To start seeds indoors with LED grow lights, sow in a seed-starting mix, keep it warm and moist until seeds sprout, then give seedlings 14 to 16 hours of gentle light a day with the light close enough that they do not stretch. Raise the light as they grow, and harden them off before they go outside.
Why start seeds indoors?
Starting seeds indoors gives you a head start on the season, a wider choice of varieties and healthier plants than a windowsill can usually manage. Even a sunny window in late winter delivers short, weak days, so seedlings stretch toward the glass and grow thin. An LED grow light fixes that by providing steady, even light for as many hours as you choose. See who would use an LED grow light.
What you need
- Seed-starting mix and clean trays or pots with drainage
- Seeds, plus labels
- An LED grow light with adjustable height or a dimmer
- A timer
- A small fan for airflow
- Optionally a seedling heat mat for warm-season crops
Step 1: Sow and germinate
Follow the packet for depth and spacing. Most seeds need moisture and warmth to germinate, not light, so a tray can sit under a light or on a warm shelf until sprouts appear. A few species need light to germinate, which the packet will say. Keep the mix evenly damp, not soggy. A clear dome or plastic wrap helps hold moisture until the first sprouts show.
Step 2: Give seedlings light the moment they sprout
The moment seedlings break the surface they need light, or they will stretch and weaken. Remove the dome and turn the light on, ideally on a timer.
- Hours: 14 to 16 hours of light a day is the usual advice from extension services and botanical gardens, followed by a dark period.
- Intensity: gentle. Seedlings and cuttings do well at a modest PPFD, often cited at about 200 to 400 µmol/m²/s and often less in the first days. See what PPFD is.
- Color: full-spectrum white light with plenty of blue helps keep seedlings short and sturdy. See why red light and blue light are important for plants.
Step 3: Set the height
Distance changes intensity. Fluorescent seed-starting lights are kept 4 to 8 inches above seedlings, while LED fixtures are generally hung higher because they are brighter per fixture. Start high or dimmed and bring the light closer if seedlings begin to stretch. If you can dim the light, you can keep the height steady and change the intensity instead. Read how far to hang LED grow lights from plants.
Step 4: Keep seedlings warm, but not hot
Most seedlings like a gentle warmth, and a heat mat under the tray speeds germination for warm-season crops such as tomatoes and peppers. Once seedlings are up, normal room temperature is fine. LED lights run cooler at the plant than HID lamps, so they will not scorch seedlings the way a hot lamp can. See LED vs HID grow lights.
Step 5: Add airflow
A small fan on low for a few hours a day moves air around, strengthens stems and reduces the damp conditions that cause damping-off, a fungal disease that topples young seedlings. Do not point the fan straight at the seedlings.
Step 6: Water and feed carefully
- Water from below, or gently from above, when the top of the mix starts to dry.
- Do not feed for the first one or two weeks, since seed-starting mix and the seed itself supply what a seedling needs.
- When the first true leaves appear, start a weak liquid fertilizer if the label recommends it.
Step 7: Raise the light as seedlings grow
Keep the distance from the light to the top of the seedlings about the same as they grow. Raising the light a little every few days, or turning the dimmer up, makes sure that taller seedlings do not scorch and short ones do not stretch.
Troubleshooting seedlings
| What you see | Likely cause | What to do |
|---|---|---|
| Tall, thin, leaning seedlings | Light too far or too weak | Lower the light or raise the dimmer |
| Bleached or curled leaves | Light too close or too strong | Raise the light or dim it |
| Seedlings collapse at the soil line | Damping-off from damp, still air | Improve airflow and water less |
| Pale, slow growth | Not enough light or nutrients | Check hours and intensity; feed lightly after true leaves appear |
Step 8: Harden off before moving outside
Seedlings grown under a grow light have never felt wind or direct sun. Over 7 to 10 days, put them outdoors in a sheltered, shaded spot for a few hours a day, then gradually give them more sun and wind before transplanting. This avoids sunburn and shock.
When to start different seeds
Timing depends on the crop and on your climate, so use the seed packet as your guide. These are rough patterns:
| Crop | When to sow indoors |
|---|---|
| Tomatoes | About 6 to 8 weeks before transplanting outside |
| Peppers | About 8 to 10 weeks before transplanting; slower to sprout |
| Lettuce and greens | About 3 to 5 weeks before transplanting |
| Herbs such as basil | About 4 to 6 weeks before transplanting |
Work backward from your last frost date, or sow indoors any time if you plan to keep the plants inside.
Pot up and thin
- Thin crowded seedlings to the strongest one per cell by snipping the others at the soil line.
- Pot up tomatoes and peppers into larger pots when they have several true leaves, so roots have room.
- Handle seedlings by their leaves, not their stems.
Questions about starting seeds under LED lights
Do seeds need light to germinate?
Most do not. They need moisture and warmth. A few species need light to germinate, and the packet will say so.
Can seedlings have light 24 hours a day?
It is better not to. A dark period helps healthy development, and sources such as extension services recommend 14 to 16 hours.
My seedlings are tall and weak. What happened?
They stretched toward too little light. Lower the light, raise the dimmer or lengthen the day, and add a gentle fan. Weak seedlings that are tall are hard to fix, so thin them and give the rest more light.
Can I use an ordinary LED bulb for seedlings?
Bright white household bulbs work for a few small seedlings if they are very close. A purpose-built full-spectrum fixture covers a tray evenly and can be dimmed as the seedlings grow.
Key takeaways
- Most seeds need warmth and moisture, not light, to germinate.
- As soon as seedlings sprout, give them 14 to 16 hours of gentle light a day.
- Keep the light close enough that seedlings do not stretch, and raise it as they grow.
- Add a gentle fan to strengthen stems and prevent damping-off.
- Harden seedlings off over 7 to 10 days before moving them outside.
Seedling schedule at a glance
- Sow and keep warm and damp; no light needed for most seeds.
- On sprouting: light on, 14 to 16 hours a day, gentle intensity.
- First true leaves: bring the light closer or increase the intensity a little; start weak feed.
- Growing on: raise the light as plants grow; use airflow.
- Before planting out: harden off over 7 to 10 days.
My LED Grow Lights have a dimmer switch, so you can start seedlings gently and raise the intensity as they grow. See all of our LED grow lights, or return to growing with LED lights.
Related reading
- LED Grow Lights for Hydroponics
- Which stage of vegetable/fruit growth can your LED lights be used at?
- Back to Growing With LED Lights