How many hours should a grow light run for seedlings?
Seedlings on a windowsill in February stretch toward the light, grow pale and floppy, and often never recover, which is why a grow light has become standard kit for serious seed starters. The question is never just how bright the lamp is, but how long it runs, because plants care about the total light received over a whole day, a figure growers call the daily light integral, or DLI. The calculator on this page converts your target DLI and your lamp's brightness into run hours, so you can set a timer once and stop guessing.
The maths is straightforward once the units are clear. PPFD measures how many photons land on a square metre every second, and multiplying it by the seconds in an hour and the hours the lamp runs gives the daily total, expressed in moles. A lamp delivering two hundred micromoles per square metre per second for sixteen point seven hours delivers twelve moles for the day, which is a solid target for most seedlings. The same daily total can come from a brighter lamp running fewer hours or a dimmer lamp running longer, which is why brightness alone never tells the whole story.
What DLI do seedlings actually need?
Most vegetable and flower seedlings grow well at ten to fifteen moles per square metre per day, with leafy crops like lettuce happy at the lower end and fruiting crops like tomatoes and peppers wanting the upper end. Microgreens and herbs can manage on eight to twelve, while seedlings destined for a sunny garden need hardening to much higher outdoor levels eventually, which is a separate process from indoor lighting. Too little light is the common failure, producing leggy stems, but too much light also stresses plants, bleaching leaves and wasting electricity, so the target range matters in both directions.
Natural daylight counts toward the total, which complicates windowsill setups. A seedling on a bright south-facing sill may already receive several moles from the sun on a clear spring day, and almost nothing on a dull winter one, so a fixed timer that ignores the weather will over-light on bright days and under-light on dull ones. A dedicated indoor setup, where the lamp is the only light source, is far easier to calculate and control, which is one reason growers who start many trays move them under lights and away from the window entirely.
Understanding PPFD and lamp height
PPFD is not a property of the lamp alone; it depends on how far the lamp hangs above the plants, and it falls quickly as distance increases. A lamp that delivers three hundred micromoles at fifteen centimetres may deliver only half that at thirty centimetres, because light spreads as it travels. Manufacturers usually publish PPFD at several heights, and the honest figure to use is the one measured at your actual hanging height, over the area your trays occupy, not the brightest spot directly under the centre of the lamp.
Coverage matters as much as centre brightness. A small, intense lamp over a large tray leaves the edges in shadow, and seedlings at the margins stretch sideways toward the bright centre, which defeats the purpose. Measure or estimate the average PPFD across the whole tray, not just the peak, and if the edges are much dimmer, lower the lamp slightly, add a second lamp, or use reflective sides around the growing area to bounce light back onto the plants. White walls and foil-lined boxes are cheap ways to improve uniformity.
| Crop | Target DLI | Lamp PPFD | Hours per day |
|---|---|---|---|
| Lettuce seedlings | 8 mol | 150 µmol | 14.8 hours |
| General seedlings | 12 mol | 200 µmol | 16.7 hours |
| Tomato seedlings | 15 mol | 300 µmol | 13.9 hours |
| Pepper seedlings | 13 mol | 250 µmol | 14.4 hours |
| Herb seedlings | 10 mol | 180 µmol | 15.4 hours |
Photoperiod: plants need darkness too
Running a grow light around the clock seems logical if light is good, but plants need a dark period for respiration and normal development, and most seedlings do best with fourteen to sixteen hours of light and eight to ten hours of darkness. Seedlings given twenty-four hour light often grow fast at first and then stall, with stressed leaves and poor root development, because the dark reactions of photosynthesis and the plant's internal clock both need the night. A simple plug-in timer that switches the lamp off overnight is one of the cheapest and most effective pieces of growing kit.
Consistency beats intensity. Seedlings respond to a regular daily rhythm, and a timer that delivers the same hours every day produces steadier growth than a lamp switched on and off by hand whenever someone remembers. Set the light period to match your routine, for example six in the morning to ten at night, so the plants' day aligns with the household's, and avoid moving trays in and out of light during the day, which confuses the rhythm and wastes the carefully calculated hours.
Choosing a grow light
Modern LED grow lights are the sensible choice for seedlings, because they run cool, use little electricity, and last for years, whereas old fluorescent tubes work but are bulkier and dimmer for the same wattage. Full-spectrum white LEDs, which look like ordinary daylight, are easiest to live with and perfectly adequate for seedlings, while purple-coloured grow lights may be slightly more efficient on paper but make the room unpleasant and hide the true colour of your plants, so problems are spotted later. For seedlings, a modest LED panel is ample; the expensive high-intensity lights sold for flowering plants are overkill.
Wattage on the box is a poor guide to brightness, because efficient LEDs produce far more light per watt than cheap ones, and some manufacturers quote the wattage of the diodes rather than the power actually drawn. Compare lamps by PPFD at a stated height and by coverage area, not by watts, and be sceptical of claims without measurements. A lamp with a dimmer is genuinely useful, because it lets you match brightness to the growth stage rather than buying a second lamp when seedlings graduate to larger plants.
Hanging height and leggy seedlings
Leggy seedlings are almost always a light problem, and the cure is to get the lamp closer, increase the hours, or both, rather than feeding more, which only makes soft growth softer. Hang the lamp fifteen to thirty centimetres above the seedlings for most LED panels, and raise it as the plants grow, keeping the distance roughly constant, because a lamp left at its original height becomes steadily less effective as the plants rise toward it, which sounds backwards until you remember the plants grow into the light and then scorch. Check daily and adjust weekly.
If seedlings are still stretching with the lamp close and the hours correct, suspect temperature: warm air makes seedlings grow fast and weak, while cooler air, around sixteen to eighteen degrees after germination, produces stockier plants. A small fan blowing gently across the trays also strengthens stems, mimicking outdoor wind and triggering the plant to invest in a sturdier stem, and it reduces damping off at the same time. Light, temperature, and air movement work together, and fixing only one of the three rarely fixes leggy seedlings.
Grow light checklist
- Set a target DLI: 10 to 15 mol for most seedlings.
- Use PPFD measured at your real hanging height, averaged across the tray.
- Run the lamp 14 to 16 hours, never 24, and use a timer.
- Keep the lamp 15 to 30 cm above the plants and raise it as they grow.
- Add a gentle fan and cool air for stocky, strong stems.
Good seedling light is a calculation, not a guess: pick a target, know your lamp's real brightness, and let a timer deliver the hours. The calculator does the arithmetic; the timer and the daily check do the growing.