How to set up a seed starting shelf, step by step
A shelf, a light you can raise, a timer and a way to keep air moving. The numbers below come from extension sources, with the disagreements noted.
Pick the shelf and measure it
Measure length, depth and the gap between shelves. You need room for the tray, the plants as they grow and the hanging distance of the light. Put your numbers into the calculator to see how many fixtures and trays fit. For a 48 x 24 inch shelf, the University of Saskatchewan shows two 4-foot fixtures. See also the 4-foot shelf page.
Choose a light
Extension sources say plain shop lights work for most home growers if they are bright enough and hang close. See shop lights for seedlings.
Hang the light so you can raise it
Start close to the plants and raise the light as they grow. Distances range by fixture type. See how far above seedlings to hang lights and how to hang lights on a wire shelf.
Set a timer
Most sources say 14 to 16 hours a day. Utah State says 12 to 14 hours or more, and Illinois says 16 with an 18 hour maximum. See hours per day.
Sow and cover
A humidity dome keeps seed trays moist while they germinate. Utah State says to remove the dome once about half of the seeds have germinated.
Add warmth only if needed
Utah State says to use a heat mat for warm-season crops if needed and to unplug it if things get too warm. See heat mats and thermostats.
Add gentle airflow
Utah State recommends a small fan at plant level for short, gentle periods, and letting the soil dry between waterings, to help prevent damping-off. See fans and damping-off.
Watch the plants and adjust
Tall, thin seedlings that lean mean the light is too weak, too far or one-sided. Utah State says to add light close to the plants and rotate them. See leggy seedlings.
Know your running cost
Watts x hours x days / 1,000 gives kWh. See what a shelf costs to run.
Sources
- University of Saskatchewan: Indoor garden, Little Green Thumbs style
- 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: What type of lights should I use to start seedlings indoors?
- University of Maryland Extension: Care of vegetable seedlings