How to Choose a Grow Light for Seedlings

Seedlings don't need expensive grow lights. Plain white LED shop lights hung 2–4 inches above the plants and run 12–16 hours a day grow stocky transplants. Here's what to buy and how to set it up.

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Why a window isn't enough

Most seeds are started in late winter, when days are short and the sun is low. Even a south window gives seedlings far less light than they'd get outdoors, and only from one side. They stretch toward the glass and grow tall, pale, and weak: "leggy." A light directly overhead fixes that, and it's the single biggest upgrade to a seed-starting setup. When to start each crop depends on your frost date; see when to start seeds indoors.

What kind of light to buy

• White LED shop lights: the best value for seedlings. A plain 4-foot fixture with daylight-white LEDs is inexpensive, runs cool, and gives seedlings what they need. Fluorescent tubes work too; University of Minnesota Extension notes that a standard two-tube "cool white" fixture gives adequate light.
• LED grow light bars or panels: brighter, often adjustable, and sized for shelving. Worth it if you start a lot of seedlings or want to grow herbs or greens under lights past the seedling stage.
• Skip the purple lights for seedlings. Red-and-blue "blurple" lights work, but they make it hard to see whether plants look healthy, and full-spectrum white light grows seedlings just as well.
• Skip single clip-on bulbs for anything more than a few pots. They light a small circle and leave the edges of a tray stretching.

How many lights you need

A 4-foot fixture spans two standard 10×20-inch seed trays laid end to end. For wider shelves, hang two fixtures side by side about 6 inches apart. Plan one fixture per pair of trays and you'll rarely go wrong.

How close to hang them

Closer than most people expect. University of Minnesota Extension recommends keeping lights no more than 4 inches above the tops of the seedlings, with 2 inches ideal, for shop-light-style fixtures. Brighter grow panels may list a larger distance; follow the manufacturer for those.

Seedlings grow fast, so the light has to move up with them. Hang fixtures from chains or adjustable rope hangers so you can raise them an inch at a time.

How long to run them

Seedlings need 12 to 16 hours of light a day, and a dark period each night. Don't leave lights on around the clock. An outlet timer turns them on and off so you don't have to remember. Plug a heat mat into a separate outlet, not the light timer: it should stay on at night.

Signs the setup is wrong

• Tall, thin stems leaning sideways: not enough light. Lower the fixture or add a second one.
• Pale, bleached, or crispy spots on the top leaves: the light is too close or too strong. Raise it an inch or two.
• Seedlings at the tray edges leggier than the middle: the fixture is too narrow for the tray. Add another light or rotate trays daily.

The rest of the setup

Lights are one piece. Warm soil for germination, a sterile mix, trays that drain, and a little air movement matter too. See seed-starting supplies: what you actually need.

Sources

• Starting seeds indoors, University of Minnesota Extension
• Grow Your Own Transplants at Home, Utah State University Extension