Grow Lights for Seed Starting

Seed starting is the one indoor-growing situation with a hard number attached. University of New Hampshire Extension puts the target at about 12 moles of light per square meter per day, and everything else — which fixture, how high, how many hours — is arithmetic downstream of that figure. It is why this section leads with numbers rather than with products.

The reason seedlings are unforgiving is that they have no reserves. A mature houseplant under-lit for a fortnight looks slightly duller. A tomato seedling under-lit for four days is permanently stretched: it throws its energy into stem length reaching for a brighter source, and you cannot shorten a stem afterward. Nearly every seed-starting disappointment traces back to that first week.

How this category divides

Three form factors, and the right one is decided by the shape of what you are lighting, not by brand:

• Linked bars and strips. A 4 ft bar matches a 4 ft shelf, and most link in series so a whole rack runs off one plug. This is the default for trays, and it is what most people should buy.
• Panels. Square output over a square area, usually 100 to 150 W with a dimmer. Excellent over a dense block of pots, wasteful over a long shallow shelf, and generally too intense at close range for bare seedlings unless dimmed.
• Bulbs and clip lights. One pot, one desk. They have a genuine place; they are simply not a tray solution, and buying one for a full tray is the commonest expensive mistake in this category.

What decides the price

Very little of it is spectrum. The price of a seed-starting fixture tracks three things: how many photons it produces, how efficiently it produces them, and whether it has a dimmer. Efficacy across the fixtures we have published specs for runs from 1.39 to 2.7 µmol/J — nearly a factor of two on the same electricity bill.

But because a seed-starting season is only about eight weeks, that gap works out at roughly $15 over the season at the US average electricity rate. For seed starting specifically, the cheaper fixture usually wins on total cost. Efficiency matters far more when you run the same light all winter for houseplants.

The mistake nearly everyone makes

Hanging the fixture too high, and leaving it there. Intensity falls with roughly the square of distance, so a light at 12 inches delivers about a quarter of what it delivers at 6. UNH Extension puts the practical consequence starkly: the same LED bar needs 8 hours a day at 8 inches, but 16 hours a day at 20 inches to deliver the same daily total. Hang everything on adjustable chains and raise it weekly — that one habit does more for seedling quality than any upgrade.

If you want to check a specific fixture against your own tray before buying, the coverage calculator does the division, and PPFD targets by plant type gives the target for everything else that might share the shelf.