How to Choose a Grow Light for Starting Seeds Indoors
Windowsill light isn't enough for strong seedlings. Here's the grow light I actually use, how close to hang it, and how many hours a day it needs to run.
My first two winters of seed-starting, I grew every tray on the sunniest windowsill in the apartment and wondered why the seedlings always looked the same way — pale, leaning hard toward the glass, stems so thin they’d flop over if you breathed on them. I thought that was just what indoor seedlings looked like. It isn’t. It’s what seedlings look like when they don’t have enough light, and a January windowsill in the Pacific Northwest almost never has enough.
A grow light fixed that faster than anything else I’ve changed about how I start seeds. You don’t need a grow room or a four-figure setup — you need one small clip-on light, hung close, running on a timer. Here’s exactly how I choose one and set it up on a balcony-sized budget.
Why a windowsill usually isn’t enough
A window filters and weakens light more than it looks like it does. Even a south-facing window loses a big share of usable light through the glass, and a PNW winter sky is overcast more days than not. Seedlings respond to weak light by stretching — reaching for whatever brightness they can find, which is why windowsill starts get tall, thin, and pale instead of short and sturdy.
You can tell a light-starved seedling at a glance: the stem is thin enough to see light through, the space between leaves is stretched out, and the whole thing leans hard toward the glass. None of that is a fixable habit once it’s happened — a leggy seedling stays leggy. The fix is more light, from the start, not a windowsill you rotate more often.
What actually matters when you’re buying one
You don’t need to learn lumens, PAR values, or spectrum charts to start seeds well. Three things matter for a home setup, and none of them require a horticulture degree:
Full spectrum, not a single color. A light labeled “full spectrum” or “daylight” (usually somewhere around 5000–6500K if it lists a number) covers what a seedling needs. Skip the ones sold purely as red-and-blue “flowering” lights — that’s for a later growth stage you’re not at yet.
Clip or clamp mounting. A small clip light with a flexible gooseneck attaches to a shelf, a bookcase, or the edge of a table, which matters if — like me — you don’t have a spare grow rack sitting around. Mine clamps to a wire shelf in a spare closet and moves with the trays as they grow.
Adjustable height. Seedlings need the light close — two to four inches above the leaves for most compact grow lights — and they’ll get taller by the week. A light you can raise as the tray grows saves you from either scorching tiny seedlings or leaving tall ones stretching again.
How close, and how long
This is the part people get wrong even after they’ve bought a good light. Hang it two to four inches above the top of the seedlings, not a foot up like you would with a bright window. Too far away and you’re back to the stretching problem; too close and the leaf tips can scorch, especially on tender starts like tomatoes and peppers.
Run it 14 to 16 hours a day on a cheap outlet timer. Seedlings don’t need round-the-clock light — they actually grow better with a dark rest period — but they need more consistent hours than a window can give you in December and January. I plug mine into a $10 timer and never think about it again until spring.
Feeding what’s under the light
Light solves the leggy-seedling problem, but a seedling still needs the right medium underneath it. Use a real seed-starting mix, not potting soil and definitely not garden soil — it’s lighter, sterile, and drains in a way that keeps young roots from damping off, which is the other thing that kills more indoor seed trays than anything else.
Seedling trays under a grow light dry out faster than you’d expect, because the light adds a little warmth right at soil level. I check mine with a soil moisture meter instead of guessing — trays this small go from soaked to bone-dry in a day, and the surface can look dry while the root zone underneath is still plenty wet. Water from the bottom when you can, setting the tray in an inch of water and letting it wick up; it keeps the tiny stems from getting knocked over and reduces damping-off.
What I skip
I don’t run a full shelf unit with multiple tiers of lights — one clip light over one tray is plenty for a balcony-scale seed-starting habit, and it’s the setup that actually got used every winter instead of gathering dust in a closet after one ambitious February. If you’re only starting a dozen or two seedlings a year, buy the one light, not the four-shelf rack.
I also skip the lights that come bundled with a tiny plastic greenhouse dome and a proprietary tray system. They’re fine, but you’re paying extra for parts you can get separately and cheaper, and the dome usually needs to come off within a week anyway once seedlings germinate and need airflow.
Sturdy little seedlings by transplant time
A grow light is one of the cheapest fixes in container gardening for a problem that otherwise ruins a whole season’s starts before they ever reach a pot. Hang it close, run it long, keep the soil evenly damp underneath, and you’ll set out stocky little seedlings in spring instead of the pale, leaning ones I grew for two winters before I figured this out.
If you’re building out a full indoor seed-starting routine — what to start when, how to harden everything off for a balcony, the whole calendar — I walk through it start to finish in Small-Space Big Harvest. This post is the light. The book is the rest of the system around it. Related reading if you’re setting up a windowsill station generally: how to start seeds indoors on a windowsill and growing herbs indoors in winter. (Otto has already investigated the new closet setup twice and found it unremarkable, which I’m choosing to take as approval.)
