Grow Light Duration Companion

DLI (Daily Light Integral) measures the total amount of photosynthetically active radiation (PAR) your plants receive in a single day, expressed in mol/m²/day. It combines both light intensity (PPFD) and duration into one meaningful number. Think of DLI as your plant's daily "light calorie count" — it determines how much energy your plant has for photosynthesis, growth, and flowering. A proper DLI ensures compact internodes, robust stems, and maximum yields.

You have several options:

• PAR Meter (Quantum Sensor): Most accurate. Devices like the Apogee MQ-500 give precise PPFD readings at canopy level. Expect to pay $150–500.
• Smartphone Apps: Apps like Photone or PPFD Meter can estimate PPFD using your phone's camera sensor. Accuracy varies but is often sufficient for hobbyists.
• Manufacturer PPFD Maps: Most quality LED brands provide PPFD distribution charts at various hanging heights. Use these as a starting point.
• Lux-to-PPFD Conversion: If you have a lux meter, you can roughly convert: for white LEDs, divide lux by 60–70 to estimate PPFD.

Recommended DLI ranges for photoperiod cannabis:

• Seedling / Clone: 12–20 mol/m²/day — gentle light, 18–24 hours. PPFD around 200–300 μmol/m²/s.
• Vegetative: 22–38 mol/m²/day — 18 hours of light. PPFD around 400–600 μmol/m²/s. Higher end with CO₂.
• Early Flowering: 28–38 mol/m²/day — 12 hours of light. PPFD around 650–900 μmol/m²/s.
• Late Flowering / Ripening: 30–45 mol/m²/day — 12 hours. PPFD 700–1000+ μmol/m²/s. Some growers push higher with CO₂.

Note: Autoflowering varieties can handle 18–20 hours of light throughout their lifecycle, resulting in higher total DLI.

Many plants use night length (not day length) to determine when to flower. This evolutionary mechanism helps plants bloom at the right season:

• Short-Day Plants (cannabis, poinsettia, chrysanthemum): Flower when nights exceed ~12 hours. They require uninterrupted darkness — even a brief light leak during the dark period can prevent or disrupt flowering.
• Long-Day Plants (spinach, lettuce, wheat): Flower when nights are shorter than ~10–12 hours, typically in late spring/summer.
• Day-Neutral Plants (tomatoes, cucumbers, many vegetables): Flower based on maturity, not photoperiod.

For indoor cannabis growers, switching from 18/6 to 12/12 triggers the flowering response in photoperiod varieties.

Yes, light stress is real. Signs of excessive DLI include:

• Leaf bleaching or yellowing (especially upper leaves)
• Leaf edges curling upward ("tacoing" or "canoeing")
• Bleached or white bud tips (light burn)
• Stunted growth despite adequate nutrients and water
• Excessive internode stretching (plants trying to escape the light)

Without CO₂ supplementation, most plants cannot efficiently use DLI above 45 mol/m²/day. The extra light energy becomes heat stress rather than productive photosynthesis. If you're pushing high DLI, ensure proper ventilation, temperature control, and consider CO₂ enrichment.

Use the "Find Duration" mode in this calculator:

1. Measure or look up your light's PPFD at canopy level.
2. Determine your plant's target DLI from the reference table above.
3. Switch to "Find Duration" mode and enter both values.
4. The calculator will tell you exactly how many hours of light you need per day.

This is especially useful when you have a fixed light fixture and want to optimize your timer settings for each growth stage.

• PAR (Photosynthetically Active Radiation): The spectrum of light (400–700nm) that plants use for photosynthesis. It's a range, not a measurement.
• PPFD (Photosynthetic Photon Flux Density): The intensity of PAR light hitting a surface per second, measured in μmol/m²/s. Think of it as "how much usable light is arriving right now."
• DLI (Daily Light Integral): The total accumulation of PAR light over a full day, measured in mol/m²/day. DLI = PPFD × hours × 0.0036.

Analogy: PPFD is like your water faucet's flow rate (liters per second). DLI is how much water fills the bucket by the end of the day (total liters).

Using 24/0 lighting (continuous light) is a debated topic:

• Pros: Maximizes DLI, promotes rapid seedling growth, stable temperature/humidity, no timer needed.
• Cons: Higher electricity costs, some plants benefit from a dark period for metabolic processes (respiration), potential for light stress if PPFD is too high.

Many growers prefer 18/6 or 20/4 for autoflowers and seedlings, providing a short dark period while still delivering high DLI. If using 24/0, reduce PPFD slightly to keep DLI in the optimal range and watch for signs of stress.