DLI Targets by Growth Stage

What DLI Actually Is

Daily Light Integral (DLI) is the total number of photosynthetically active photons (400–700 nm) that land on one square meter over 24 hours. It's measured in moles per square meter per day (mol/m²/day).

The math is simple: DLI = PPFD × photoperiod (hours) × 0.0036. So 600 µmol/m²/s for 18 hours = 600 × 18 × 0.0036 ≈ 38.9 mol/m²/day.

DLI matters because plants integrate light over time. A clone under 200 PPFD for 18 hours gets the same dose as a clone under 300 PPFD for 12 hours. Cannabis, like most C3 crops, responds to total daily photon dose up to a saturation point that depends on CO2, temperature, water, and nutrient availability ^[1][2].

Why Growers Use DLI Targets

Three reasons:

1. It collapses two variables into one. Instead of arguing about PPFD vs. hours, you target a dose.
2. It prevents the two most common lighting mistakes: under-lighting clones into stretchy, weak plants, and over-lighting flowering plants into bleached, terpene-degraded tops.
3. It scales across fixtures and rooms. A DLI of 40 mol/m²/day means the same thing under HPS, LED, or sunlight.

Greenhouse vegetable research has used DLI as a planning tool for decades ^[3]. Cannabis-specific work is newer but converging on similar logic: higher DLI = higher yield, with diminishing returns and bleaching risk above roughly 65 mol/m²/day in ambient CO2 ^[1][4]. Strong evidence

DLI Targets by Stage

These ranges reflect published cannabis trials plus widely used commercial practice. Treat them as starting points, not gospel.

| Stage | PPFD (µmol/m²/s) | Photoperiod | DLI (mol/m²/day) | |---|---|---|---| | Clones / seedlings (week 1–2) | 100–300 | 18–24 h | 6–22 | | Early veg | 300–600 | 18 h | 20–40 | | Late veg | 400–700 | 18 h | 26–45 | | Early flower (week 1–3) | 600–900 | 12 h | 26–39 | | Peak flower (week 4–6) | 800–1000 | 12 h | 35–43 | | Late flower / ripening | 700–900 | 12 h | 30–39 |

With CO2 enrichment (1000–1200 ppm), flowering plants can use 1200–1500 PPFD, pushing DLI to 50–65+ mol/m²/day before saturation ^[1][4]. Without CO2, more than ~900 PPFD is usually wasted electricity. Strong evidence

A peer-reviewed greenhouse trial by Rodriguez-Morrison et al. (2021) found cannabis flower yield increased roughly linearly with light intensity up to 1800 µmol/m²/s under elevated CO2, but cannabinoid concentrations (% THC) did not increase with light — yield per plant did ^[1]. So more light grows more flower, not stronger flower. Strong evidence

How to Hit Your DLI Target (Step-by-Step)

1. Measure, don't guess. Buy a quantum PAR meter (Apogee MQ-500 is the reference; cheaper options from Photone, UNI-T, or Hyperlux exist but vary in accuracy). Phone apps using the camera are usable for relative comparisons but can be off by 20%+ on white LEDs.

2. Map your canopy. Take PPFD readings at canopy height in a 9-point grid (corners, edges, center). Average them. Note the spread — if your center reads 900 and corners read 400, you have a coverage problem, not a DLI problem.

3. Calculate current DLI. Average PPFD × hours × 0.0036.

4. Compare to target for stage. If you're at 25 mol/m²/day in peak flower, you're under-lit. If you're at 55 without CO2, you're over-lit and probably bleaching.

5. Adjust one variable at a time. Raise or lower the fixture, dim the driver, or change photoperiod (veg only — don't mess with flower photoperiod). Re-measure.

6. Verify environment supports the dose. Higher DLI demands higher VPD tolerance, more water, more CO2, and tighter nutrient management. Cranking lights without raising the rest of the stack causes deficiencies and stress. Strong evidence

7. Log it. Record DLI weekly alongside yield and quality so you build your own data.

Common Mistakes

• Reading PPFD at the fixture, not the canopy. Light falls off with the square of distance. A 1000 PPFD reading 12 inches from the lamp can be 500 at the actual canopy.
• Assuming wattage equals DLI. A 600W LED and a 600W HPS deliver different photon outputs. Look at PPF (µmol/s) on the spec sheet, not watts.
• Pushing flower DLI above 40 mol/m²/day without CO2. You'll see bleaching, fox-tailing, and terpene loss without yield gain ^[1].
• Forgetting clones are shade plants. Clones rooting under 600 PPFD often stall or burn. Start at 150–250 PPFD. Strong evidence
• Believing the 'more light = more THC' marketing. Controlled trials show yield scales with light; cannabinoid percentage does not ^[1]. You get more grams of similar-potency flower, not stronger flower. Strong evidence
• Ignoring DLI consistency. A plant that gets 40 mol one day and 20 the next performs worse than one that gets 30 every day. Stable beats peak.

Related Techniques

DLI works best as part of a calibrated environment. Pair it with:

• VPD Targets by Growth Stage — light dose only pays off if transpiration can keep up.
• CO2 Supplementation — the only way to use DLI above ~45 mol/m²/day productively.
• PPFD and PAR Basics — the instantaneous measurement underneath DLI.
• Light Burn vs. Bleaching — how to recognize over-DLI in real time.
• Defoliation and Canopy Management — getting your measured DLI to the right leaves.

Sources

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