardization enables the carpenter to build an opening the item will fit in, without having the item at the time of building.
Similarly, DLI is a measurement. It’s how we quantify light. Like the doors above, plant varieties have “standard” DLI requirements. For instance, buttercrunch lettuce has a DLI requirement of 14 to 17 for optimal growth. Knowing that ahead of time, you can build a hydroponic system and select a grow light that will yield a DLI of 14 to 17.
You don’t need the plant ahead of time. You already know its lighting needs.
So, how do you measure DLI? The next section covers a couple of common ways to measure DLI.
PPFD & DLI Measuring Devices
Like the carpenter needs a measuring tape, you’ll need a device to measure PPFD. Why only PPFD? Because PPFD can be used to calculate DLI. If that’s sounding too technical, don’t worry, I’ve built a calculator you can use to quickly convert PPFD to DLI. Keep reading and you’ll get to it!
In order to get extremely accurate measurements, you need an expensive meter like an MQ-500 from Apogee Instruments. If you’re like most hobbyist gardeners, you’re not in the market for a $550 light meter.
A lot of phone apps claim to be capable of measuring PPFD accurately, but most of them are terrible. Fortunately, I’ve found an extremely accurate app for both iPhone and Android. It consistently produces readings comparable to the MQ-500 mentioned above.
The app is called Photone. You can download it for free, but you’ll need to pay to unlock different light sources.
Light Measurements Made Easy
Photone is an easy to use, accurate light meter. With it you can quickly measure PPFD and DLI outputs from multiple types of grow lights.
The app measures PPFD, calculates DLI, and also has Lux and Kelvin temperature sensors. The app can be used in natural sunlight, or under artificial lighting like LED, HPS, CMH, MH, fluorescent, and compact fluorescent.
If you’re using the app on an iPhone, you’ll need to cover the front (selfie) camera with a strip of white copy paper (approximately 20lb weight paper). The strip of paper acts as a diffuser to help provide more accurate readings. If you’re using an Android device, no diffuser is needed.
Using the app is simple. Open the app, place the homemade paper diffuser over the front camera, if needed, select the light type, and hold the phone directly over the plant at the same level as the top leaves. Photone will automatically start showing PPFD levels as soon as it picks up light. You can swipe lefts and right to swap between modes (PPFD, DLI, LUX, etc). Take readings over all your plants to make sure everything is getting proper lighting.
At a fraction of the cost compared to expensive meters, Photone is well worth the few dollars you’ll pay to unlock its expanded features.
Now that you have the tools to measure DLI, it’s time to dial in that perfect lighting.
How To Achieve Perfect Plant Lighting
Getting the perfect lighting for your plants comes down to five steps. These five steps are a looping process, meaning that you can perform them over and over to find that perfect lighting scenario for your plants, in your environment.
Step 1: Research
Research the plant to find its optimal DLI. If you need help finding the DLI range for a specific plant, jump down to the plant DLI chart. The values on the chart will get you into the ballpark.
Step 2: Measure
Measure the light intensity at the plant surface (top of leaves).
Step 3: Adjust
Adjust the height of your light and/or how long the lights stay on to dial in a DLI that matches your plants’ needs.
Balance is key here. Unless you’re growing a light-loving plant, like peppers or tomatoes, shoot for around 12 to 14 hours of light and adjust the DLI by raising or lowering the light.
Running extremely intense lights for short amounts of time, or extremely weak lights nonstop, is harmful to plants. Mimic how plants interact with the sun.
Step 4: Document
Make notes about the adjustments you make, and the target DLI you’re aiming for. This information will help you continue to adjust your lighting until your plants are producing at their very best.
I use graph paper to document each grow cycle. The grow area is mapped out with the plant type and the initial DLI reading at the plant (based on a 14hr light cycle). I also document nutrient strength, pH. Any other important notes are also kept on this page, such as how much water the plants use per week, what date they matured on, when they bolted, etc.
Step 5: Observe
Observe the response of the plant. If you want to get super sciencey, you can even document the dry weight of the plant along with leaf count at harvest to compare with future DLI adjustments.
If you don’t want to get that technical, watch the plant. If the plant is getting leggy (long and lanky) like it’s reaching for more light, increase the intensity and DLI. If the plant looks like it’s burning, curling, or otherwise running away from the light, decrease the intensity and DLI.
Rinse & Repeat
This process may take several cycles to dial in, but you’ll end up with very specific data related to the plants in your environment. If you use the plant DLI chart below, I recommend starting somewhere in the lower half of the recommended DLI range and working up from there.
Plant DLI Chart
The recommendations in this chart are a compilation of my own growing research, along with the research of numerous university students. Within the DLI chart, you’ll find a recommended DLI range and a recommended lighting duration. The lighting duration recommendation is in hours. For instance, a “Light On” range of 12-14 means having the grow light on for 12 to 14 hours per day. The lighting duration is a recommendation that helps provide a balance of intensity over time to prevent “burning” plants by running super-intense light for short periods of time.
This calculator will help you determine your target PPFD range. I use the chart by first inputting a lighting duration that falls within the recommended range from the chart above, then taking a PPFD measurement in the grow area. By entering that PPFD value into the calculator I know if the PPFD is too high or low based on the DLI value returned by the calculator. If the DLI is too high, raise the light higher. If the DLI is too low, lower the light closer to the plants.
How to Plan for Perfect Plant Lighting
If you are in the planning stage, you can use steps one & two from above to make a smart buying decision.
First, decide on the plants you want to start growing. Butter lettuce is a common “first” hydroponic plant. Once you pick your plants, head over to this plant DLI table to find out the lighting needs of the plant.
Next, it’s time to research grow lights. Reputable manufactures publish PPFD maps for each grow light. These maps show the PPFD output of the light, along with the area the light will cover. The map will also show what size growing area the light is suitable for.
In the image below, you can see PPFD values for a ViparSpectra P1000. Along the left and bottom sides of each map, the grow area size is listed. For this light, the recommended grow area is 2-foot by 2-foot. Notice that PPFD output drops as the light gets closer to the edges of the growing area.
Use the PPFD maps along with this DLI calculator to make sure the grow light will deliver enough light to meet the daily requirements of the plants you intend to grow. It’s really that simple.
How to Calculate DLI
If you read the section above, you already know the Photone app can calculate DLI automatically which is really handy. If you’re using a different meter that outputs PPFD but not DLI, here’s how to manually calculate DLI.
To calculate DLI, you need to know two things: the PPFD reaching your plants’ leaves, and how long you want to run the grow lights every day.
PPFD = The reading on the meter
PHOTOPERIOD = How long the lights will be on each day
3600 = The number of seconds in an hour
1,000,000 = Converts micromoles (PPFD) to moles (DLI)
(PPFD) x (3600 x photoperiod)
Keep in mind that light and nutrient requirements are directly related. As the light intensity and/or duration increases, more nutrients/water will be consumed by the plant. The opposite is also true, if the light intensity/duration is reduced the rate of nutrient uptake will also be reduced.
Grow Light FAQs
Now that you know how to measure light intensity, you can answer most of these questions for yourself.
How Close Should Seedlings be to Grow Lights
For young seedlings, adjust the height of the grow light so that it provides a DLI of 6 to 10 with a 12-hour light cycle. As seedlings mature, adjust the grow light to provide a DLI of 10-15 over a 12-hour cycle.
Seedlings of light-loving plants like peppers and tomatoes will thrive in higher DLI settings, even as young seedlings. For instance, a young tomato seedling will grow aggressively with a DLI of 20-25.
What Color Lights do Plants Grow Best in
A light that provides wavelengths from the full photosynthetically active radiation (PAR) spectrum (400 to 700nm) will provide the most useful light for plants. Such lights are called “full-spectrum” lights.
How Many Watts Per Square Foot for LED Grow Lights
A more important measure is PPFD output. Don’t worry as much about wattage. Instead, take the PPFD output of the LED grow light and calculate that to DLI output. Match the DLI output of the grow light with the DLI requirements of your garden.
How Long to Keep Grow Lights on Plants
Different types of plants prefer different light cycles. I’ve color-coded plants based on preferred lighting duration, you can check that out in this plant pairing guide.
How Far Should Grow Lights be From Plants
Grow lights generate heat. A general gauge is to place your hand on top of your plant while the grow light is on. If your hand becomes uncomfortably hot within 1 minute, the grow light is likely burning the plant. If you can comfortably keep your hand in place, the grow light is far enough away.
Now that the grow light is in a “safe” position, measure the PPFD output at the plants’ leaf level. Use that PPFD measurement to calculate the DLI output of the light. Cross-reference the DLI output of the light with the DLI requirement of the plant. Make adjustments as necessary.
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