DLI (Daily Light Integral)
The total amount of photosynthetically active radiation (PAR) delivered to a plant over a full day, measured in moles of photons per square meter per day (mol/m²/day). The most useful single metric for matching light conditions to crop requirements.
Daily Light Integral (DLI) measures the total quantity of photosynthetically active light (PAR, 400-700nm wavelengths) reaching a plant surface over a 24-hour period. The unit is moles of photons per square meter per day (mol/m²/day).
DLI is more useful than instantaneous light measurements (lux, foot-candles, or PPFD) because it accounts for both intensity and duration. A grow light at 500 µmol/m²/s run for 8 hours delivers a different DLI than the same light run for 16 hours. An outdoor location that’s partially shaded gets a lower DLI than a full-sun location even if instantaneous readings during full-sun periods are similar.
How DLI is Calculated
DLI = PPFD (µmol/m²/s) × photoperiod (hours) × 3.6 / 1000
A clear summer day in the US Midwest delivers a peak outdoor DLI of 50-65 mol/m²/day. Most vegetable gardens receive 20-50 mol/m²/day depending on cloud cover, latitude, and season. Winter outdoor DLI at 40°N latitude can drop below 10 mol/m²/day on overcast days.
DLI Requirements by Crop
USDA and university horticultural research programs have established DLI requirements for vegetable crops:
| Crop | Minimum DLI | Target DLI |
|---|---|---|
| Lettuce and leafy greens | 12-14 | 14-17 |
| Herbs (basil, parsley, cilantro) | 10-14 | 14-20 |
| Seedlings (vegetable transplants) | 10-12 | 15-20 |
| Cucumber | 20 | 25-30 |
| Tomato (production) | 20-22 | 25-35 |
| Pepper | 18-20 | 25-30 |
| Strawberry | 12 | 15-20 |
| Microgreens | 8-12 | 12-16 |
These thresholds have practical implications. A tomato plant receiving DLI below 20 mol/m²/day will grow but produce substantially less fruit than the same plant at DLI 30. A seedling starting setup targeting DLI 15-20 produces compact, well-formed transplants; DLI under 10 produces stretched, weak seedlings.
DLI in Outdoor Siting
A site receiving 4 hours of direct sun per day delivers a much lower DLI than a full-sun site, even though both might read the same peak lux during the sunny period. A shaded spot that receives 200 µmol/m²/s for 5 hours delivers DLI = 200 × 5 × 3.6 / 1000 = 3.6 mol/m²/day - adequate only for shade-tolerant herbs and greens, not for fruiting crops.
Full outdoor sun in summer delivers enough DLI for any vegetable crop. The issue arises in sites with buildings, trees, or other obstructions that shorten the effective sunlight period. When evaluating a potential garden site, tracking when direct sun arrives and leaves - and totaling those hours across a clear day - gives a rough DLI estimate.
DLI and Artificial Light
Indoor grow lights must compensate for the absence of sunlight. A seedling starting room without windows needs enough artificial light to deliver 12-20 mol/m²/day (typically achieved with 14-16 hours of lighting at 200-400 µmol/m²/s). For fruiting crops indoors, delivering 25+ mol/m²/day requires either very high-intensity fixtures, very long photoperiods, or both - which is why serious indoor tomato production uses commercial-grade LED fixtures.
The impracticality of matching outdoor summer DLI indoors is one reason cherry tomatoes are more suitable for indoor production than large slicing tomatoes - they require lower DLI to produce acceptable yields.
Measuring DLI
Consumer-grade PAR meters (quantum sensors) read PPFD in µmol/m²/s and are available for $50-350. Some smartphone apps estimate PPFD using the phone camera sensor, but accuracy is limited (±30-50%) and varies significantly by device and conditions. For serious grow light evaluation, a calibrated quantum sensor is worthwhile.
For outdoor site assessment, the Sager Weathercaster or university extension DLI maps show typical seasonal DLI values by region.