Grow lights for a seed starting shelf: how to size, hang and time them
A shelf of seedlings needs lights that cover the whole shelf, hang close to the plants, and run on a timer for most of the day. The numbers vary by light type, so this guide shows the ranges university extension services give and where they differ.
The short answer
A common layout is two 4-foot fixtures over a 24 x 48 inch shelf, which is how the University of Saskatchewan's classroom setup works. Hang the fixtures from chains so you can raise them as the seedlings grow, and put them on a timer for 14 to 16 hours a day. Everything else is detail about those three choices.
To get a count for your own shelf, use the seed starting shelf calculator.
How many lights does a shelf need?
Coverage matters more than total wattage on a shelf. The fixtures have to reach every tray, so the question is how many rows of lights fit across the shelf depth and how many fixtures fit along its length.
Our calculator assumes about one row of fixtures for every 12 inches of shelf depth. That is our own planning rule, drawn from shelf examples such as the Saskatchewan setup, not an extension standard. A 24 inch deep shelf gets two rows, an 18 inch shelf gets two, and a 12 inch shelf gets one. You can change the row width in the calculator if your lights have a wider or narrower spread.
Saskatchewan's guide notes that fruiting plants grown to maturity, such as tomatoes, may want up to four lights on a shelf rather than two.
How many hours a day?
Iowa State, UConn, Maryland and Oklahoma State all give 14 to 16 hours a day for seedlings. Two details are worth knowing:
- One Iowa State answer about a standard two-tube fluorescent shop fixture says 12 to 14 hours.
- UConn says herbs and other greens can do with about 12 hours, and that plants need some hours of darkness rather than 24 hours of light.
Light amount depends on both intensity and time. A UNH Cooperative Extension fact sheet reports that one LED light bar it tested needed about 8 hours a day at 8 inches above the crop, but 16 hours a day at 20 inches, to give the same total light. Closer lights can do the same job in fewer hours. That result is specific to that fixture, so treat it as an illustration, not a rule.
How close should the lights hang?
Extension sources do not agree on one distance, because fixtures differ in output. Here is what they say:
| Source | Light type | Distance above seedlings |
|---|---|---|
| University of Maryland | Fluorescent | 1 to 2 inches |
| Iowa State | Fluorescent | 4 to 6 inches |
| Oklahoma State | Fluorescent | About 6 to 8 inches |
| Iowa State | LED | About 8 to 12 inches |
Rule of thumb: follow the manual for your fixture, start close, and raise the light as the seedlings grow. Iowa State and Mississippi State both recommend hanging lights from chains or cords so the height is adjustable. Leggy, stretched seedlings usually mean the light is too weak or too far away.
Do you need a "grow light"?
Often you do not. Iowa State says a plug-in fluorescent or LED shop fixture is a good choice for most home gardeners and that grow lights are often more expensive and not necessary. For LED shop fixtures it suggests full-spectrum models of at least about 3,000 lumens. Mississippi State says shop lights work well for starting seeds, with 2,000 to 3,000 lumens as a minimum, and notes that shop lights need to sit close to the plants.
The spectrum matters less than people think for seedlings. Iowa State explains that a full-spectrum bulb gives enough red and blue light for seedling growth at a lower cost than a purple-looking grow light.
A note on short tubes and bars
Oklahoma State points out that light intensity drops off over roughly the last 3 inches at each end of a fluorescent tube, so one 4-foot tube lights more usable area than two 2-foot tubes end to end. That finding is about fluorescent tubes. Check the manufacturer's coverage information before assuming the same for short LED bars.
Timer and airflow
Use a timer so the photoperiod is the same every day. Iowa State recommends a timer set for 14 to 16 hours and also advises good air circulation, partly to carry away heat from the fixtures. A small fan on low is enough. Check that the timer is rated for the total load of the fixtures plugged into it and that its plug type matches.
Do seeds need light to germinate?
Some do and many do not. Iowa State advises putting trays under lights right after sowing only if the seed needs light to germinate. Otherwise, turn the lights on once the seedlings emerge.
What to check before buying a light
- Length. A 4-foot fixture suits a 48 inch shelf. Measure the shelf, not the package.
- Brightness. For LED shop fixtures, look for full-spectrum white light and at least about 3,000 lumens, per Iowa State.
- Adjustable hanging. Chains or cords so you can raise the light as plants grow.
- Linking. If you will plug several fixtures into one timer, check how many the maker allows to be linked.
- Power. Add up the wattage of all fixtures and confirm the timer or power strip can carry it.
- Keep water away. Seedling trays get watered. Route cords and fixtures so drips cannot reach them.
Common mistakes
- Hanging lights too far above the seedlings, which leads to stretched, weak stems.
- Switching lights by hand, so the day length changes from one day to the next.
- Judging a light by wattage alone. Wattage tells you what the fixture draws from the wall, not how much usable light reaches the plants.
- Buying a light that is much shorter or longer than the shelf.
Go deeper
- How many grow lights a 4-foot wire shelf needs and for a narrow 24-inch shelf
- How many seed trays fit on a wire shelf
- How far above seedlings to hang lights and how many hours to run them
- Can you use shop lights? and LED vs fluorescent
- Leggy seedlings, heat mats and fans
- Step by step shelf setup, seed starting mix, watering, fertilising and hardening off
- When to start seeds indoors (frost-date calculator), germination temperature chart and seedling problems
- By crop: tomatoes, peppers, lettuce and herbs
- What a shelf costs to run
- Buying guides: 4-foot shelf lights, T5 LED bars, heat mats, trays and domes, gear shortlist
Every page is also listed on the site map.
Sources
- 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?
- 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)
- UNH Cooperative Extension (Jonathan Ebba): Growing seedlings under lights (PDF)
- Mississippi State University Extension: LED lights for starting seeds indoors
- Mississippi State University Extension column, "Shop lights are an economical way to sow seeds indoors" (reprinted in the Sequim Gazette)
- University of Saskatchewan: Indoor garden, Little Green Thumbs style
Extension pages change. If a number here differs from the current page, trust the source.