Seedlings and clones need far less light than flowering plants: 200-400 µmol/m²/s PPFD, delivered over an 18-20 hour photoperiod. Dial in too much intensity and you get bleaching, heat stress, and stunted root development. The key decision is LED vs. fluorescent, where modern T5 LED strips give you the even distribution fluorescents are known for with better efficiency and cooler operation. Both work; the right pick depends on whether you want a dedicated propagation station or a single fixture that carries plants from seed through flower.
What PPFD Do Seedlings Actually Need?
PPFD (photosynthetic photon flux density, measured in µmol/m²/s) is the number that matters for seedlings. The target range for cannabis seedlings and clones is 200-400 µmol/m²/s. That is lower than most growers expect, and well below what vegetative or flowering plants require.
Pushing above 400 µmol/m²/s at this stage does not speed up development. Instead, it causes photoinhibition, where the plant's photosystems are overwhelmed before roots are established enough to support vigorous growth. The result is bleached leaf tips, upward leaf curl, and plants that look stressed rather than thriving. Keep intensity conservative until roots are visible and the plant is actively pushing new growth.
Note that cannabis seedlings sit at the high end of the propagation spectrum. Most vegetable or herb seedlings target 100-200 µmol/m²/s. If you are starting cannabis specifically, aim for the 200-400 range rather than the softer baseline common for ornamentals.
| Growth Stage | PPFD Target | Hours/Day |
|---|---|---|
| Seedling / Clone | 200-400 µmol/m²/s | 18-20h |
| Vegetative | 400-600 µmol/m²/s | 18h |
| Peak Flower | 800-1,200 µmol/m²/s | 12h |
LED vs. Fluorescent for Seedlings: Which Is Better?
Both LED and fluorescent fixtures have a long track record in propagation. The choice comes down to your setup, budget, and whether the light will serve double duty beyond the seedling stage.
LED advantages:
- Lower heat output keeps temperatures stable under humidity domes
- Better energy efficiency over a full grow cycle
- Many models are dimmable, which allows precise PPFD control without changing hang height
- Some fixtures include far-red and UV diodes that become useful in later stages
Fluorescent (T5/T8) advantages:
- Extremely even light distribution across a full propagation tray
- Low upfront cost relative to panel LEDs
- Time-tested for cloning and seedling propagation across decades of cultivation
- Replacement bulbs are widely available
The practical verdict: For dedicated propagation and clone trays, modern T5 LED strips are the strongest option. They deliver the even, gentle light distribution that fluorescents are known for, while running cooler and consuming less power. Full-spectrum panel LEDs make more sense when the same fixture will carry plants from seedling through early veg. If you want one light that handles propagation and the first few weeks of vegetative growth without swapping hardware, a dimmable panel at 30-40% intensity is the more flexible path. If you want a clean, low-heat propagation station, LED T5 strips are the cleaner answer.
Top Seedling Grow Lights
These picks cover dedicated propagation setups through early veg transition:
| Light | Key Specs | Price | Why It Stands Out |
|---|---|---|---|
| 100W, full spectrum Samsung LM301H diodes, 2x2-2x4 footprint, controller-compatible dimming | -- | 100W keeps seedling-stage PPFD in range without dimming hardware; integrates with AC Infinity fan controllers for automated light schedules; compact footprint fits propagation shelves; scales into early veg with the same fixture | |
| T5 LED strips, full-spectrum Sun White, 4FT length, 2-pack, even canopy coverage | -- | T5 form factor produces the most even light distribution across a full propagation tray; low heat signature ideal for humidity dome setups; Sun White spectrum closely matches natural daylight; low barrier to entry for a dedicated clone and seedling station. Active Grow strips are a grower's standard for this reason. | |
| 300W (dimmable), Samsung LM301H, 4x4 coverage, WiFi/Bluetooth dimming via app | -- | Dimmable to 20-30% for seedling-stage PPFD; one fixture carries through seedling, veg, and early flower without changing hardware; smart app control for scheduled intensity ramp-ups as plants develop. Overkill for a dedicated seedling station, but the right call if transitioning the same space. Browse the Spider Farmer lineup for the full range. | |
| 300W foldable bar design, full spectrum, dimmable, 3x3-4x4 coverage | -- | Dimmable for lower PPFD seedling phase; foldable bar design gives even coverage across the canopy; strong price-to-performance ratio. A good option if budget is the primary constraint and you are growing in a 3x3 or 4x4. Mars Hydro has a wide catalog if you need a different footprint. |
The seedling grow light category includes additional options sized for propagation shelves, mother rooms, and clone trays. The broader LED grow light lineup covers everything from 100W propagation fixtures through commercial-scale systems.
Setting Up Grow Lights for Seedlings: Hang Height and Schedule
Getting PPFD into the right range is partly a function of the fixture, but hang height matters just as much. The closer the light, the higher the intensity reaching the canopy.
Recommended hang heights:
- T5 LED strips: 6-12 inches above the top of the canopy or humidity dome
- Full-panel LEDs at low intensity: 24-36 inches above the canopy
- If your light is dimmable, start at 30-40% output and raise hang height if seedlings show stress before adjusting the dial
Light schedule: Run lights 18-20 hours on and 4-6 hours off. A dark period matters even at the seedling stage. Plants use the rest period for root development and cellular repair. Avoid 24-hour light cycles, which can cause mild stress and produce weaker root systems than a proper dark period allows.
Signs of too much light:
- Bleaching or yellowing at leaf tips or centers
- Leaf curl upward or downward (tacoing)
- Stunted growth or slow root development despite healthy-looking soil
Signs of too little light:
- Leggy, stretched stems reaching toward the light (etiolation)
- Pale or light-green leaves lacking deep color
- Slow root development and delayed transition to vegetative growth
If you are using a dimmable fixture, start at 30-40% and increase intensity gradually over the first two weeks as roots establish. This approach works well with panel LEDs like the Spider Farmer G3000 Smart, where the app makes incremental adjustments easy to schedule.
For a full breakdown of spectrum science and PPFD targets at every stage, see Full Spectrum Grow Lights Explained.
Related Guides
- Full Spectrum Grow Lights Explained
- Best LED Grow Lights for Indoor Growing
- How Long Should Grow Lights Be On?
- Grow Room Temp and Humidity Chart for Perfect Climate Control
Frequently Asked Questions
- What grow light is best for seedlings?
- Low-wattage full-spectrum LEDs and T5 LED strips are the best fit. Target 200-400 µmol/m²/s PPFD. The AC Infinity Ionboard S22 at 100W and Active Grow T5 LED strips are both well-matched to propagation without risking light burn on young plants.
- Can seedlings get too much light?
- Yes. Exceeding 400 µmol/m²/s at the seedling stage causes bleaching, heat stress, and slowed root development. Keep lights at the upper end of the recommended hang height range and dial down intensity if the fixture is dimmable. Watch for leaf curl or pale tips as early warning signs.
- How long should grow lights be on for seedlings?
- 18-20 hours on, 4-6 hours off. A dark period is important even at the seedling stage. Avoid 24-hour light cycles, which can produce weaker root systems and mild chronic stress compared to plants given a proper rest period.
- Is LED or fluorescent better for seedlings?
- Modern T5 LED strips give you the practical best of both: the even distribution fluorescents are known for, combined with the efficiency and longevity of LED. Full-panel LEDs make more sense if you plan to grow plants through veg and flower in the same space and want a single fixture that handles every stage with the dial turned down during propagation.
- How far should grow lights be from seedlings?
- T5 LED strips: 6-12 inches above the canopy or humidity dome. Full-panel LEDs running at low intensity: 24-36 inches. Watch for leaf curl or bleaching as a sign the light is too close, and leggy, stretched growth as a sign it is too far away.
- What wattage grow light do I need for seedlings?
- 50-150W is sufficient for a 2x2 to 2x4 propagation area. Higher-wattage lights work if they are dimmable: run them at 20-40% output during the seedling stage and increase as plants transition into vegetative growth. A 300W fixture dimmed to 25% is a reasonable substitute for a purpose-built propagation light if you already own it.
