Three things stand in for a grow light. A brighter window, an ordinary LED shop fixture, or a shelf you build yourself. The grow light alternatives that hold up are the ones you match to the job, and your window is the weakest of the three. A 4 foot (1.2 m) full spectrum daylight LED shop fixture rated at 3,000 lumens or more does the work your window cannot.
My pepper seedlings leaned so far toward the glass by week three that two of them folded over at the soil line. That was my first winter of seed starting, before the shelf went up, and that sill was the brightest one in the house. Indoors, even your best windowsill carries a fraction of the light intensity you get outside.
Every substitute has a ceiling you cannot argue with. University of Maryland Extension puts a north window at 25 to 100 foot-candles, an east or west window at 100 to 500, and a south window at 500 to 1,000 foot-candles. Give a tray four to six hours of sun on the leaves and you clear 1,000. Seedlings want the top of that range, so a DIY grow light shelf beats your sill for anything you plan to grow on for weeks.
A Brighter Window
- Best case: A south window with four to six hours of sun on it pushes past 1,000 foot-candles, the only window band that gets you near a fixture.
- Real limit: North and east windows fall far below that, which keeps your pothos going and starves a tray of seedlings.
- Cost: Nothing at all, so save the sill for sprouting seeds and for plants you have already grown to size.
LED Shop Fixture
- What to buy: A 4 foot (1.2 m) full spectrum daylight LED rated at 3,000 lumens or better.
- Why it wins: You get the same output on the fifth cloudy day in a row, and no window in your house can promise that.
- Cost: A finished light cart runs into the hundreds of dollars, while a shop fixture and a frame you already own do the same job.
DIY Shelf
- Frame options: Wire shelving, a plastic shelving kit, 2x4s and plywood, PVC, or a cabinet you were going to give away.
- Reflectors: Mylar or white polymer film on the sides pushes stray light back onto your plants.
- Room to grow: Add a shelf and a second fixture when you fill the first one up.
The frame is where you save. A purchased light cart runs into the hundreds of dollars, and the fixture is the only piece you cannot fake, so you can hang it on almost anything you own. I built mine from wire shelving. A plastic shelving kit works, and so does a frame of 2x4s and plywood, a run of PVC, or a cabinet nobody in your house wants. I lined the sides with Mylar and white polymer film, and the light that would have washed your wall goes back onto your plants.
Match the substitute to the job and you skip half the spending. Seeds that need light to sprout take only 2 to 15 micromoles per second, so a windowsill or a low intensity lamp you run 12 hours a day carries them through germination. South facing window plants that are done growing hold their shape on a sill too. The stretch in between, from sprout to stocky transplant, is where your window quits on you.
So buy the shop fixture first and build the frame out of what you already have. Whether a plain LED bulb gives your plants the right spectrum is a separate question, and I take that one apart in its own article. Of all the grow light alternatives, a north window is the one you should rule out for seedlings. The fixture does the work and your shelf just holds it up.
Read the full article: Grow Lights for Indoor Plants: What Really Works
