How far above seedlings should grow lights hang?
It depends on the type of light. Extension sources give anything from 1 inch to 12 inches because they are talking about different fixtures.
What the sources say
| Source | Light | Distance |
|---|---|---|
| University of Maryland | Fluorescent | 1 to 2 inches above the plants |
| Utah State | Fluorescent shop lights | 2 to 3 inches, and watch for leaf scorch from lights that give off more heat |
| University of Illinois | Fluorescent | A few inches above the plant tops |
| Iowa State | Fluorescent | 4 to 6 inches |
| Oklahoma State | Fluorescent | 6 to 8 inches |
| Iowa State | LED | About 8 to 12 inches |
| University of New Hampshire | LED bar example | 8 hours a day at 8 inches, or 16 hours a day at 20 inches |
Why the numbers differ
Light gets weaker quickly as you move away from the fixture, so a distance only makes sense for one fixture. The fluorescent figures are the closer ones, and LED bars vary in brightness. The UNH example shows the trade: a farther bar needs more hours to give the same light.
A practical way to set the height
- Start with your fixture's manual if it has a distance. Use the sources above only when it does not.
- Put the lights near the lower end of the range when seedlings first sprout, then raise them as the plants grow. Utah State and Illinois both describe raising lights as seedlings grow.
- Check the leaves after a few days. Scorched or bleached leaves mean too close. Tall, thin stems leaning toward the light mean too far or too dim (see leggy seedlings).
- Hang the fixture on something adjustable such as chains or cord ratchets, so you can change the height without taking it down.
Fluorescent tube ends
Oklahoma State says the ends of a fluorescent tube lose roughly 3 inches of intensity. Do not put seedlings in the last few inches under each end of a tube.
Shelf height
You need room to raise the light. If seedlings will grow to 6 inches before transplanting, the space between the tray and the shelf above needs to allow for the plant height plus the hanging distance for your light.
Sources
- University of Maryland Extension: Care of vegetable seedlings
- Utah State University Extension: Starting vegetable seeds indoors, seeding culture and transplanting (2025)
- University of Illinois Extension: Starting the garden, supplemental lighting for indoor seed starting (2020)
- Iowa State University Extension: How do I set up supplemental lights for my seedlings?
- Iowa State University Extension: What type of lights should I use to start seedlings indoors?
- Oklahoma State University Extension: Lighting can be tricky when starting seeds indoors (January 2024)
- UNH Cooperative Extension (Jonathan Ebba): Growing seedlings under lights (PDF)