Seedlings need light as soon as they break through the soil surface. The moment you see a tiny green loop or sprout emerging, turn on your grow lights and keep them running 14 to 16 hours per day. Waiting even a few days can produce weak, stretched-out stems that never fully recover.
Before Sprouts Emerge: Light During Germination
Most vegetable and flower seeds germinate best in darkness. Species like onions and common garden vegetables don’t need light until after they sprout, and light can actually inhibit germination in some cases. A few exceptions exist: begonias, primulas, and coleus require light to trigger germination, so their seeds are typically sown on the soil surface and left uncovered. If you’re unsure, check your seed packet. For the majority of garden seeds, you can keep them in a warm, dark spot until they sprout, then move them under lights immediately.
What Happens Without Enough Light
A seedling that doesn’t get light right away enters a survival mode called etiolation. The stem stretches rapidly upward, growing tall and spindly as the plant desperately reaches for a light source. The cotyledons (those first two small leaves) stay folded and pale. This isn’t just cosmetic damage. The plant is burning through its stored energy reserves without producing any new food through photosynthesis. Etiolated seedlings have thin, weak stems that often can’t support the plant later, and they’re more vulnerable to disease.
Even a delay of two or three days can cause noticeable stretching. If your seedlings are already leggy, you can bury the stem slightly deeper when you transplant, but prevention is far easier than correction.
How Many Hours of Light Per Day
Aim for 14 to 16 hours of light followed by 8 to 10 hours of complete darkness. A simple plug-in timer makes this effortless. The dark period matters more than most people realize. Research on young tomato plants found that dark periods shorter than 3 hours actively hampered growth. Plants given fragmented light and dark cycles (like 3 hours on, 1 hour off, repeated throughout the day) showed lower chlorophyll levels, reduced photosynthesis, and slower overall growth compared to plants on a single 18-hours-on, 6-hours-off cycle.
Running lights 24 hours a day is a common temptation, but plants use darkness to carry out essential internal processes, including moving sugars from leaves to roots. Continuous light doesn’t give seedlings an advantage and can cause stress over time. Stick to one uninterrupted light period and one uninterrupted dark period each day.
How Close to Place Your Grow Lights
The right distance depends on the wattage of your light. For low-wattage LEDs (100 to 300 watts), start with the lights 18 to 24 inches above the tops of your seedlings. Medium-wattage panels (300 to 500 watts) should sit 24 to 30 inches away. High-wattage fixtures (600 watts or more) need 30 to 36 inches of clearance. LEDs run cooler than older HPS bulbs, so they can generally sit closer to plants without causing heat damage.
These are starting points. Watch your seedlings closely during the first week and adjust. If stems are stretching and leaning toward the light, move the fixture a few inches closer. If the upper leaves start yellowing, curling upward, or developing a bleached, papery look, raise the light 6 to 12 inches higher. Light burn always shows up on the topmost leaves first, the ones closest to the fixture, which makes it easy to distinguish from other problems.
Why Light Quality Matters
Not all light is equally useful to a seedling. Blue light wavelengths (roughly 400 to 500 nanometers) play a key role in keeping seedlings compact and sturdy. Plants grown with adequate blue light are shorter with thicker, darker green leaves compared to plants grown without it. Blue light essentially acts as a natural growth regulator, discouraging the kind of leggy stretching you’re trying to avoid. This effect is especially pronounced under indoor lighting, where blue light is the only source of these wavelengths.
Most full-spectrum LED grow lights and T5 fluorescent tubes include enough blue light for seedlings. If your light has a visibly warm, orange-toned output, it may lack the blue wavelengths seedlings need for stocky growth. A “cool white” or “daylight” fluorescent, or any LED marketed for vegetative growth, will work well.
How Much Light Intensity Seedlings Need
If your grow light lists its output in PPFD (a measure of usable light reaching the plant surface), seedlings of most vegetables and herbs do well at 100 to 200 micromoles per square meter per second. Lettuce and herbs can thrive at the lower end, around 100. Peppers and other flowering plants tolerate up to 300 during the seedling stage but don’t need it. You don’t need a light meter to grow healthy seedlings, but if you have one, these numbers help you dial in the right distance without guessing.
Signs You Need to Adjust
Healthy seedlings are compact, with short distances between leaf sets and sturdy stems that hold themselves upright. Two common problems signal that your lighting needs adjustment:
- Leggy, stretched stems: The light is too far away, too dim, or not running enough hours. Move the fixture closer gradually and confirm you’re hitting 14 to 16 hours daily.
- Yellowing or bleaching on top leaves: The light is too close or too intense. Upper leaves turning yellow, white, or crispy while lower leaves look fine is a classic sign of light burn. Raise your fixture 6 to 12 inches and monitor for improvement over a few days.
Moving Seedlings From Grow Lights to Sunlight
Seedlings raised under artificial light haven’t been exposed to UV radiation, wind, or temperature swings. Placing them directly in full outdoor sun can scorch their leaves within hours. The transition, called hardening off, takes about 7 to 10 days. Start by setting seedlings in dappled shade or bringing them outside on an overcast day for a few hours. Each day, increase the amount of direct sunlight they receive and the time they spend outdoors. By the end of the week, they should tolerate a full day of sun and be ready for transplanting into the garden or outdoor containers.