Can you start seeds in a sunny window?

You can germinate almost anything in a window, and you can grow quick, short-duration seedlings there such as lettuce or brassicas destined to go out in three weeks. What a window struggles with is a tomato or a pepper held indoors for six to eight weeks in late winter, because the day length is short, the sun angle is low, and glass filters part of the spectrum. The result is a tall, pale, weak plant that transplants badly even though it germinated perfectly.

Do seeds need light to germinate?

Most vegetable seeds do not — they need warmth and moisture, and germinate perfectly in the dark under a covering of medium. A few small-seeded crops such as lettuce, celery and some herbs are light-dependent and should be surface-sown or barely covered, which is why seed packets specify. The light requirement begins the moment the first seedlings break the surface, and that is the point at which a tray on a shelf in a dim room starts stretching within a day or two.

How far above seedlings should a grow light be?

Two to four inches for a typical LED or T5 fluorescent shop light, raised as the seedlings grow to hold that gap. That sounds close, and it is: the intensity of light falls with the square of the distance, so a fixture at 12 inches delivers roughly a ninth of the light it delivers at 4 inches. If leaves start to bleach or look scorched, raise it an inch. Higher-output horticultural fixtures need more clearance, so follow the specification for the model.

How many hours a day should grow lights run?

Twelve to sixteen hours, on a timer. More than about sixteen offers little benefit for vegetable seedlings and some plants need a dark period, while less than twelve tends to produce stretch in late winter. Sixteen hours is a good default for tomatoes, peppers and eggplant, and fourteen suits brassicas and lettuce. A cheap outlet timer is the accessory that makes the whole system work, because manual switching inevitably slips.

Are expensive full-spectrum grow lights worth it for seedlings?

For seed starting, no. Seedlings are not fruiting, so they do not need the light intensity a flowering plant does, and an ordinary LED shop light producing a bright, broadly white output at a few inches above the canopy grows excellent seedlings. Spend the money on a second fixture to cover more trays rather than on a higher specification over one. Where higher output does earn its price is growing plants indoors to harvest, which is a different job.

Do You Need Grow Lights to Start Seeds?

Written & researched by Elena Marsh Gardening & Landscaping Editor

Updated October 2026

Overhead view of a person harvesting ripe tomatoes into a basket in a backyard vegetable garden, in soft early-morning light

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If you are starting seeds indoors more than about four weeks before transplanting, yes — you need a grow light. A bright south-facing window in late winter supplies only a fraction of the light a seedling needs, because the days are short and the sun sits low, and glass cuts it further. Seedlings respond by stretching: long pale stems, widely spaced leaves, and plants that flop and transplant poorly. A basic LED or T5 shop light held 2 to 4 inches above the canopy for 12 to 16 hours a day solves it completely.

When is a window actually enough?

There are real cases where you can skip the light, and it is worth knowing them rather than buying equipment you do not need.

Situation	Window sufficient?	Why
Germinating seeds	Yes	Most seeds need warmth, not light
Lettuce or brassicas, 3 weeks indoors	Usually	Short indoor period, tolerant crops
Tomatoes, 6 - 8 weeks in Feb or Mar	No	Long hold in the darkest part of the year
Peppers or eggplant, 8 - 10 weeks	No	Longest indoor period of any crop
Unobstructed south window, April sowing	Often	Longer days, higher sun angle
East or north window, any season	No	Too few hours of direct light
Heated greenhouse or sunroom	Yes	Overhead light from all directions
Cold frame from late March	Yes	Full outdoor light with frost protection

A useful test costs nothing: put a tray in the window and check whether the seedlings lean noticeably toward the glass within two days of emerging. Strong phototropic lean means they are hunting for light they are not getting, and rotating the tray daily manages the lean without fixing the cause.

Wide view of a person harvesting ripe tomatoes into a basket in a backyard vegetable garden, in soft early-morning light

What kind of light do I actually need?

Seed starting is one of the least demanding indoor growing jobs, because seedlings are building leaves rather than fruit. Three things matter, in this order.

Proximity. Light intensity falls with the square of distance, so a fixture at 4 inches delivers roughly nine times what the same fixture delivers at 12 inches. Getting close is worth more than any upgrade in fixture quality.
Duration. 12 to 16 hours daily on a timer. Sixteen suits tomatoes, peppers and eggplant; fourteen suits brassicas and greens.
Coverage. A 4 foot shop light covers two standard 10 by 20 trays. Seedlings at the ends of a fixture always stretch more than those in the middle, so allow overhang.

An inexpensive LED shop light with a bright, broadly white output does the job. Purple-heavy blurple panels work but make it hard to judge leaf color, which is a genuine practical disadvantage when you are trying to spot nutrient problems or disease. Specialty full-spectrum horticultural fixtures are built for flowering and fruiting plants and are overspecified for a seedling tray; buy a second ordinary fixture instead and cover more trays.

Low-angle view of a person harvesting ripe tomatoes into a basket in a backyard vegetable garden, in bright midday sun

How should the light be set up?

Hang the fixture on chains or an adjustable frame so you can raise it weekly. Fixed-height setups always end up too far away.
Set the gap to 2 to 4 inches above the tallest leaves and keep it there as the seedlings grow.
Put it on a timer. This is the single component most likely to be skipped and most likely to cause failure.
Add reflective sides if you have them — white board or foil on the outer edges reduces the stretch of the outermost seedlings.
Run a small fan a few hours a day. Air movement thickens stems and reduces damping off, which is the other main cause of seedlings collapsing, covered in why seedlings fall over.
Drop night temperature a few degrees. Warm nights promote stem elongation, so cooler nights make stockier plants.
Rotate trays every few days so every seedling gets time near the center of the fixture.

Close-up detail of the material being worked in a seeds & vegetable growing project, in soft overcast light

What does it cost to run?

Less than most gardeners expect, which is the argument that usually settles the question. A typical LED shop light of the kind used over a seedling shelf draws roughly 40 watts. Run 16 hours a day for eight weeks, that is around 36 kilowatt-hours, or about $6 at the US average residential rate of roughly 17 cents per kilowatt-hour reported by the Energy Information Administration. Two fixtures over four trays for a full seed-starting season still comes in around the price of a single flat of nursery transplants.

The saving on transplants is the real return, along with access to varieties no garden center carries. If you are setting up from scratch, the equipment comparison is in best seed starting kits, the sowing sequence in how to start seeds indoors, and timing by region in when to start seeds by zone. If your current seedlings are already stretched, the rescue is in fixing leggy seedlings, and the transplant timeline is in when to move seedlings outside.

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