How many hours a day to run seedling lights
Most extension sources say 14 to 16 hours a day. A few give lower numbers, and the reasons are worth knowing.
What the sources say
| Source | Hours per day |
|---|---|
| Iowa State, UConn, University of Maryland, Oklahoma State | 14 to 16 hours for seedlings |
| University of Illinois | About 16 hours, with 18 as the most for seedlings |
| Utah State | 12 to 14 hours or more for actively growing seedlings |
| Iowa State (standard fluorescent shop fixture) | 12 to 14 hours |
| UConn (greens and herbs) | About 12 hours can be enough |
| University of New Hampshire (LED bar example) | 8 hours at 8 inches, or 16 hours at 20 inches |
How to choose
- With lights a few inches above the plants, 14 to 16 hours is the common answer.
- If the light is far from the plants, as in the 20 inch UNH example, the plants need more hours to make up for lower intensity.
- Use a timer. Illinois and Utah State both recommend one, and the dark period is part of the schedule.
- Do not run lights around the clock. The sources set maximums, and Illinois gives 18 hours as the most for seedlings.
Setting up a timer
A basic plug-in timer is enough for one shelf if the total power is within the timer's rating. Add up the watts on the shelf with the calculator and compare with the rating printed on the timer. If you link several bars, they can usually run from one plug, within the limits on the fixture.
Sources
- Iowa State University Extension: How do I set up supplemental lights for my seedlings?
- Iowa State University Extension: Will a fluorescent light fixture provide sufficient light for seedlings?
- UConn Home & Garden Education Center: Grow lights
- University of Maryland Extension: Care of vegetable seedlings
- Oklahoma State University Extension: Lighting can be tricky when starting seeds indoors (January 2024)
- University of Illinois Extension: Starting the garden, supplemental lighting for indoor seed starting (2020)
- Utah State University Extension: Starting vegetable seeds indoors, seeding culture and transplanting (2025)
- UNH Cooperative Extension (Jonathan Ebba): Growing seedlings under lights (PDF)
Related
- How far above seedlings should grow lights hang
- What a seed starting shelf costs to run
- Seed starting shelf gear shortlist
- Can You Use Regular Shop Lights for Seedlings?
- Leggy Seedlings: Is It the Light?
- Cool White vs Red-Blue Grow Lights for Seedlings
- LED vs Fluorescent for Seed Starting
- Seed starting shelf light calculator
- Main guide to grow lights for a seed starting shelf