Last updated on 22 Feb 2025
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Author: Leslie F. Halleck
Kaya Leaf’s Key Urban and Tropical Horticulture Takeaways
Doesn’t a bright window provide more than enough light for all of your plants to survive?
Sadly, it’s not that straightforward.
You done all that you’ve can to maximise watering and soil conditions. Yet your plants are struggling to thrive indoors or in a particularly shady part of the garden.
That’s the reason why we did our research with this book to get specific about light measurements and how to bring Kaya Leaf readers’ green thumbs to the next level!
Whether you want to skip ahead to the quick cheat sheet of Daily Light Integral (DLI) figures or dive into the science, we’re here to ensure your plant babies get the light they need to flourish.
Why Isn’t Your Sunny Window Enough?
To us, a bright room may feel like paradise, but to plants, it’s often a dim, low-energy cave.
Most indoor ambient light is neither intense enough nor of the right spectrum for healthy plant growth. If you’ve been wondering why your plants look tired, not measuring the exact amount of unusable light for your plants could be the problem.
What Is PAR, and Why Should You Care?
Light isn’t just light—it’s a spectrum of wavelengths, from violet and blue to yellow and red. For plants, the key range is Photosynthetically Active Radiation (PAR), measured between 400 to 735 nanometers (nm).
PAR is where the photosynthesis magic happens. Within this range:
- Blue Light (450–495 nm) drives vegetative growth and controls stem stretching. It also boosts antioxidants in crops like lettuce.
- Red Light (620–750 nm) encourages flowering and fruiting.
Together, they form the foundation of most grow lights’ purple hue. For a balanced growth strategy, you’ll need a tailored mix of both.
And what about green light (495-570 nm)?
While it’s true that plants reflect much of it (hence their green color), contrary to popular belief, plants do absorb and use some green light.
Some green light is vital for seedling and leaf development, flower initiation and stem growth.
Green light also penetrates deep into the canopy, keeping lower leaves alive and photosynthesizing.
Plants Need Sleep Too
Like us, plants need rest. Many species are photoperiodic, requiring specific durations of light and darkness to trigger different growth stages.
For example, poinsettias need at least 11 hours and 45 minutes of uninterrupted darkness to flower. A single flash of red light during the dark period resets their internal clock and delays blooming!
To optimise growth, it’s essential to know your plants’ photoperiodic needs.
Understanding Plant Light Metrics: The Big Three
When it comes to light, lumens and lux measurements are meaningful for human eyes— not plant growth. For plants, you’ll need to understand and measure these three essential metrics:
1. PPF (Photosynthetic Photon Flux)
Measured in micromoles per second (µmol/s), PPF tells you how much PAR light your lamp emits every second. It’s essentially the total light output available for photosynthesis.
2. PPFD (Photosynthetic Photon Flux Density)
PPFD measures the density of PAR light that actually reaches your plants, reported as micromoles per square meter per second (µmol/m²/s). You can think of this as the “light intensity” your plants experience.
Use a PAR quantum flux meter to measure PPFD over a specific area and distance from the light source.
3. DLI (Daily Light Integral)
The crown jewel of plant lighting, DLI measures the cumulative amount of PAR light a plant receives in a day. It’s expressed in moles per square meter per day (mol/m²/d).
Here’s how to calculate DLI:
DLI = PPFD x photoperiod (hours of light per day) x 0.0036
Where 0.0036 converts seconds in an hour (3600) to the mole scale (dividing by one million). This ensures your final DLI figure reflects the total number of light photons absorbed daily.
Want to increase your DLI? You can move plants closer to the light source, add artificial lights, and/or increase the hours of light exposure.
Quick Cheat Sheet: DLI Targets for Plants
To accurately measure your plant’s light needs, skip the household lux and footcandle meters. Instead, invest in a PAR quantum flux meter—a tool that measures PAR output accurately across the 400–700 nm spectrum.
Here’s a breakdown of how much DLI and light your plants need per day, based on their light preferences:
- High-light plants (e.g. tomatoes): 18–30 mol/m²/d
- Medium-light plants (e.g. lettuce): 12–16 mol/m²/d
- Low-light plants (e.g., seedlings and cuttings): 6–10 mol/m²/d
- Heavy-shade plants (e.g. tassel fern): 3–6 mol/m²/d
A PAR quantum flux meter is a must-have for serious growers and many models today can even calculate DLI figures for you directly!
Kaya Leaf Takeaways
Growing healthy, thriving plants indoors is a blend of art and science.
By understanding PAR, DLI, and how plants respond to light, you’ll create an environment where your plants can flourish.
Happy growing Kaya Leaf friends!