Various academic and online sources suggest wide-ranging PPFD values between 50 and 400 µmol/m²s for microgreens and in our own research we have seen healthy crop growth with PPFD values as low as 25 to 85 µmol/m²s on a 14-hour photoperiod.

In our early research, where we typically had a 14-hour photoperiod and an average PPFD of 50 µmol/m²s, we could calculate our DLI as:

50 µmol/m²s x 14 hours/day x 3600 s/1,000,000 µmol = 2.5 mol/m²d.

This is lower than values published here (6 to 12 mol/m²d) for microgreens and here for vegetable seedings (6 to 10 mol/m²d). A 2021 study by Gao et al. stated the following with a 12-hour light cycle:

The contents of soluble protein, soluble sugar, free amino acid, flavonoid, vitamin C…in broccoli microgreens were higher under 70 µmol/m²s. Overall, 50 µmol/m²s was the optimal light intensity for enhancement of growth of broccoli microgreens, while 70 µmol/m²s was more feasible for improving the phytochemicals of broccoli microgreens in an artificial light plant factory.

The DLI in the case of the Gao et al. (2021) study, using 70 µmol/m²s as a value, would be:

70 µmol/m²s x 12 hours/day x .0036 = 3.02 mol/m²/d

This value is 16% higher than our research values of 2.5 mol/m²d, but significantly lower than other published values shown above. An important consideration here could be the difference in light distance from the crop.

A higher-intensity light that is further away from the crop could see less PPFD increase through the growing cycle (with PPFD increasing as the crop grows closer to the lights) relative to a crop that is closer to a lower-intensity source. In our light mapping research exercises, PPFD could increase by over 25% in some spots as the crop grew from 4 cm to 10 cm in height.

6.4. Sample Calculation 1 with Adjustments

In our early research, as we showed above, we used a 14-hour photoperiod and an average PPFD of 50 µmol/m²s. We could thus calculate our DLI as:

50 µmol/m²s x 14 hr/day x 3600 s/hr x 1 mol/1,000,000 µmol  = 2.5 mol/m²d

If we want to increase our DLI value to 5 mol/m²d with our existing set up, we have multiple ways to do that:

1. Increase our photoperiod (currently 14 hours)
   1. We would need to double our photoperiod in order to double our DLI, so obviously this action alone would not be sufficient
2. Move lights closer to the crop (decrease shelf spacing)
   1. We would want to measure the new projected values with the PAR meter before moving the shelf to ensure the change will meet our needs
3. Improve reflectivity to reduce light loss
   1. This could direct 15 to 20% or more light to the crop
4. Replace the lights
   1. If you cannot get to the desired PPFD values with your current light setup, then you would need to replace your lights with ones which have a higher light output.

From the above notes, it appears we might be best off with a combination of things to avoid replacing the lights:

1. Increase photoperiod from 14 hours to 16 or 18 hours
2. Raise our shelf by 5 to 10 cm to be closer to the lights
3. Add white reflectors to the back and side of the bench to better prevent light loss

In this case, we would make the changes and then take another set of measurements. If you have met your goal – great! But if you do all these things and still can’t get the desired DLI, what do you do next? In cases like this, you typically need to replace your fixtures. It might be that the lights you bought were not intense enough to begin with, or they may have lost intensity over time and thus need to be replaced.

6.5. Sample Calculation 2 with Adjustments

In this example we will use one of our research shelves and LED light setups to do some calculations. Here are the details of this shelf:

• Vertical Shelf Spacing: 30 cm
• Lights: 2 x 31 W lights with built-in reflectors
• Light distance from top of crop at uncovering time: 19 cm
• Light distance from top of crop at maturity: 15 cm
• ~PPFD at uncovering height: 172 µmol/m²s
• ~PPFD at maturity: 197 µmol/m²s
• Photoperiod: 14 hours
• DLI: 180 µmol/m²s x 14hr x .0036 = 9 mol/m²d

In this case we have a much higher DLI than our first example, but is it too much? Crop health and crop growth can help determine that. In our trials, crops did very well with this lighting set up. And at 62 Watts per shelf, this lighting set up was also very efficient.

Determining PPFD, Photoperiod, and DLI Values to Use

A great place to start with lighting selection is determining what PPFD and DLI values to use. From our trials and secondary research we recommend a PPFD between 125 and 175 μmol/m²s and a DLI between 6 and 10 mol/m²d. We find we have the best growth results in this range. You will also notice there are a number of lights on the market that will provide these values, or close to.

You can use these PPFD and DLI values to determine the necessary photoperiod using the chart below. Aim for a photoperiod between 12 and 16 hours. In doing so, you can see the area in bold below has the values which meet PPFD, DLI, and photoperiod recommendations. Full chart here.

Now you need to find the lights that best get you into that range. You can certainly use higher or lower PPFD and DLI values if your experience shows they are producing suitable results, but to do so properly you need to ensure you are taking into consideration multiple quality factors. Some key things to look for in your crops under different lighting conditions include yield, color, flavor, eating texture, stem length, stem-to-leaf ratio, days to maturity, and storability. Keep in mind that temperature, seeding rate, and light spectra will also affect these crop quality factors.

Too Much Light

While light is essential for plant growth, more is not always better. Both excessive light intensity (high PPFD) and overly long photoperiods can lead to stress, wasted energy, and poor crop quality. Here’s how to tell when your lighting might be overdoing it:

Signs of Excessive PPFD (Light Intensity)

• Leaf yellowing or bleaching: This may look like sunburn and often starts at the top of the canopy.
• Stunted growth or overly short stems (hypocotyls): Extremely compact plants can indicate stress from too much light.
• Leaf curling or edge burn: Especially in tender crops like basil or arugula.
• Uneven crop development: Crops directly under the light may appear shorter, paler, or more stressed, while edge crops (getting slightly less light) may look healthier.
• Increased water use: High light boosts transpiration, which can dry out trays faster and increase watering needs.

Signs of Excessive Photoperiod (Light Duration)

• Reduced pigment levels: Some crops, like radish or arugula, may lose color (e.g., less anthocyanin or chlorophyll) with very long days.
• Nutrient imbalances: Longer light exposure increases metabolic demand, potentially leading to deficiencies.
• Wasted energy: Past a certain point, more light hours offer diminishing returns—especially if PPFD is already high.
• Disrupted circadian rhythms: Just like people, plants benefit from dark periods to reset internal processes.

What’s the Limit?

• For most microgreens, a photoperiod of 12–16 hours and PPFD between 125–175 µmol/m²/s gives the best results.
• Pushing beyond 18–20 hours per day or >200 µmol/m²/s may be useful in some cases, but should be tested carefully and monitored closely.

If you’re seeing signs of over exposure, try:

• Raise your lights slightly to reduce PPFD.
• Shorten the photoperiod by 1–2 hours and observe crop response.
• Add reflectors or diffusers to spread light more evenly across the tray.
• Measure PPFD at crop height to verify intensity isn’t exceeding optimal range.

As with nutrients and water, the key to lighting is balance: enough to drive growth, but not so much that you stress your crop or waste energy.