A sunny window is not enough for seedlings in February. The light is weak, it comes from one side, and the days are short. Seedlings stretch toward the glass, flop over, and never recover. A grow light fixes all three problems. The good news is that a basic LED light now costs less than a flat of nursery transplants.
This guide covers the lights that work for seed starting, how to hang them, and how long to run them. None of these are high-power lights for growing tomatoes to harvest indoors. They are for the six to eight weeks between sowing and transplanting.
What a seedling needs from a light
Seedlings want bright light for 14 to 16 hours a day, close to the leaves. Brightness drops fast with distance. A light that is plenty at 3 inches (8 cm) is weak at 12 inches (30 cm). That is why cheap LED strips hung low beat an expensive fixture hung high.
Colour matters less than people think. Any full spectrum white LED grows fine seedlings. Purple lights work too, but they make it hard to spot yellowing leaves or pests. I prefer white.
Watts tell you how much power the light uses, not how much light it makes. Compare lights by the area they cover. A 2-foot (60 cm) fixture lights one standard 10 by 20 inch tray. A 4-foot (120 cm) fixture lights two trays side by side.
Best grow light for a shelf of trays: Barrina LED shop lights
Barrina 4-foot LED grow lights come four to a pack, link end to end, and hang from the included chains or clips. Each 4-foot (120 cm) tube lights two trays. Hang two tubes per shelf, 6 inches (15 cm) apart, and you cover a full shelf of four trays. They run cool, so the tubes can sit 2 to 3 inches (5 to 8 cm) above the leaves without scorching.
This is the setup most seed starters settle on after trying something smaller first. Wire shelving units from the hardware store hold them well. Add an outlet timer and the whole shelf runs itself.
For a shorter shelf, the 2-foot Barrina strips cover one tray each and link the same way.
Best grow light for one or two trays: a clip-on lamp
If you start a tray of tomatoes and a tray of peppers, you do not need a shelf. A gooseneck clip-on LED grow light clamps to a table edge or shelf and bends to sit right over the tray. Most have a built-in timer and three brightness settings. Use the brightest setting. The lower settings are for houseplants, not seedlings.
Another simple option is a SANSI full spectrum LED bulb in any desk lamp or clamp lamp. It screws into a standard socket. Point it straight down from 6 to 8 inches (15 to 20 cm) above the tray.
Best all-in-one kit: humidity dome with built-in light
The AC Infinity humidity dome kit stacks a tray, a clear dome, and an LED light in one unit. The light sits in the lid, so it is always the right height. It takes up the space of a shoebox on a counter. Good for a kitchen, an apartment, or a first try at seed starting. You will outgrow it the year you decide to start 200 seedlings, but it keeps working for herbs and lettuce after that.
Best grow light for growing on after transplanting
Once seedlings are potted up into 3 or 4 inch (8 to 10 cm) pots, they want more light than a strip gives. The Spider Farmer SF1000 and the VIVOSUN 100W LED panel are compact panels that cover a 2 by 2 foot (60 by 60 cm) area with real intensity. They are also enough to grow lettuce, herbs, and peppers to harvest indoors. Hang them 12 to 18 inches (30 to 45 cm) above seedlings and dim them to half power for the first week. Both have a dimmer knob. The VIVOSUN store sells the same panel direct if Amazon is out of stock.
How high to hang a grow light
Hang LED strips and tubes 2 to 4 inches (5 to 10 cm) above the tops of the seedlings. Raise the light every few days as they grow. If stems stretch and lean, the light is too high or too weak. If leaves curl down or look bleached, the light is too close.
Panels like the SF1000 are stronger. Start them at 18 inches (45 cm) and lower them week by week. The back of your hand held at leaf height is a fair test. If it feels warm after a minute, raise the light.
How many hours to run grow lights
Run lights 14 to 16 hours a day. Seedlings need a dark period, so 24 hours is worse than 16. A timer is the single best add-on for a grow light. Set it once and forget it.
Turn the light on the day the first sprout shows. Before that, seeds want warmth, not light. A seedling heat mat does that job. The full list of gear is in my seed starting supplies checklist.
Do seedlings need a fan too
Yes, once they have true leaves. A small clip fan on low keeps the air moving, which prevents damping off and makes stems thicker. Run it a few hours a day. Point it across the trays rather than straight at them.
What a grow light costs to run
A 4-foot (120 cm) LED strip draws about 20 watts. Two strips running 16 hours a day use roughly 0.6 kilowatt hours, which is less than a dime a day in most of North America. A full shelf of eight strips for eight weeks of seed starting costs about the same as one flat of nursery seedlings. The light pays for itself the first spring and keeps paying every year after.
Older fluorescent shop lights still work, and plenty of gardeners run them. They use about twice the power, the tubes dim after a couple of seasons, and they give off more heat, so keep them a bit higher. If you already own them, use them. If you are buying new, buy LED.
What to grow under these lights
Tomatoes, peppers, eggplant, onions, leeks, celery, and most flowers started indoors all do well under a strip light. Read when to start seeds indoors for timing. Microgreens and sprouts grow fine under the same light all winter, so the shelf earns its keep in the off season. My post on pendant grow lights covers the prettier fixtures for houseplants in living spaces.
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