Run grow lights 12–16 hours a day for most vegetable seedlings, and give them at least 8 hours of darkness. Lettuce and spinach are the exception: keep them to about 11 hours, or they may bolt.
Open this example in the Grow Light Planner
Why the number of hours matters
Seedlings respond to the total light they get in a day, not just how bright the light is. That total is the daily light integral, or DLI: brightness × hours. Run a light longer and it delivers more light per day; run it shorter and it has to be brighter to deliver the same amount.
Minnesota recommends 12–16 hours of light a day (Starting Seeds Indoors), and Maryland says seedlings need about 8 hours of darkness (Starting Vegetable Seeds Indoors). So 16 hours is the most to run lights for most crops. Lettuce and spinach are the exception: Wisconsin says to give them no more than 11 hours, or they may bolt (Check Soil Temperature).
Hours of light by crop
These are the settings the BedMath Grow Light Planner uses. Most crops get 16 hours, the top of Minnesota's range, so modest LED bars can reach the daily light target.
| Crop | Lights on | Dark |
|---|---|---|
| Tomatoes | 16 h | 8 h |
| Peppers and eggplant | 16 h | 8 h |
| Broccoli, cabbage, and kale | 16 h | 8 h |
| Lettuce and greens | 11 h | 13 h |
| Onions and leeks | 16 h | 8 h |
If you start greens and tomatoes together, give the greens their own shelf and timer. One timer at 16 hours would give lettuce too long a day.
Worked example: 4 trays of lettuce
Four 1020 trays of lettuce on one 48 × 24 in shelf, under 2-ft LED bars. The planner gives:
Now switch the crop to tomatoes, with the same 4 trays and shelf. The lights stay on for 16 hours, so the target of 12 mol/m²/day needs only 208 µmol/m²/s. The same 10 bars deliver 14.6 mol/m²/day.
The takeaway: cutting lettuce to 11 hours didn't cut the number of lights. A shorter day means each hour has to be brighter. It does cut electricity: 1.10 kWh a day for lettuce versus 1.60 for tomatoes.
Setting the timer
- Use an outlet timer so the lights go on and off at the same time every day.
- Plug heat mats into their own thermostat, not the light timer (Starting Seeds Indoors).
- If your lights can't reach the target in 16 hours, add lights rather than hours. Seedlings still need their dark period (Starting Vegetable Seeds Indoors).
Run your own numbers
The free BedMath Grow Light Planner works this out for your trays, shelves, crop, and lights, and shows every formula and source. This link opens it with the example above filled in.
Sources
Published 2026-11-03. Worked examples come straight from the planner's current math.