What is PPFD and Why Does It Matter?
PPFD stands for Photosynthetic Photon Flux Density — a measurement of the amount of light actually available to plants for photosynthesis, expressed in micromoles of photons per square metre per second (μmol/m²/s). Unlike lux or lumens, which measure light as the human eye perceives it, PPFD specifically measures the wavelengths (400–700 nm) that drive plant growth.
For indoor herb growing, PPFD is the most useful metric because it directly predicts how quickly your plants will photosynthesise. Too low a PPFD and plants stretch toward the light source, become pale and flavour-poor, and may bolt earlier than usual. Too high and leaves bleach, curl, and develop "light burn" — a form of photo-oxidative stress. The goal is the optimal range for your specific herb.
PPFD Targets for Common Herbs
Different herbs have meaningfully different light requirements. Basil, as a Mediterranean annual that evolved in high-sun environments, needs significantly more light than mint, which grows naturally in shaded, moist habitats. Matching the light intensity to the herb's natural preference produces the strongest, most flavourful plants.
Relative PPFD Requirements — Basil, Coriander, Mint
| Herb | PPFD Range (μmol/m²/s) | Light Hours / Day | Approx. LED Distance | Notes |
|---|---|---|---|---|
| 🌿 Basil | 200–400 | 14–16 hours | 15–25 cm | Higher end of range produces more intense flavour and essential oil content |
| 🌿 Coriander | 150–300 | 12–14 hours | 20–30 cm | Higher PPFD increases bolt risk — keep at lower end of range if bolting is a problem |
| 🌿 Mint | 150–250 | 12–14 hours | 25–35 cm | Tolerates lower light better than basil; excess light reduces mint's cooling menthol content |
| 🌿 Parsley | 150–300 | 12–14 hours | 20–30 cm | Similar to coriander; germinates slowly but once established is tolerant of some variation |
| 🌿 Rosemary / Thyme | 250–450 | 14–16 hours | 15–20 cm | Mediterranean herbs — need the most light of any common culinary herbs; lower PPFD produces weak, leggy growth |
| 🌿 Chives | 150–250 | 12–14 hours | 25–35 cm | Tolerates lower light; useful for window-sill level setups with supplemental lighting |
How to Set the Correct Grow Light Height
Grow light distance affects PPFD dramatically — moving a light 50% further away typically reduces PPFD by approximately 75%, following the inverse square law of light. This means small distance adjustments have large effects on what your plants receive.
Start with the Manufacturer's Recommended Distance
Most quality LED grow lights include a distance chart in the packaging that shows recommended height for different growth stages (seedling, vegetative, flowering). Start at the vegetative recommendation for herbs — this is usually in the right range for most culinary herbs and gives a safe starting point before fine-tuning.
Watch Leaf Posture — The Most Reliable Signal
Plant leaves are highly responsive to light intensity and give clear visual signals long before any damage becomes apparent. Healthy herbs receiving the right PPFD will hold their leaves in an upright, slightly angled position — often called "praying" — to maximise light capture. This posture indicates the plant is actively and productively photosynthesising.
If leaves lie flat and horizontal, the plant may be receiving optimal to slightly high light. If stems are elongating rapidly and leaves are small and pale, the light is too far away. Use these cues to adjust the height 2–3 cm at a time and monitor over 2–3 days before adjusting further.
Use a Lux Meter App as a Proxy
Free lux meter apps (such as Lux Light Meter Pro on iOS or Android) use the phone's front camera to measure approximate light intensity. While not as accurate as a calibrated PPFD meter, they are useful for consistent relative measurements — checking that your plants are receiving more or less light than yesterday, and comparing light levels at different heights above the canopy.
As a rough guide for broad-spectrum LED: 10,000 lux ≈ 200 μmol/m²/s. Use this ratio as a starting approximation — exact conversion varies by light spectrum, so verify against leaf posture rather than relying solely on lux readings.
Keep Notes and Adjust Incrementally
Indoor growing conditions change continuously — as plants grow taller, the distance between leaf canopy and light decreases. Check height every 3–5 days and raise the light to maintain the correct distance as plants develop. A small piece of tape on the light's hanging cable at the correct height makes it easy to maintain a consistent position after raising.
Track your observations: date, light height, canopy height, and leaf posture. Over 2–3 weeks a clear pattern emerges showing how your specific light and plant combination respond, making future adjustments intuitive.
Which Grow Light Type is Best for Herbs?
Not all lights support plant growth equally — the spectral output matters as much as raw intensity. Here is how the three most common light types compare for indoor herb growing.
Full-Spectrum LED
Provides the complete wavelength range plants need for photosynthesis and secondary metabolite production (including essential oils that give herbs their flavour). Energy-efficient, low heat output, long lifespan. The best choice for all indoor herbs.
Best choiceFluorescent / CFL
Adequate for seedlings and low-light herbs like mint and chives. Lacks the red spectrum needed for strong vegetative growth in basil and rosemary. Can work well if multiple tubes are used close to the canopy (5–10 cm). Cost-effective for starting out.
Works for low-light herbsRegular Incandescent / LED Bulb
Standard household bulbs emit almost no blue spectrum — the wavelength most critical for vegetative herb growth. Plants will grow toward the light but will be leggy, pale, and flavour-poor. Not recommended even as a supplement; invest in a dedicated grow light.
Not suitableHow to Tell If Your Light Is Too Far or Too Close
⬆️ Light Too Far Away — Symptoms
- Stems stretching rapidly upward (etiolation)
- Leaves small, pale green, or slightly yellow
- Weak, floppy stem structure
- Flavour and essential oil content reduced
- Coriander bolts earlier than expected
- Slow growth rate despite otherwise good conditions
⬇️ Light Too Close — Symptoms
- Leaf bleaching — pale white or yellow patches
- Leaf edges curling upward or browning
- Crispy texture on upper leaf surface
- Growth slowing despite all other conditions being correct
- Tips of new growth appearing burnt or dried
- Heat stress if light also radiates significant warmth
FAQs
Standard LED bulbs are not suitable replacements for grow lights. They lack the blue spectrum (400–500 nm) that drives vegetative growth and the red spectrum (600–700 nm) that supports flowering and fruiting. Plants will survive under regular LEDs but will be leggy, pale, and produce very little flavour. A purpose-built full-spectrum grow light — even an inexpensive 20–30W panel — will produce dramatically better herb growth. The difference in results is substantial and visible within 2 weeks.
The most reliable method is observing leaf posture — upright, slightly angled "praying" leaves indicate correct intensity for most herbs. For a more quantitative check, use a free lux app on your smartphone and aim for 10,000–20,000 lux at canopy height for basil, or 8,000–15,000 lux for coriander and mint. A dedicated PPFD meter gives the most accurate reading if you want to fine-tune precisely to the μmol/m²/s targets in the table above.
Most culinary herbs thrive with 12–16 hours of light per day under grow lights. Basil, rosemary, and thyme need the higher end (14–16 hours) as they evolved in high-sun environments. Mint, coriander, and chives do well at 12–14 hours. Always use a timer — consistent daily photoperiod prevents the erratic light cycles that can trigger premature bolting in coriander and encourage legginess in basil. A 24-hour rest period (all lights off) is also important for normal plant physiology.
Yes, significantly. The essential oils and secondary metabolites that give herbs their flavour are produced in direct response to light intensity. Basil grown at the higher end of its PPFD range (300–400 μmol/m²/s) produces measurably more essential oil and has stronger flavour than the same variety grown at 150 μmol/m²/s. Similarly, mint's menthol content increases with appropriate light levels. The right grow light distance is therefore not just about plant health — it directly improves the quality and intensity of the herbs you harvest.
Yes — this is one of the most commonly missed adjustments. As herbs grow taller, the canopy-to-light distance decreases, and PPFD at the canopy level increases accordingly. Without periodic adjustment, a light correctly positioned at potting time will be too close (and potentially causing light burn) by week 4–6 as the plant has grown 10–15 cm taller. Check the canopy-to-light distance every 3–5 days and raise the fixture to maintain the correct distance. Adjustable hanging cables or a simple measuring stick make this straightforward.
Start at the Right Height — Adjust from What the Plant Tells You
Getting grow light distance right is a process of starting at the manufacturer's recommendation, reading your plants' responses, and making small adjustments over the first 1–2 weeks. The PPFD targets and distance guidelines in this guide give you a solid scientific framework — but the most reliable tool is consistent observation of leaf posture, colour, and growth rate.
Set a timer for consistent daily photoperiod, use a lux app for periodic checks, and raise the light incrementally as plants grow. With these adjustments, you can grow compact, intensely flavoured basil, coriander, and mint indoors year-round — completely independent of outdoor conditions or available sunlight.