DLI Targets by Growth Stage
Light dose targets by stage, from clones through harvest maturity.
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DLI collapses photoperiod and intensity into one number—total daily photons. It's useful for consistency, but it's not magic; it only works if your environment (VPD, CO2, nutrients, temperature) can actually support the dose. Most cannabis growers under-light clones and seedlings; fewer over-light flower without CO2. Measure actual canopy PPFD, don't guess from the fixture.
What DLI Actually Is
Daily Light Integral (DLI) is the total number of photosynthetically active photons (400–700 nm wavelength) that land on one square meter over 24 hours, measured in moles per square meter per day (mol/m²/day).
The calculation is straightforward:
DLI = PPFD × photoperiod (hours) × 0.0036
For example: 600 µmol/m²/s for 18 hours = 600 × 18 × 0.0036 ≈ 38.9 mol/m²/day.
Why this matters: plants respond to cumulative light dose over time, not just the intensity at any moment [1]. A seedling receiving 200 µmol/m²/s for 18 hours accumulates the same daily photon dose as one receiving 300 µmol/m²/s for 12 hours—both land around 13 mol/m²/day. Cannabis, like most C3 crops, shows photosynthetic response (and eventually saturation) based on total daily photons, mediated by CO2, temperature, water availability, and nutrient supply [1][2].
Why Growers Use DLI Targets
DLI is a practical planning tool because it solves three problems:
1. It combines intensity and duration into one number. Instead of debating whether 500 PPFD for 20 hours or 800 PPFD for 12 hours is better, you specify a dose: 36 mol/m²/day. Both can achieve it.
2. It prevents the two most common lighting mistakes:
- Under-lighting clones and seedlings into stretchy, weak plants with sparse nodes
- Over-lighting flowering plants in ambient CO2 (below 600 ppm), causing photoinhibition, bleaching, and terpene degradation
3. It scales across equipment. A DLI target of 40 mol/m²/day means the same thing under LED, HPS, fluorescent, or greenhouse sunlight. You can swap fixtures without recalculating.
A peer-reviewed trial by Rodriguez-Morrison et al. (2021) on cannabis found that flower yield increased roughly linearly with light intensity up to 1800 µmol/m²/s under elevated CO2 (1200 ppm), but cannabinoid concentration (% THC) remained static [1]. This confirms that higher DLI grows more flower, not stronger flower—a distinction often blurred in grower folklore Strong evidence.
DLI Targets by Stage
These ranges are informed by published cannabis light trials and commercial greenhouse practice. They are starting points; adjust based on your cultivar, environment, and yield data.
| Stage | PPFD (µmol/m²/s) | Photoperiod | DLI Target (mol/m²/day) |
|---|---|---|---|
| Clones / seedlings (week 1–2) | 100–300 | 18–24 h | 6–22 |
| Early vegetative (week 3–4) | 300–600 | 18 h | 20–40 |
| Late vegetative (week 5–8) | 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 (week 7–9) | 700–900 | 12 h | 30–39 |
Without CO2 enrichment (ambient ~400 ppm), cannabis photosynthesis plateaus around 900–1000 PPFD; anything higher is metabolically unutilized and wastes electricity Strong evidence. Canopy temperatures, VPD, and nutrient availability become limiting before photosynthetic capacity.
With CO2 enrichment (1000–1200 ppm), flowering plants can use 1200–1500 PPFD productively, pushing DLI to 50–65+ mol/m²/day before saturation occurs [1]. At these levels, light is no longer limiting; the bottleneck shifts to water transpiration (VPD), nutrient uptake, and heat dissipation Strong evidence.
How to Hit Your DLI Target (Step-by-Step)
Step 1: Get an accurate PAR meter. Buy a quantum PAR meter—Apogee MQ-500 is the industry reference. Cheaper alternatives (Photone, UNI-T, Hyperlux) exist but often vary ±10–20% on white LEDs Weak / limited. Phone apps using the camera sensor are usable for relative comparisons ("is that brighter than yesterday?") but unreliable for absolute PPFD values.
Step 2: Map your canopy. At actual canopy height, take PPFD readings at nine points: four corners, four edges, one center. Average them. Note the spread—if center reads 900 µmol/m²/s and corners read 400, you have a coverage or fixture-mounting problem, not a DLI problem. Uneven coverage wastes energy on over-lit center leaves while under-lighting edges.
Step 3: Calculate your current DLI. Use the average PPFD: DLI = avg PPFD × photoperiod hours × 0.0036. Record it.
Step 4: Compare to target for your stage. If you're in peak flower with a target of 35–43 mol/m²/day and measuring 25, you are under-lit. If you're measuring 55 without CO2 enrichment, you are over-lit and likely seeing bleaching or yellowing of upper leaves.
Step 5: Adjust one variable at a time. To raise DLI:
- Lower the fixture closer to the canopy (increases PPFD, but check for heat stress and light-leaf contact)
- Increase lamp power or add a second fixture (increases PPFD)
- Extend photoperiod in veg only (do not change flower photoperiod; maintain 12h/12h)
To lower DLI:
- Raise the fixture
- Reduce lamp power or use fewer fixtures
- Shorten photoperiod in veg only
Re-measure 24–48 hours after each change to allow leaves to acclimate.
Step 6: Verify the rest of your environment can support the dose. Higher DLI demands higher water availability (higher VPD), more CO2, stricter nutrient precision, and better heat dissipation. Raising lights from 40 to 60 mol/m²/day without raising humidity, CO2, or feeding will cause calcium/magnesium deficiencies and stress Strong evidence. This is why DLI works best as part of an integrated environment plan.
Step 7: Log and iterate. Record DLI, photoperiod, avg PPFD, and canopy temperature weekly. Track final dry yield and visual quality (bud density, color, terpene aroma notes). Over one or two crops, you'll calibrate your specific environment.
Common Mistakes
Reading PPFD at the fixture instead of canopy. Light intensity falls off with the square of distance. A reading 12 inches from the lamp can be 1000 PPFD, but 24 inches away (at the actual canopy) it may be 250 PPFD. Always measure at leaf level.
Confusing watts with DLI. A 600 W LED and a 600 W HPS deliver different PPF (photons per second). A 600 W high-efficiency LED might emit 2.7 µmol/s; a 600 W HPS might emit 1.8 µmol/s. Check the spec sheet for µmol/s (PPF), not watts.
Over-DLI in flower without CO2. Pushing flower DLI above 40–45 mol/m²/day in ambient CO2 causes photoinhibition: bleached leaf tips, fox-tailed bud structure, and terpene loss with no yield gain [1]. You'll see it first on the tallest buds closest to the light.
Starting clones at full-strength light. Clones and seedlings are shade plants. Rooting cuttings under 600+ PPFD often abort, bleach, or stall. Start at 150–250 PPFD (roughly 6–15 mol/m²/day at 18h) and increase as leaves harden Strong evidence.
Believing more light increases THC percentage. Controlled trials consistently show that yield (dry weight) scales with light, but cannabinoid percentage does not [1]. You end up with more grams of similar-potency flower, not stronger flower. This is grower folklore, not reality Strong evidence.
Ignoring DLI consistency. A plant that receives 40 mol/m²/day on Monday and 20 mol/m²/day on Tuesday performs worse than one that gets 30 mol/m²/day both days. Stable light dose outperforms variable extremes. Use timers and consistent fixture height to maintain DLI within ±5 mol/m²/day day-to-day.
Related Techniques
DLI is most effective when paired with other quantified targets:
- VPD by Growth Stage — Light dose only pays off if vapor pressure deficit is in range; otherwise, stomata close and photosynthesis stalls, wasting the photons you paid for.
- PPFD Targets by Stage — The instantaneous light intensity that underlies DLI calculation; understanding PPFD helps you troubleshoot uneven canopy coverage.
- Light Height by Stage — Practical fixture mounting to reach your PPFD target without heat stress.
- EC Targets by Stage — Nutrient concentration scaled to growth rate; higher DLI increases plant demand for nitrogen, potassium, and calcium.
- Vegetative Stage Feeding — Feeding strategy to support faster growth under higher DLI.
CO2 supplementation is the only way to productively use DLI above ~50 mol/m²/day; beyond that, CO2 becomes the limiting factor, not light.
Sources
- Peer-reviewed Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment. Frontiers in Plant Science, 12, 646020.
- Peer-reviewed Chandra, S., Lata, H., Khan, I. A., & ElSohly, M. A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants, 14(4), 299–306.
- Peer-reviewed Faust, J. E., & Logan, J. (2018). Daily Light Integral: A Research Review and High-resolution Maps of the United States. HortScience, 53(9), 1250–1257.
- Peer-reviewed Eaves, J., Eaves, S., Morphy, C., & Murray, C. (2020). The relationship between light intensity, cannabis yields, and profitability. Agronomy Journal, 112(2), 1466–1470.
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Related
- EC Targets by Stage — Staged electrical conductivity targets for nutrient feeding throughout cannabis growth.
- Light Height by Stage — Vertical light distance control for stage-appropriate intensity.
- PPFD Targets by Stage — Optimize light intensity at each stage to maximize photosynthesis and yield.
- Vegetative Stage Feeding — Nutrient strategies during cannabis growth phase before flowering initiation.
- VPD by Growth Stage — Adjust humidity and temperature to match plant transpiration needs at each stage.