Grow Lights for Seedlings

Vegetable seedlings need far more light than a windowsill gives them. Extension services recommend running lights 12 to 16 hours a day. Fluorescent tubes should sit just a few inches above the leaves. LEDs vary so much in output that no single hanging height is right for all of them. If your seedlings are tall, pale and floppy, University of Minnesota Extension puts it plainly: "Lack of light is the major cause of elongated, skinny stems."

How much light seedlings actually need

Plant scientists measure light for growth as the daily light integral (DLI): the total amount of photosynthetically useful light a plant gets in a day, in moles per square meter per day. Purdue's Ariana Torres and Roberto Lopez describe DLI in their guide to measuring daily light integral as a function of both intensity and duration. That means a dim light for many hours can deliver as much as a bright light for a few. Outdoors across the U.S., they report daily values from 5 to 60 mol·m⁻²·d⁻¹ depending on location and season.

UNH Extension's Growing Seedlings Under Lights fact sheet uses a target of about 12 mol·m⁻²·d⁻¹ for sun-loving seedlings and compares how long different setups take to deliver it:

Two lessons follow. First, ordinary fluorescent shop lights are weak, which is why they have to be run close and long. Second, moving the same LED from 8 to 20 inches doubled the hours needed. Light intensity, UNH notes, "decreases rapidly" with distance.

You can estimate this for your own setup if you know a light's rated intensity at a given distance (its PPFD, in µmol·m⁻²·s⁻¹). Purdue's conversion multiplies a 24-hour average by 0.0864. For a light on only part of the day, the same arithmetic works out to PPFD × hours on × 0.0036. A light delivering 200 µmol·m⁻²·s⁻¹ at leaf height for 16 hours gives about 11.5 mol·m⁻²·d⁻¹.

A 12-mole target is a reasonable goal, not a hard requirement. Purdue notes that propagation of young plants has benefited from as little as 4–11 mol·m⁻²·d⁻¹ in greenhouse work. Seedlings under less light will grow, just more slowly and with longer stems.

Why a windowsill usually isn't enough

UMN Extension discourages windowsills for seed starting. Its reasons are that the light is weak compared with artificial sources, the temperatures swing to extremes, and the seedlings "tend to be too tall, with thin, bent stems." Light through a window comes from one side and only for part of the day. It also comes in late winter and early spring, when days are short and the sun is low.

Distance and hours: what extension services say

The close-distance advice was written mostly with fluorescent tubes in mind. For LEDs, the honest answer is that it depends on the fixture. A Minnesota Master Gardener answering a reader through Ask Extension acknowledged that published guidance is "contradictory." She suggested starting close and raising the light if seedlings look bleached.

Use the plants as your meter:

• Stretched, pale, leaning: not enough light. Lower the light or add hours.
• Bleached or whitish patches on top leaves: too intense or too close. Raise the light.
• Short, stocky, deep green: about right.

Keep the light close as plants grow. UNH advises keeping lamps "close to the tops" and raising them as seedlings get taller. Chains or adjustable hangers make that easy.

LED vs. fluorescent

K-State's comparison of lighting options for seed starting is a useful reality check:

• Some LEDs produce "more than twice the amount of light per watt" of fluorescents, and LEDs last two to four times longer.
• For a home gardener running a couple of lights for a few weeks, the energy savings may be only "a few dollars." Fluorescent bulbs rated for 10,000–20,000 hours are typically used under 1,000 hours a season.
• For fluorescents, cooler "5000-6500K" bulbs are recommended. A standard 10-by-20-inch tray needs about four tubes, or two shop light fixtures, to cover it.
• Some LED models degrade if run more than 18 hours a day.

Either kind works if it delivers enough light. UMN notes that ordinary "cool white" fixtures are adequate, and you don't need expensive "grow" branded lights.

Other setup details

• Use a timer. UMN recommends one so lights turn off on schedule. Don't plug a heat mat into the light timer. UMN specifically advises against it.
• Temperature. UMN recommends air above 60°F. Utah State suggests nights of 60–65°F with days about 10 degrees warmer for most vegetables, and cooler conditions (about 55°F nights, 65°F days) for cool-season crops.
• Air movement. The Ask Extension answer mentions a small fan across seedlings to strengthen stems and reduce fungal disease. That comes from a volunteer's answer, not a trial.
• Timing. Starting too early leaves you with overgrown plants waiting for warm weather. UMN's Minnesota timing is about 5–6 weeks indoors for tomatoes and 9 for peppers.

Once seedlings are stocky and a few weeks from transplant, the next step is Hardening Off Transplants. For germination temperatures, see Soil Temperature for Germinating Vegetable Seeds. The whole spring sequence is in Seed Starting and Season Extension.