Grow Lights for a Greenhouse

For about three months a year a greenhouse sits below the light level its crops want, and for the other nine it does not. If you are closing that gap over a bench of roughly two feet square, the VIVOSUN LumaLight 100W is the fixture we would buy, because it is the only board we cover that publishes an ingress rating for a damp structure. For a long bench, two GE Grow 24in fixtures end to end are the damp-rated answer.

We have not tested any of these fixtures. What follows is published specification, extension guidance quoted and named, and the subtraction that tells you whether you need a light at all.

The finding that surprises people: the glazing

A greenhouse is darker inside than the garden outside, and by far more than most owners assume. Writing in Vegetable Growers News, Erik Runkle of Michigan State University puts it plainly: a greenhouse’s “structure and glazing materials commonly reduce light transmission by 35 to 50 percent”, before you count hanging baskets, heat pipes or a winter’s worth of dust on the panels.

That loss is constant all year. It is invisible in June, when there is light to spare, and decisive in December, when there is not.

How much light a greenhouse is actually short by

Runkle gives the three figures that make this a subtraction rather than a guess. On a dark winter day outdoor DLI “could be less than 5 mol·m-2·d-1”. A common target minimum inside a greenhouse “is 10 to 12 mol·m-2·d-1”. And in the northern half of the US, greenhouse DLI stays below 12 for about three months of the year.

Now turn that into a fixture. A 2 × 2 ft bench is 0.3716 m², and we assume 80% of a fixture’s photons land inside its footprint — the same convention used across this site.

Sixty-five micromoles per second is a small fixture — one 4 ft strip, or a single grow bulb at close range. That is the genuinely useful result on this page, and it runs against the instinct to buy the biggest panel available: a greenhouse light is a top-up, not a sun replacement, and it is sized accordingly. Compare a room with no window at all, where the same bench needs the whole daily total and the fixture roughly doubles — that arithmetic is here.

Do you need grow lights in a greenhouse at all?

For most of the year, no, and Runkle is explicit about the season: ornamentals at northern latitudes typically receive no supplemental light from April through September, because natural DLI is usually at least 12 by then.

So the honest answer is seasonal. A greenhouse grow light earns its place roughly from late November to mid-February in the northern half of the US, and is switched off the rest of the year. If you are buying one in May to improve growth, the fixture is not your constraint.

Two further cases where the answer is still no. If the crop is cold-hardy and slow by design — overwintering greens, garlic, hardy herbs under a cloche — the low winter DLI is part of the plan rather than a problem to fix. And if the greenhouse is unheated and sitting near freezing, light is not the limiting factor; temperature is.

Heat: the thing a grow light will not fix

This is the single most common misconception about greenhouse lighting, and it is worth stating bluntly. A grow light does not warm a greenhouse to any useful degree.

The arithmetic is unkind. A 100 W LED fixture dissipates at most 100 W of heat — about 341 BTU per hour — into a structure that loses heat through its entire glazed surface on a cold night. A small 6 × 8 ft greenhouse needs a heater rated in thousands of BTU to hold frost off. Four grow lights would not substitute for one small heater, and they would cost more to run. The general case is in do LED grow lights get hot.

The corollary matters for the opposite reason: because LED fixtures add so little heat, you can hang them close to a crop in a greenhouse without the scorch risk the old high-pressure sodium lamps carried.

The rating that decides this page

A greenhouse is a damp location in the electrical sense at minimum — condensation forms on every surface overnight, including the fixture — and in a structure you hose or mist, parts of it are a wet one. That narrows the field more sharply than any photon figure.

IP65 means dust-tight and protected against water jets, which is a meaningful claim over a misted bench and is published by exactly one board we cover. “Not published” is not a claim that a fixture is unsafe; it is a statement about what the manufacturer documented. In a dry living room the distinction barely matters. In a structure that condenses every night it is the first thing to check.

Two things no rating covers. The first is the connection at the other end of the cord: a damp-rated fixture plugged into an unprotected indoor extension lead on a wet floor is still a hazard, and a greenhouse circuit should be GFCI protected with outdoor-rated cable. The second is cold. No manufacturer on this page publishes an operating temperature range, so an unheated greenhouse in a hard frost is outside every published specification here. That is a real gap in the available data and we are not going to fill it with an estimate.

When to run them, which is not all day

Supplemental light in a greenhouse is worth most when natural light is worth least, and Runkle’s guidance is specific: operate lamps during a portion of the night and on cloudy winter days, and in the early morning and late afternoon on sunny winter days.

In practice that is a timer with two blocks — a few hours before dawn and a few after dusk — rather than a fixture running through the middle of a bright day, when you are paying to add photons to a bench that already has enough. A cheap plug-in timer does this better than a built-in one, for the reasons in grow lights with timers.

The glass cabinet case, which is a different problem

A good share of people searching for a greenhouse grow light mean a glazed cabinet indoors — the converted display cabinet that houses a humidity-loving collection. That is not this page’s arithmetic. A cabinet in a room gets whatever the room gets, which in winter is often nothing usable, so the fixture supplies the entire daily total rather than a top-up, and the enclosure makes heat and airflow the design problem instead of rain. The sizing for that case is in grow lights for a room with no windows, and the small-fixture shortlist is in grow lights for terrariums.

1. VIVOSUN LumaLight 100W — the only IP65-rated board

Publishes an IP65 rating, a 2 × 2 ft coverage area and 521 µmol/m²/s at 12 inches — a PPFD figure with the distance attached, which most boards omit. For a bench that gets misted or condenses heavily, it is the only board here whose maker has said anything about water at all.

Best for: a 2 ft square bench in a structure that gets damp, where you want a rating rather than an assumption.

The downside: VIVOSUN publishes no PPF in µmol/s and no efficacy in µmol/J, so this board cannot be ranked against the others on photons per watt, and we will not derive a PPF from the PPFD because that calculation is not valid. It is also far more light than the 65 µmol/s a 2 ft square bench needs as a top-up, so expect to run it on a low dimmer step.

2. GE Grow 24in fixture — a damp-rated run along a bench

72 µmol/s from 40 W, damp-rated, and designed to be linked end to end — which is exactly the shape of a greenhouse bench. One fixture is slightly more than our 2 ft square top-up figure; two in line cover a 4 ft bench with 144 µmol/s against the 129 the arithmetic asked for.

Best for: the standard case — a 4 ft bench, a damp structure, a three-month season.

The downside: 1.80 µmol/J, which is well under the 2.5 µmol/J commercial efficacy minimum, and GE states explicitly that it is not rated for outdoor use — a greenhouse counts as indoors for this purpose only if it is a proper enclosed structure. Non-dimmable, so your only intensity control is height.

3. SuncoGrow 4ft 40W — damp-rated, by the linear foot

The only cheap 4 ft strip we have found that publishes a PPF at all (48.42 µmol/s), damp-rated and ETL listed, linkable up to four fixtures from one plug. For a bench run, that combination of a real number and a real rating is rare.

Best for: a long bench on a budget, where you want to add strips as the shortfall demands.

The downside: 1.21 µmol/J is the lowest efficacy of anything on this site, so you pay for that certainty every month on the meter. One strip over a 4 × 1 ft run averages about 104 µmol/m²/s, which is a genuine top-up but not a replacement; two strips is the configuration we would actually buy.

4. Spider Farmer SF1000 — the most photons per watt, under cover

249.21 µmol/s from 100 W at a published 2.5 µmol/J, which meets the commercial efficacy minimum exactly and is roughly twice the photons per watt of the damp-rated options above. Dimmable, which matters when your required top-up is a fifth of what the board can deliver.

Best for: a dry, heated greenhouse or a conservatory, where running cost over a long season matters more than an ingress rating.

The downside: Spider Farmer publishes no location or ingress rating of any kind, so on the one specification that decides this page it is silent. In a structure that condenses nightly, that is a judgment you are making rather than one the manufacturer has supported.

The honest summary

Check the season first: if it is not December, January or February and you are not in the northern half of the country, the shortfall may not exist. If it does, measure the bench, take 8 mol/m²/day as the gap, and the fixture you need is smaller than the category wants to sell you — about 65 µmol/s over a 2 ft square. Then choose on the rating rather than the photon figure, because in a greenhouse that is the specification doing the real work.