DLI Targets for Indoor Leafy Greens, Herbs and Microgreens Under LED

The Daily Light Integral (DLI) is the total photosynthetically active photons a crop receives in 24 hours, expressed in mol/m²/day. For indoor leafy greens, herbs and microgreens it is the single most useful planning number because it rolls photoperiod and intensity into one comparable value. Most leafy greens perform well in a DLI band of roughly 12-17 mol/m²/day, herbs a little higher, and microgreens lower because of their short production cycle. This article shows how to choose a DLI, convert it to the PPFD your LEDs must deliver, and where the published extension guidance comes from.

What DLI actually measures

DLI is the area under the PPFD curve over a full day. If PPFD is constant, the conversion is straightforward. The relationship is one of the three formulas we use across this site, and it is pure unit conversion — no assumptions about any crop or product.

Crop-stage differences

DLI needs change through production. Seedlings and transplants need lower DLI to avoid stretching before they are spaced; mature leafy greens use the higher end of the band; herbs such as basil tolerate and often benefit from more light than lettuce. Microgreens, harvested in 7-21 days, accumulate DLI over a short window, so a higher daily PPFD is typical to hit a useful total in a few days. These are planning ranges consistent with controlled-environment guidance such as MSU Extension and e-GRO publications on greenhouse and indoor lighting.

• Lettuce and leafy greens: commonly 12-17 mol/m²/day across the crop.
• Herbs (basil, mint): often toward the higher end, sometimes above 17.
• Microgreens: short cycle, so a higher daily PPFD to reach a useful total fast.
• Seedlings/transplants: lower DLI early to control stretch before spacing.

Turning DLI into a fixture plan

Once you pick a DLI and photoperiod, the table above gives a target PPFD. From there the fixture-count estimate ties it to your canopy area. Keep the three numbers — DLI, photoperiod, PPFD — consistent so the plan is internally coherent. Our indoor growing industry page carries a full worked layout example.

Worked example: converting a DLI target into a daily schedule

This example is illustrative. Assume a basil bench with a planning DLI of 18 mol/m2/day and two candidate photoperiods: 16 hours and 20 hours. Rearranging DLI = PPFD x h x 0.0036 gives PPFD = DLI / (h x 0.0036).

• At 16 hours: PPFD = 18 / (16 x 0.0036) = 312 umol/m2/s.
• At 20 hours: PPFD = 18 / (20 x 0.0036) = 250 umol/m2/s.
• The 20 hour plan needs roughly 20 percent lower intensity for the same DLI, which usually means fewer fixtures or more dimming headroom.
• The 16 hour plan concentrates the same light into fewer hours, which raises peak fixture demand but leaves a longer dark period for respiration and for scheduling other operations.
• Fixture count check: for a 40 m2 bench at 250 umol/m2/s with a uniformity factor of 1.15 and 1,600 PPF fixtures, count = (250 x 40 x 1.15) / 1,600 = 7.2, rounded to 8 fixtures.

Both schedules deliver the same daily light integral, yet they differ in fixture count, electrical demand profile and crop response. That is why DLI planning and photoperiod choice are one decision, not two.

Crop-by-crop planning bands and where they come from

The DLI bands used in extension literature are starting points drawn from research and commercial practice, not guarantees for a specific cultivar. Typical indoor planning bands look like this: lettuce and other baby leaf greens around 12-17 mol/m2/day, basil and most culinary herbs around 15-25, microgreens much lower at 6-12 because the cycle is short and density high. Within a band, the low end protects quality (for example, less bitterness in lettuce) and the high end pushes growth speed at higher energy cost.

Treat the band edges as information, not as commands. The most reliable path is to run one bench at the middle of the band, record germination-to-harvest time, quality observations and energy use, then adjust in steps of about 10 percent. Because light interacts with temperature, CO2 and spacing, change one input at a time and keep written records so results are attributable.

Questions to ask before you buy

• Can the supplier state PPF and PPE at the drive current your DLI plan requires, not only at maximum output?
• Is the spectrum published as relative intensity by wavelength so you can compare across suppliers?
• What dimming range is supported, and does PPFD stay linear with the control signal across that range?
• Do fixture dimensions and mounting hardware fit your bench spacing and ceiling height?
• What is the warranty, and is it valid at the ambient temperature and humidity of your grow room?
• Can the supplier provide photometric files for layout simulation before you commit to a quantity?

Common mistakes in DLI planning

These are the errors most often found when a room underperforms its plan. Each one is cheap to avoid and expensive to discover late:

• Planning from the datasheet PPFD instead of the mapped canopy PPFD, so the real DLI is 10-20 percent below the plan in the dark zones.
• Fixing the photoperiod first and the intensity second, which locks the fixture count before the operations constraints (labour windows, tariffs, HVAC) are known.
• Applying one DLI number to a mixed crop room; basil and lettuce bands overlap, but the upper basil band will burn quality out of sensitive lettuce cultivars.
• Ignoring the shoulder effect: the first and last hour of the photoperiod often run dimmed or disrupted, so the delivered DLI is below the nominal calculation.
• Treating a DLI band as a target to exceed; beyond the upper edge, extra light is mostly extra cost and quality risk.
• Changing two variables at once (light and feed, light and spacing), which makes the result unattributable and the lesson useless.
• Never re-measuring after changes: fixtures move, trays move, and the DLI the room actually delivers drifts away from the DLI the plan describes.

Measuring the DLI you actually deliver

A practical sensor plan

One logging quantum sensor per representative zone, mounted at canopy height on a fixed stand, records the real PPFD every few minutes. Multiplied by the photoperiod, the daily sum is your delivered DLI, and a weekly review catches drift early. Keep the sensor clear of shading from trellises and workers, and re-check its calibration annually; a drifting sensor quietly rewrites every number built on it.

Interpreting the gap between plan and delivery

When measured DLI runs below plan, the cause is usually uniformity (edges and corners), dimming that never returned to setpoint after a manual override, or a photoperiod shortened by control faults. When it runs above plan, look for fixtures left at full output after a trial or a stacking error in the layout. Both directions cost money, and both are invisible without a logged sensor.

The sheet format matters more than the example numbers: one row per zone forces the plan to answer for every zone, and the required-PPFD column is what the fixture layout has to deliver at canopy level, verified by measurement rather than assumed from the quote.

References you can verify

• MSU Extension / e-GRO — greenhouse and controlled-environment lighting guidance
• MSU College of Agriculture & Natural Resources
• University of Florida IFAS — horticulture extension
• Royal Horticultural Society — light and plants

Frequently asked questions