Grow Lights for Seed Starting: Hours, Height, and Watts
Run seedling lights 14 to 16 hours a day. Hang fluorescent tubes 2 to 4 inches above seedlings and LEDs 4 to 12 inches, then raise them as plants grow.
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seed starting grow lights indoor
Grow Lights for Seed Starting
A windowsill is not enough light for seedlings. University extension sources agree that fluorescent or LED lights over the trays give sturdier plants. This guide covers how long to run them, how far above the plants to hang them, and what the watts number means.
Plan your own shelf on the Seed Starting Setup Desk.
How Many Hours
Run lights 14 to 16 hours a day. Iowa State, the University of Maryland, and Oklahoma State all give that range. Minnesota says 12 to 16 hours and Clemson says 16 to 18.
Do not leave lights on all day and night. Plants need a dark period (Minnesota). Plug the lights into an outlet timer so you never have to remember.
How Far Above the Seedlings
Extension sources give different distances, so here is each one.
| Light | Source | Distance |
|---|---|---|
| Fluorescent tubes | Minnesota | No more than 4 in, as close as 2 in is ideal |
| Fluorescent tubes | Clemson | 2 to 4 in, raised as plants grow |
| Fluorescent tubes | Iowa State | 4 to 6 in |
| Fluorescent tubes | Nebraska | No further than 4 to 6 in |
| Fluorescent plant grow tubes | Oklahoma State | About 6 to 8 in |
| LED lights | Iowa State | About 8 to 12 in for many fixtures |
| LED lights | Minnesota | 4 to 6 in for seedlings |
| LED lights | Nebraska | 3 to 6 in above the leaves |
| LED lights | Maryland | 2 to 6 in |
For LED grow lights, follow the maker's height guide first. Fixtures differ.
Start low when seedlings are tiny, then raise the light as they grow. Hang the fixture from chains, or put trays on blocks you can take away one at a time (Iowa State).
What Watts Mean
Watts measure the power a light uses. They do not measure how much light reaches the plants (Minnesota). Look for a PPF number on the label, which measures plant usable light.
For LED, Oklahoma State and Nebraska Extension give a rule of thumb: 16 watts per square foot of grow space for low light plants and 25 watts for high light plants. Multiply your shelf width by its depth in inches and divide by 144 to get square feet. The setup desk does this for you.
Which Bulbs to Pick
- Cool white fluorescent tubes (6500K) and broad spectrum LEDs that mimic daylight are good options (Maryland).
- A cool white tube plus a warm white tube in one fixture gives even growth (Maryland, Nebraska).
- Skip regular incandescent bulbs. They give little blue light, which causes stretching, and they give off heat that can hurt seedlings (Oklahoma State).
- Light fades in the last 3 inches of each fluorescent tube, so a 4 foot tube beats two 2 foot tubes (Oklahoma State).
- Dust the tubes monthly. Dust cuts the light (Oklahoma State).
Setup Tips
- Run a small fan. It carries heat off the fixture (Iowa State).
- Clemson suggests attaching aluminum foil to the fixture to reflect light onto the plants.
- Clemson grows seedlings under 48 inch, 40 watt fluorescent tubes on a timer, and fits four 128 cell trays under one fixture.
- Seedlings that stretch tall and thin need more light. Lack of light is the major cause of elongated, skinny stems (Minnesota). See how to fix leggy seedlings.
Next Steps
- Check your hang height and watts on the Seed Starting Setup Desk.
- Find your sow dates on the seed-starting calendar.
- Learn when you need a heat mat.
- Read how to start seeds indoors.
Sources: University of Minnesota Extension, Starting seeds indoors, and Lighting for indoor plants and starting seeds. Clemson Extension HGIC 1259. Iowa State Extension, supplemental lights for seedlings. University of Maryland Extension, Starting Vegetable Seeds Indoors. Oklahoma State University Extension, Lighting can be tricky when starting seeds indoors, and LED Grow Lights for Plant Production. Nebraska Extension in Lancaster County, Using LED Lights for Houseplants and Transplants, and Nebraska Extension G1650.