Grow Lights for a Room With No Windows

With no window, the fixture has to deliver the plant’s entire daily light integral rather than top up what the glass already gave you — roughly twice the work a winter supplement does. Because the consequence is long hours rather than brute intensity, the fixture we would buy is the AC Infinity IONBOARD S22: at a published 2.7 µmol/J it is the cheapest thing here to run for the fourteen to sixteen hours a windowless room requires.

We have not tested these fixtures. The sizing below is published specification and extension targets, divided out in view.

What changes when the window is gone

Every other sizing page on this site solves a subtraction: your window supplies some light, the plant wants more, and the fixture covers the difference. In a basement, an interior office or a hallway, the first term is zero. The fixture is not a supplement. It is the whole supply, and it is the only thing between the plant and the dark.

That has three consequences, and only the first is obvious.

• The fixture gets bigger. If your window had been supplying half the target — a fair description of a decent north room in winter — the windowless version does exactly twice the work.
• Reliability starts to matter. A supplement that fails for a week is a dim week. A sole light source that fails for a week is a week of darkness, which is why a timer you can hear clicking and a plant you walk past daily are worth more here than anywhere else.
• Hours become the cheapest lever you have. More on this below, because it is the part that saves money.

The sizing, for a shelf

Pick the target honestly. Iowa State University Extension places low-light foliage houseplants at 3 to 6 mol/m²/day and foliage and flowering houseplants at 6 to 10. A windowless room suits the first group, and attempting the second is a decision to run a much larger fixture for longer.

For a single plant rather than a shelf, the figure drops sharply, because the area does.

Hours are the cheapest photons in the building

The daily total is intensity multiplied by time, and in a windowless room you control both. Time is far cheaper.

There is a limit, and it is not a cost limit. The dark period is a physiological requirement, not a courtesy: plants use it, and running a light around the clock damages many species rather than accelerating them. Fourteen to sixteen hours is the working range for foliage, and the published guidance is set out in grow light hours per day. Never twenty-four.

Which plants will accept a windowless room

A fixture reliably moves a dark room into the low-light band. It does not reliably move it into the bright one, and that distinction decides what you should put in the room.

The practical reading: a windowless room is an excellent home for the plants that evolved on a forest floor, a workable one for a flowering houseplant if you are willing to run a board, and a poor one for anything you intend to eat. Flowering specifically needs a band more than surviving does, which we work through in will a grow light make my plant flower.

If the room is a basement

Basements add two variables that have nothing to do with light, and both can make a correctly sized fixture look like a failure.

Temperature. An unheated basement often sits in the mid to high fifties Fahrenheit. Tropical foliage slows markedly there regardless of how much light it receives, so a plant that stops growing in a lit basement may be cold rather than dark. This is also why a basement is a genuinely good place to overwinter dormant plants, where slow is the objective — that case has its own page.

Damp. Many basements are damp locations in the electrical sense. Of the fixtures here, the SuncoGrow strips and the GE fixture publish damp ratings; the LED boards publish nothing. If your basement has a dehumidifier running, treat the rating as a requirement rather than a nicety — the same table that governs a greenhouse applies, and it is in grow lights for a greenhouse.

If the room is a windowless office

The constraint here is not the plant, it is the person sitting next to it for eight hours.

A fixture sized for a shelf of foliage is bright enough to be unpleasant at eye level, so the two things worth paying for are dimming and a position above the sight line. A board under a shelf aimed down at the plant is comfortable; the same board on a desk aimed across one is not. On looking at the fixture itself, the published photobiological-safety position is covered in are grow lights safe for eyes and pets.

The other office-specific problem is the schedule. A plant wants fourteen hours and the building may switch the circuit off at seven. Put the light on its own plug timer rather than the room’s switch, confirm the socket is not on a lighting circuit the cleaner kills at night, and choose the schedule for the plant rather than the working day.

1. AC Infinity IONBOARD S22 — long hours, lowest cost per photon

A published 2.7 µmol/J, which clears the 2.5 µmol/J commercial efficacy minimum and is the highest figure anyone here publishes, with dimming in 20% steps. In a room where the fixture runs sixteen hours a day, twelve months a year, efficacy is not an abstraction — it is the only specification that compounds.

Best for: a 2 ft square of plants that will be lit every day indefinitely, where the running cost over years exceeds the purchase price.

The downside: AC Infinity publishes no PPF in µmol/s at all, and its “1098 PPFD” claim carries no measurement distance, so it is the one fixture here you cannot size directly — we back out roughly 311 µmol/s from the efficacy and the wattage, and label that as ours. No location rating either, which matters in a damp basement.

2. Spider Farmer SF1000 — one shelf, output you can verify

249.21 µmol/s published, at a published 2.5 µmol/J that the division confirms. Against our 92 µmol/s shelf requirement it is two and a half times oversized, which with a dimmer is the right kind of wrong: turn it down, or hang it higher and cover more shelf.

Best for: anyone who wants to compute the answer before buying, and a fixture with room to grow into.

The downside: no PPFD at any distance, so the height has to come from your own division rather than a published curve. No location rating. And at 100 W run sixteen hours it is $8.80 a month, which is the real cost of a windowless room: the light never gets a summer off.

3. SuncoGrow 4ft 40W — a long bench, a strip at a time

48.42 µmol/s published per strip, damp-rated, ETL listed, and linkable up to four from one plug. For a windowless room the incremental shape is genuinely useful: start with two strips over a shelf, measure the plants’ response, add a third if growth is thin.

Best for: wire shelving in a basement, where the shelf is long and thin and the room is damp.

The downside: 1.21 µmol/J is the lowest efficacy on this site, and in a room where the light runs all year that is the worst place to accept it — two strips at 80 W for 16 hours is $7.04 a month against $8.86 for a board producing three times the light. Non-dimmable, so height is your only control.

4. SANSI 36W BR30 — one plant in an interior corner

65.6 µmol/s and, uniquely among bulbs, a published PPFD with the distance attached — 265.58 µmol/m²/s at 1 ft. For a single plant in a hallway or a dark corner with a lamp already in it, that is the whole solution, and the only fixture here you can place by arithmetic before you buy.

Best for: one plant, one lamp, in a room with no shelf to light.

The downside: 1.82 µmol/J, well under the commercial bar, and SANSI states that it is “not recommended to be used in tightly closed fixtures” and weighs a pound — so the lamp matters, as set out in can you put a grow light bulb in a regular lamp. One bulb lights one plant; it will not rescue a collection.

The honest summary

A windowless room is a solvable problem and a surprisingly cheap one, provided you accept the band it puts you in. Take 5 mol/m²/day for foliage, run fourteen to sixteen hours on a timer, size the fixture by the area rather than the room, and spend the money on efficacy rather than output, because the thing that makes this room expensive is not brightness — it is that the light never gets to stop.