In horticulture, understanding PPFD (Photosynthetic Photon Flux Density) and DLI (Daily Light Integral) is critical for optimizing plant growth. These metrics help growers determine the amount of light plants need at different growth stages, whether for vegetables, flowers, seedlings, or specialized crops like cannabis. In this article, we’ll explore these concepts in detail and provide insights into calculating light requirements and show how to calculate it.
Plants and Light Absorption
Plants primarily absorb light within the 400-700 nm range of the visible spectrum—commonly referred to as the Photosynthetically Active Radiation (PAR) range. This light is critical for photosynthesis, during which plants convert carbon dioxide (CO₂) and water (H₂O) into glucose and oxygen. The absorption efficiency at different wavelengths varies depending on the plant’s cellular structure, making light requirements species-specific.
What Is PPFD and Why Is It Important?
PPFD measures the number of light photons (µmol) hitting a square meter per second within the 400–700 nm range (Photosynthetically Active Radiation, or PAR). It is an instantaneous measure of light intensity and directly impacts photosynthesis. However, PPFD alone is not sufficient for evaluating plant growth needs—it must be combined with the duration of light exposure to calculate the DLI.
PPFD and Plant Growth Phases:
- PPFD for Vegetables:
Vegetables like tomatoes, peppers, and cucumbers typically require 120–350 µmol/s·m², depending on the growth stage. - PPFD for Flowering Plants:
Flowering crops, such as roses or cannabis during the flowering phase, often need higher PPFD levels, ranging from 650–1,600 µmol/s·m². - Best PPFD for Flowering:
For flowering plants like cannabis, a PPFD of around 1,600 µmol/s·m² is considered optimal under controlled conditions.
What Is DLI and Why Does It Matter?
DLI (Daily Light Integral) measures the total amount of light (photons) delivered to plants over a 24-hour period. It is expressed in moles of photons per square meter per day (mol/m²/d). Unlike PPFD, which is a snapshot of light intensity, DLI considers both intensity and duration, offering a cumulative measure of light exposure. It’s an essential metric for determining the daily light needs of plants. Most plants require DLI values between 6-18 mol/m²/d, depending on their species and growth stage.
DLI Requirements for Different Crops:
- DLI for Seedlings:
Seedlings typically need a low DLI of 6–12 mol/m²/d, depending on the species. - DLI for Vegetative Growth:
Vegetative crops like leafy greens or cannabis in the vegetative phase require 12–17 mol/m²/d. - DLI for Flowering:
Flowering crops often require 20–40 mol/m²/d for optimal growth and yield.
How to Calculate DLI:
The formula for calculating DLI is:
DLI = PPFD × light hours per day × (3600 ÷ 1,000,000)
For example:
If the light intensity (PPFD) is 400 µmol/s·m² and the photoperiod is 16 hours/day:
DLI = 400 × 16 × (3600 ÷ 1,000,000) = 23.04 mol/m²/d.
Calculating DLI and PPFD
Example: Converting DLI to PPFD
To calculate PPFD from a given DLI value, consider the following example:
- Target DLI: 12 mol/m²/d
- Light photoperiod: 16 hours/day
Calculation:
-
Convert DLI to moles per second:
12 mol/m²/d ÷ 16 hours ÷ 60 minutes ÷ 60 seconds = 0.000208 mol/s/m².
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Convert to μmol/s·m² (PPFD):
0.000208 × 1,000,000 = 208 μmol/s·m².
Thus, to achieve a DLI of 12 mol/m²/d with 16 hours of light, a PPFD of 208 μmol/s·m² is required.
Accounting for Natural Light
In greenhouse environments, the contribution of natural sunlight must be subtracted from the target DLI value when designing artificial lighting systems. This ensures that the combined natural and artificial light does not exceed the plant’s optimal DLI range.
PPFD and DLI for Specific Plant Stages
PPFD and DLI for Seedlings
- How Much PPFD for Seedlings?
Seedlings require lower light intensity, typically 100–300 µmol/s·m², to avoid light stress while promoting healthy root and stem development. - Best PPFD for Seedlings:
A PPFD of 200 µmol/s·m² is often optimal for most seedlings. - DLI for Seedlings:
Target a DLI of 6–12 mol/m²/d.
PPFD and DLI for Vegetative Growth (Veg)
- How Much PPFD for Veg?
Vegetative crops require moderate light intensity, typically 280–550 µmol/s·m², depending on the species. - Best PPFD for Veg:
A PPFD of 350 µmol/s·m² is ideal for cannabis and many other crops in the vegetative phase. - DLI for Veg:
Aim for a DLI of 12–17 mol/m²/d to promote vigorous growth.
PPFD and DLI for Flowering
- How Much PPFD for Flowering?
Flowering plants demand higher light intensity, with a PPFD range of 650–1,600 µmol/s·m². - Best PPFD for Flowering:
A PPFD of 1,600 µmol/s·m² is recommended for optimized flowering in cannabis and other high-light crops. - DLI for Flowering:
Target a DLI of 20–40 mol/m²/d, depending on the crop type.
PPFD Ranges for Key Crops
Horticultural Crops
- Tomato:
- Minimum: 170 μmol/s·m²
- Maximum: 350 μmol/s·m²
- Typical: 270 μmol/s·m²
- Pepper: Typical PPFD of 230 μmol/s·m².
- Cucumber: Typical PPFD of 230 μmol/s·m².
Potted Plants
- Orchid (Phalaenopsis): Typical PPFD of 160 μmol/s·m².
- Bromeliads: Typical PPFD of 90 μmol/s·m².
- Kalanchoë: Typical PPFD of 90 μmol/s·m².
Cut Flowers
- Chrysanthemum: Typical PPFD of 140 μmol/s·m².
- Rose: Typical PPFD of 220 μmol/s·m².
- Lily: Typical PPFD of 90 μmol/s·m².
These ranges are critical for providing plants with the optimal light intensity for photosynthesis, growth, and flowering.
Cannabis DLI and Lighting Recommendations
Cannabis Lighting Needs:
-
Cannabis DLI Chart:
- Seedlings: 6–12 mol/m²/d
- Vegetative growth: 12–17 mol/m²/d
- Flowering: 20–40 mol/m²/d
-
PPFD for Cannabis:
- Vegetative growth: 280–550 µmol/s·m² (best PPFD: 350 µmol/s·m²).
- Flowering: 650–1,600 µmol/s·m² (best PPFD: 1,600 µmol/s·m²).
Conversions and Practical Applications
To achieve the desired DLI for a specific crop, growers must balance PPFD and lighting duration, taking into account the contributions of natural light in greenhouses. For indoor farming, artificial lighting systems must be precisely calculated to meet the target DLI without excess energy use.
For example:
- If growing tomatoes indoors with a target DLI of 20 mol/m²/d, and a photoperiod of 16 hours/day, the required PPFD would be:
20 ÷ 16 ÷ 60 ÷ 60 × 1,000,000 = 347 μmol/s·m².
Mol to Micromol Conversion
1 mole = 1,000,000 micromoles (µmol). This conversion is often required when working with DLI and PPFD values.
Conclusion
Understanding PPFD and DLI is essential for modern horticulture. These metrics allow you to design effective lighting strategies tailored to the specific needs of their crops. Whether in greenhouses or indoor farms, balancing artificial and natural light based on DLI requirements ensures healthy plant growth, efficient energy use, and higher yields. By incorporating these principles, you can unlock the full potential of their plants while minimizing resource waste.
