Getting Cannabis Lighting Right, A Practical Guide to PAR, PPFD and Plant Stages
Lighting is one of those things growers think they understand, until a crop underperforms.
You followed the manufacturer’s hanging height, bought a powerful light, and everything should work. But plants stretch, tops bleach, or yields fall short. More often than not, the issue isn’t genetics or nutrients, it’s light intensity.
This article explains how much light cannabis actually needs, how to measure it properly, and how to adjust it across every stage of growth, without relying on wattage myths or brand marketing.
Why Wattage and Hanging Height Don’t Work
A common mistake is using rules like:
- “X watts per square meter”
- “Hang the light 40 cm above the canopy”
- “This light is made for flowering”
The problem is simple, plants don’t respond to watts or height, they respond to photons.
Two lights with the same wattage can deliver very different amounts of usable light at canopy level. The only way to know what your plants are actually receiving is to measure it.
PAR, PPFD and What Your Light Meter Is Really Showing
Let’s clear up the terminology without overcomplicating it.
PAR
PAR, Photosynthetically Active Radiation, describes the range of light plants can use, between 400 and 700 nanometres.
It’s a definition, not a measurement.
PAR does not have a number attached to it.
PPFD
PPFD, Photosynthetic Photon Flux Density, is the actual amount of PAR light hitting the plant canopy.
- Unit, µmol/m²/s
- This is what quantum and “PAR” meters display
- Even if the screen says “PAR”, the number is PPFD
So if your meter reads 700, that means your plants are receiving 700 µmol/m²/s of usable light.
No conversion. No calculations.
How Much Light Cannabis Needs at Each Stage
These values work regardless of:
- LED, HPS or CMH
- Brand or wattage
- Soil, coco or hydro
Recommended PPFD by Growth Stage
| Stage | PPFD Range (µmol/m²/s) |
|---|---|
| Clones / Cuttings | 100 – 300 |
| Seedlings | 200 – 400 |
| Early Vegetative | 300 – 500 |
| Late Vegetative | 500 – 700 |
| Early Flower (Weeks 1–3) | 600 – 900 |
| Mid Flower (Weeks 4–6) | 800 – 1000 |
| Late Flower (Weeks 7+) | 900 – 1100 |
Once you push past 1000 PPFD, the margin for error gets very small. Plants can handle it, but only if everything else, airflow, nutrition, temperature and humidity, is dialled in.
DLI, Why Light Hours Matter Just as Much as Intensity
PPFD tells you how strong the light is.
DLI, Daily Light Integral, tells you how much light the plant receives over the entire day.
This matters because moderate light for long hours can be just as stressful as very strong light for fewer hours.
Typical DLI Targets
| Stage | DLI (mol/m²/day) |
|---|---|
| Clones | 6 – 10 |
| Seedlings | 10 – 15 |
| Vegetative | 20 – 35 |
| Flowering | 35 – 45 |
As a rough guide:
- Veg plants under 18 hours of light usually do well around 400–600 PPFD
- Flowering plants under 12 hours usually perform best between 700–1000 PPFD
More than that doesn’t automatically mean more yield.
The Light Type Doesn’t Change the Rules
Growers often assume LEDs, HPS and CMH need different targets. They don’t.
What changes is:
- Efficiency
- Heat output
- Coverage pattern
What doesn’t change is how the plant responds to light.
Cannabis reacts to photons, not the technology producing them.
How to Measure Light Properly, Most People Get This Wrong
To get useful readings:
- Place the sensor flat at canopy height
- Sensor facing directly upward
- Take multiple readings:
- Centre of the canopy
- Each corner
- Use the average, not the highest number
Measuring too close to the light or at pot level will give misleading results.
Why Even Light Coverage Matters More Than Maximum Power
A single hot spot in the centre of the tent doesn’t grow better cannabis, it just stresses the tops.
As a rule:
- Edge readings should be at least 70–75 percent of the centre reading
Often, raising the light slightly improves coverage and overall plant performance, even if the centre PPFD drops a little.
Signs Your Light Is Too Strong or Too Weak
Plants usually tell you what’s wrong before serious damage happens.
| Symptom | Likely Issue |
|---|---|
| Leaves curling upward | Too much light |
| Pale or white tops | Severe light stress |
| Slow growth | Not enough light |
| Excessive stretch | PPFD too low |
| Yellowing only at the top | Light stress, not nutrients |
If only the top leaves look unhappy, check light before changing feed.
Safe, Reliable Targets for Most Grows, No CO₂
If you want strong results without constantly pushing limits:
| Stage | PPFD |
|---|---|
| Vegetative | 450 – 550 |
| Early Flower | 700 – 800 |
| Late Flower | 850 – 950 |
These ranges work well across most environments and reduce the risk of stress.
The Takeaway
Good lighting isn’t about buying the biggest fixture or following a chart on the box.
It’s about:
- Measuring what your plants actually receive
- Matching light intensity to growth stage
- Prioritising even coverage over raw power
Once lighting is dialled in properly, everything else, feeding, training, yield and quality, becomes easier.
Measure first, adjust second, guess never.
