Is My Grow Light Strong Enough? How to Tell
“Strong enough” only means something against a target. A light that is perfect for seedlings is far too weak for flowering, and one tuned for flowering will bleach a tray of clones. The way to answer the question is to compare the PPFD your light actually delivers at the canopy against the target for your crop’s current stage.
Target PPFD by growth stage
These are the ranges the indoor-growing community and the major fixture makers have broadly converged on, in µmol/m²/s at the canopy:
| Stage | Target PPFD (µmol/m²/s) | Notes |
|---|---|---|
| Seedlings / clones | ~100–300 | Start low; a hot light stresses young plants and bleaches new growth |
| Vegetative | ~400–600 | Enough to drive structure without wasting photons |
| Flowering | ~600–1000 | Upper end mainly pays off with supplemental CO₂ |
The ranges are guidance, not a hard line — cultivar, temperature, VPD and CO₂ all shift the sweet spot — but they are close enough to tell “strong enough” from “way off.” Pushing much past ~1000 µmol/m²/s without added CO₂ tends to hit diminishing returns rather than more yield.
If you prefer to think in daily dose, PPFD over a photoperiod gives DLI (Daily Light Integral, mol/m²/day): DLI = PPFD × seconds of light per day ÷ 1,000,000. Two lights at the same PPFD deliver different DLI if their run-times differ. Our PPFD to DLI converter does the arithmetic.
How to check what your light actually delivers
You need the PPFD at the canopy where the plants are, not the fixture’s headline number. Three ways, in order of accuracy:
- Read the manufacturer PPFD map at your real hanging height, and take the value over where the plant sits — corners included, not just the center. Watch the units and the height; see how to read a PPFD chart, and remember these maps run optimistic (are PPFD charts accurate).
- Measure with a quantum meter at canopy height. This is the accurate answer. A borrowed or rented meter for one mapping session anchors everything else.
- Estimate from a phone lux reading and a light-source conversion factor when you have no meter — a ballpark, not a substitute. See how to estimate PPFD without a quantum meter and the lux to PPFD converter.
Reading the result
Compare the canopy PPFD to the target for the current stage:
- Below target — lower the light or raise the power, then re-read. Height is the strongest single lever; see how PPFD changes at different heights.
- At target in the center, low in the corners — the light covers a smaller area than the rated footprint. Shrink the canopy, add a second fixture, or accept slower edge plants.
- Over target, especially on seedlings — raise the light or dim it. More is not better once you are past the stage’s range.
A common outcome is “strong enough for veg, short for flower.” That is normal — the same fixture can be right for one stage and under-powered for the next, which is exactly why the target moves with the plant.
A phone can get you close today
You do not need to buy a meter to get a first read on whether a light is in the right range. A phone lux reading plus the right conversion factor puts you in the ballpark for white LEDs, and it is enough to tell “strong enough” from “nowhere near.” We are building a per-phone-calibrated PPFD/PAR meter app to make that read trustworthy instead of a guess — if that is useful to you, the waitlist is on the lux to PPFD tool page.
Sources
- Target PPFD by growth stage (seedling 100–300, veg 400–600, flower 600–1000): https://www.spider-farmer.com/blog/ppfd-for-different-plant-growth-stages/ and https://marijuanapackaging.com/blogs/resources/optimal-ppfd-levels-for-seedlings-vegetative-growth-and-flowering-stages
- Flowering upper end and supplemental CO₂ context: https://www.mars-hydro.com/info/post/how-much-ppfd-for-indoor-plants-in-each-growth-stage
- DLI as PPFD × photoperiod: https://www.cocoforcannabis.com/grow-light-guide/mars-hydro-fc-6500-par-test-and-review/