Grow Lights for Vegetable Seedlings

There is a useful simplification available here that most vegetable seed-starting advice buries: at the seedling stage, brassicas, lettuce, squash, peppers and tomatoes all want about the same light. They diverge enormously later, in the garden. Under lights, in a tray, for six weeks, they do not.

University of New Hampshire Extension puts the seedling target at about 12 moles per square meter per day. That figure applies across the tray, which means you can light a mixed sowing with one fixture and one setting, and stop trying to zone your shelf by crop.

What actually differs between crops

Not the intensity — the timetable. Different vegetables reach transplant size at very different rates, and that is what determines how much shelf you need and for how long.

The practical consequence: your shelf is not one crop for eight weeks, it is a rolling succession. Size the light for a full tray, then plan which crop sits where as things come and go.

Sizing the fixture

Work backwards from the target. A daily light integral of 12 mol/m²/day at a given photoperiod implies a specific PPFD, and PPFD across your area implies a photon output.

Bars, for the same reason as always

Seed trays are long rectangles and shelves are long rectangles. A square panel centered over a 4 ft shelf lights the middle well and the ends poorly, and the seedlings at the ends will tell you so within a week by leaning. Linear bars put emitters along the whole length.

The trade-off, stated plainly: Barrina publishes no photon output for any strip in its T5 range, so a bar-based shelf cannot be sized in advance the way a panel can. We still pick it first here, because the geometry advantage is large and seedlings give you feedback within days — the full argument is in bar versus panel coverage.

Photoperiod

University of Minnesota Extension recommends 16 to 18 hours a day for seedlings; Iowa State University Extension suggests 12 to 14 generally with 16 as a ceiling for home growers. For vegetables we would run 14 to 16 and put it on a timer, which lands inside both recommendations.

The reason to prefer the longer end is economic rather than botanical: hours are the cheapest way to raise daily light. Two extra hours a day on a 120 W rack costs about $6.60 across a ten-week season — far less than the fixture you would otherwise buy to raise intensity instead.

Distance, and the one crop that punishes you

Keep bars 2 to 4 inches above the seedling tops and raise them as the plants grow. Tomatoes are the canary: they stretch within days of being under-lit and the stretched stem never shortens. If your tomatoes are leggy, every other crop on that shelf is also short of light — they are just hiding it better.

Crop-specific detail for the most common one is in grow lights for tomato seedlings, and the general distance arithmetic is in how far grow lights should be from seedlings.

What a vegetable season costs to light

That figure is worth sitting with, because it is the strongest argument for starting your own vegetables at all. Price your own rack in the running-cost calculator.

Hardening off, which the light cannot do for you

A seedling grown under a fixture at 238 µmol/m²/s has never seen full sun, which on a clear day is several times that. Moving trays straight outdoors bleaches and scorches them regardless of how well they were lit indoors. Step them into outdoor conditions over a week to ten days — this is not a lighting problem and no fixture solves it.

The honest summary

One target, one fixture, one timer, for the whole mixed tray. Size for about 166 µmol/s over a standard shelf at 14 hours, use bars because your shelf is a rectangle, keep them close, and watch the tomatoes for the first sign that anything is wrong. The whole season costs less than a flat of nursery transplants.