How Much Light Should Seedlings Get
As someone who starts dozens of seedlings every spring, I get asked this question more than any other. Too little light and seedlings stretch and weaken; too much and they can bleach or dry out. Getting the light right is one of the simplest ways to give young plants a strong start. Below I’ll walk through how many hours of light seedlings need, how bright that light should be, how to position lights, what signs to watch for, and practical tips I’ve learned over years of trial and error.
Why light matters for seedlings
Light drives photosynthesis, shapes sturdy stems, and influences when seedlings shift from leaf-building to flowering. At the seedling stage you want compact, green growth and strong stem thickness so the plants can survive transplanting. Light affects both the duration (how many hours) and the intensity (how bright), and both are important.
How many hours of light per day
Most vegetable and annual flower seedlings do best with 14 to 16 hours of light per day. Some fast-growing crops (tomatoes, peppers, brassicas) benefit from up to 16–18 hours when you need very rapid, compact growth. For many herbs and slower annuals, 12–14 hours is often sufficient.
I use a simple rule: aim for 14–16 hours from a consistent artificial source, then give seedlings 8–10 hours of darkness to rest. A timer is indispensable — it prevents accidental over- or underexposure and keeps photoperiods consistent.
How bright should the light be
Seedlings need bright, even light. Rather than obsessing over exact lumen or lux numbers, focus on practical signals and equipment choices:
- Fluorescent T5 fixtures and full-spectrum LED grow lights are ideal for seedlings.
- Provide a bright, cool white light that mimics daylight; avoid weak desk lamps that give uneven illumination.
- If you use foot-candles as a guide, aim for consistent brightness across the seed tray rather than an exact number — many growers recommend roughly 1,000–2,000 foot-candles for robust seedling growth, but the most reliable method is watching plant response.
Distance from light to seedlings
How close the light is matters as much as how long it’s on. Lights too far away encourage stretching; lights too close can scorch or dry leaves.
- T5 fluorescent tubes: position 2–4 inches above the foliage and raise as seedlings grow.
- LED grow lights: check the manufacturer’s recommended distance — many low-power LEDs work well 2–6 inches above seedlings, while higher-output fixtures may need 8–12 inches.
- Adjust height rather than increasing the time if seedlings show any signs of stress.
Signs seedlings are getting too little or too much light
Watch your plants daily; they give clear clues.
- Too little light: long, weak stems (leggy), larger gaps between leaves, pale green color, and seedlings leaning toward light sources.
- Too much light or heat: bleached or yellowed leaf tips, crisping leaves, slowed growth, or soil drying rapidly.
“Early on I learned the hard way that ‘more light’ isn’t always better. My tomato seedlings turned white and fragile under a cheap high-output lamp until I raised it an inch and cut the run time back by two hours.”
Natural light versus grow lights
Windowsill light is unpredictable. South-facing windows can work for some hardy herbs and flowers, but they often provide uneven light and heat. If you rely on natural light, expect to rotate trays, supplement with grow lights on cloudy days, and accept uneven, leggy growth for many vegetables.
I prefer to start seeds under fluorescent or LED grow lights year-round. It gives reliable results and lets me control the exact photoperiod and intensity.
Practical daily schedule and tips
Here’s a practical routine I follow and recommend:
- Set lights on a timer for 14–16 hours daily for most seedlings; 16–18 hours for fast-growing crops if you want quicker results.
- Keep lights close but not touching — adjust every few days as seedlings grow.
- Rotate trays if using a window to avoid leaning toward the light.
- Use a reflective surface (white walls, Mylar, or foil backing) to even out light across trays.
- Pair bright light with proper temperature and watering — light without warmth or moisture won’t help much.
Hardening off: transitioning to outdoor light
Seedlings raised under artificial lights must be acclimated slowly to outdoor light and wind. Begin by placing them in morning sun for 1–2 hours and gradually increase time outdoors over a week or two. This prevents sunscald and shock.
Final checklist
- Set 14–16 hours of bright, even light for most seedlings.
- Use a timer to keep photoperiod consistent.
- Place lights close enough to prevent stretching, raising as plants grow.
- Watch plants for leggy growth or bleaching and adjust intensity/distance accordingly.
- Harden off seedlings before moving them to full outdoor conditions.
In my experience, a consistent light routine is the secret to healthy transplants. When seedlings are compact, green, and stocky, they handle transplant shock and go on to produce better yields and blooms. Start with quality lights, keep things consistent, and read your plants — they’ll tell you everything you need to know.
Happy seed starting! If you want, tell me what you’re starting this season and I’ll give light and timing suggestions specific to those plants.
