The most common seedling mistake isn't overwatering — it's wrong light. Too much light and the cotyledons bleach white or curl under. Too little and the seedling stretches into a tall, weak stem that falls over under its own weight. The window between "not enough" and "too much" is narrower for seedlings than at any other stage.
This guide gives you the exact numbers: PPFD, light height, and photoperiod from germination through the transition to vegetative growth.
Why seedlings need less light
A seedling has almost no leaf surface area. The cotyledons (the first round leaves) are tiny solar panels — they can only absorb so much light before the photosynthetic machinery overloads. An adult cannabis plant in flower thrives at 800–1000 µmol/m²/s PPFD. A seedling at that intensity would bleach and die within a day.
As the plant grows leaves, it can handle more light. The progression is gradual:
| Stage | Age | PPFD target | DLI target |
|---|---|---|---|
| Emerging (cotyledons only) | Day 1–3 | 100–200 µmol/m²/s | 6–10 mol/m²/day |
| First true leaves | Day 4–7 | 200–300 µmol/m²/s | 12–16 mol/m²/day |
| Second node | Day 8–14 | 300–400 µmol/m²/s | 16–22 mol/m²/day |
| Transition to veg | Day 14+ | 400–600 µmol/m²/s | 22–35 mol/m²/day |
DLI (Daily Light Integral) is the total light a plant receives per day. PPFD × hours × 0.0036 = DLI. A seedling running 200 PPFD for 18 hours gets a DLI of 12.96 — right in the sweet spot for the first true leaves.
Light height by fixture type
You probably don't have a PAR meter for your first grow. That's fine — use distance from the canopy as a proxy.
LED panels (most common)
| Wattage | Seedling height | PPFD at that height |
|---|---|---|
| 100W (2×2 panel) | 30–36 inches | ~150–200 µmol/m²/s |
| 200W (3×3 panel) | 36–42 inches | ~150–250 µmol/m²/s |
| 300W+ (4×4 panel) | 42–48 inches | ~150–200 µmol/m²/s |
The rule: start with the light at the highest position your tent allows and dim it to 25–40% power if your light has a dimmer. That combination of maximum distance + low power gives gentle, even light that won't overwhelm seedlings.
CFL bulbs
If you're using CFL bulbs for seedlings (a valid, cheap approach):
- 23W CFL (100W equivalent): 3–4 inches from the top of the seedling
- 2–4 bulbs in a cluster: covers a small seedling tray
- CFLs produce almost no heat at reasonable distances and very low PPFD, so they're nearly impossible to overdo
T5 fluorescent fixtures
- 4-bulb T5HO fixture: 8–12 inches above seedlings
- These are the traditional nursery light — even coverage, low intensity, minimal heat
Photoperiod: how many hours
Photoperiod (regular) seeds
Run 18 hours on / 6 hours off from day one. The 6 hours of darkness isn't strictly required during the seedling stage, but it establishes the rhythm you'll maintain through veg and saves electricity. Some growers run 20/4 or even 24/0 for seedlings — both work, but the gains over 18/6 are minimal and the extra electricity cost isn't justified.
Autoflower seeds
Run 20 hours on / 4 hours off from day one. Autoflowers don't need a dark period to trigger flowering (they flower based on age, not light cycle), and the extra 2 hours of daily light compounds into meaningful growth over a 70–80 day lifecycle. Some auto growers run 24/0, but most strains benefit from at least 4 hours of darkness for recovery.
Signs your light is wrong
Too much light (light stress)
- Cotyledons curl downward — the edges fold under, making the leaf look like a taco
- Bleaching — leaves turn pale yellow or white, starting from the center, spreading outward
- Stunted growth — the seedling stops growing vertically and the leaves thicken but don't expand
- Red or purple stems — can indicate light stress (though genetics and temperature also cause this)
Fix: raise the light 6 inches and/or dim 10–20%. Check again in 24 hours. Seedlings recover quickly from mild light stress if caught early.
Too little light (stretching)
- Tall, thin stem — the seedling is reaching for light. The internodal spacing (distance between nodes) is excessive
- Leaning — the seedling leans toward the light source
- Pale green color — not enough light for vigorous chlorophyll production
- Floppy stem — the stem can't support itself and the seedling falls over
Fix: lower the light 6 inches and/or increase intensity by 10%. If the seedling has already stretched, you can bury the stem up to the cotyledons when you transplant — it will root along the buried stem.
The first 14 days: a schedule
Days 1–3: emergence
The seed has cracked and the cotyledons are emerging from the soil.
- Light: on at 25–30% intensity, maximum height (or CFL at 4 inches)
- Schedule: 18/6 (photo) or 20/4 (auto)
- Watch for: stretching (stem growing tall without opening cotyledons). If the seedling is more than 2 inches tall before cotyledons open, it's stretching — lower the light
Days 4–7: first true leaves
The serrated cannabis leaves appear between the cotyledons.
- Light: increase to 35–50% intensity, or lower the light 4–6 inches
- Schedule: same — 18/6 or 20/4
- Watch for: leaf curl (too much light) or continued stretching (not enough). The stem should be short and sturdy, and the first true leaves should be broad and flat
Days 8–14: second and third nodes
The plant now has 2–3 sets of true leaves and is entering early vegetative growth.
- Light: increase to 50–60% intensity, or lower the light another 4–6 inches
- Schedule: same — 18/6 or 20/4
- Watch for: the transition from "seedling" to "vegetative" is gradual. When the plant has 4+ nodes and the leaves are growing visibly wider each day, you can begin increasing light toward full veg intensity (400–600 PPFD)
Day 14+: transition to veg
- Light: ramp to 60–75% intensity over the next week
- Target: 400–600 PPFD by day 21
- Don't jump straight to full power — increase 10% per day and watch for stress
Using a phone app as a PAR meter
Dedicated PAR meters cost $300–500. Phone apps (Photone, Korona) use the phone's light sensor to estimate PPFD. They're not laboratory- grade, but they're accurate enough for seedling management:
- Download Photone (iOS/Android) — the free version works for LED and CFL
- Select your light type (LED, fluorescent, HPS)
- Hold the phone face-up at canopy height (top of the seedling)
- Read the PPFD value
- Adjust light height or dimmer until you hit your target
Accuracy caveat: phone sensors are better at relative measurement (is this spot 2× brighter than that spot?) than absolute measurement. Use the numbers for guidance, not as gospel. If the app says 200 µmol/m²/s and the seedling looks healthy, the number is close enough.
Common mistakes
- Blasting seedlings with full power — many new growers unbox a 300W LED and run it at 100% intensity 18 inches above a seedling. That's 3–4× more light than the seedling can handle. Start low, ramp up
- No light at all for the first few days — some guides suggest keeping seedlings in the dark until they emerge. Once the seed cracks and the white tap root is visible, it needs light the moment it breaks the soil surface (not before)
- Inconsistent light schedule — a seedling doesn't need a specific schedule as desperately as a flowering plant, but consistency helps. Set a timer and don't fiddle with it
- Forgetting to raise the light as the plant grows — the light stays fixed but the plant grows toward it. A 30-inch gap on day 1 becomes a 24-inch gap by day 7 — enough to cause stress. Check the gap every few days
- Using an HPS/MH for seedlings — high-intensity discharge lights produce massive heat and PPFD. They work for seedlings only if hung very high (4+ feet) and are overkill. Use a small LED or CFL for the first 2 weeks, then switch to the HPS for veg
For full seedling care (watering, feeding, environment), see our seedling care guide. For seedling problems and diagnosis, see seedling troubleshooting. For understanding PPFD and DLI in depth, see our lighting guide.