How to Choose a Grow Light for a Greenhouse

A greenhouse grow light adds light, it does not replace daylight

A greenhouse is a room with glazing, so plants already receive daylight through the glass or plastic. The job of a grow light in a greenhouse is supplemental: it adds light when the days are short, the sky is overcast, or the bench sits in a shaded corner. The University of Minnesota Extension notes that supplemental lighting can make up for a lack of natural sunlight, and that all plants require light for photosynthesis.

This shapes the shopping question. A greenhouse grower is not choosing a light that must carry the whole job alone, the way a tent grower does. The chosen light fills a gap in the daylight the greenhouse already provides. That is why the same fixture that suits a shelf of houseplants may also suit a greenhouse bench, and why the fixture type should match the spot you want to light.

• A bar light or panel over a bench
• A quantum board over a work table
• Strips on a shelf
• A grow light bulb in a lamp socket
• A clip-on for a single pot

Intensity and coverage: what a listing can say

Grow light listings describe a light by its wattage and its coverage area. The wattage is the draw of one light, never an equivalent figure. The coverage area is the footprint in square feet that the maker names, often tied to a tent name such as 2x4 or 4x4. These two words, watts and coverage, are the closest a listing comes to intensity.

Intensity itself, as a grower would measure it, is stated in figures such as PAR, PPFD or DLI. Those figures rarely appear in a title. They appear on the maker's manual and the maker's page for the model, where the maker states the light map over a footprint. This article reads what the listing states and leaves those figures to the manual. See PAR, PPFD and DLI and how to read a light map.

When two listings are compared, read the watts of one light first, then the coverage area, then the controls. A pack of 2 or 4 lights changes the total draw, but the watts of one light stay the same. See how to calculate coverage area.

• Light type
• Watts of one light
• Coverage area in square feet
• LED chip maker
• Dimmable and timer
• IP code
• Pack size (1, 2, 4 or 8)

Spectrum words on a listing: full spectrum, red and blue, Kelvin

Listings use spectrum words to describe the color of the light. Full spectrum is the most common word and means the light covers the visible range rather than one narrow band. Red and blue LEDs, and terms such as 660nm red, describe diodes with a narrow color. Kelvin values such as 3000K, 4000K and 5000K describe the tone of white light. See full spectrum versus targeted wavelengths and Kelvin color temperature.

For a greenhouse, the spectrum question is the same as for any grower: what do the plants need? A listing that names full spectrum or a Kelvin value tells you the tone; it does not tell you a measured spectrum chart. The maker's page for the model is where a spectrum chart, if one is published, is read.

Moisture protection: read the IP code and follow the manual

Greenhouse air is humid, especially near benches with trays of seedlings. A grow light listing may state an IP code, two digits that describe protection from solids and from moisture. This is a listing term to read: the first digit covers solids, the second covers moisture, and a higher second digit means the maker designed the housing against more moisture. The exact figure, when a title does not state it, is on the maker's page and in the manual.

A listing that omits a detail, such as an IP code or a lifespan figure, simply does not state it; the maker's page may. The manual is also where placement is described. The maker's manual and local electrical rules come first for any fixture in a humid space, and this article adds nothing beyond that sentence.

Mounting a grow light in a greenhouse: general terms

Grow lights for a greenhouse mount the same way they mount anywhere: hanging from a frame, resting on a shelf, clipped to a pot rim, or standing on a base. The listing words tell you which. A hanging light ships with hangers; a clip-on light names a gooseneck or clamp; a strip light may be linkable or magnetic. See greenhouse versus indoor tent lighting.

The spot you choose matters more than the type. A bench of seedlings needs a light positioned over the tray; a houseplant on the floor needs a light on a stand or a clip-on. The maker's manual describes how to mount that specific model, and the catalogue's stated details do not include a hanging height.

Dimmer and timer: controls that suit a greenhouse

A greenhouse gets changing daylight, so a light that can be dimmed is useful: on a sunny morning the grower can turn it down, on a dark afternoon turn it up. A timer automates on and off cycles, which matters when the grower is not in the greenhouse every day. See the dimmable grow lights guide and the grow lights with timer guide.

Listings state controls plainly: dimmable, dimmer knob, built-in timer, 4/8/12H cycles, app control. Read those words on the listing. What the catalogue cannot say is how the dimmer behaves over a season; that is in the manual. See why a built-in timer matters.

Read the maker's page next to the listing

The listing gives the title words: light type, wattage of one light, coverage area, chip maker, dimmer, timer. The maker's page for the model gives the rest: mounting instructions, the IP code if the listing omits it, any spectrum chart or light map, and the manual. Makers such as VIVOSUN and GE Lighting publish grow light lines with these details on their own pages. See the main grow light guide and how to read a grow light spec sheet.

For a greenhouse, the same rule holds as for any purchase: compare listings on stated details, then open the maker's page before you decide. The DesignLights Consortium maintains a Qualified Products List for horticultural lighting, a resource where performance claims are checked against published standards; it is a place to look up a model, not a substitute for the maker's manual.

What to pick for your job

Questions

Do greenhouse plants need a grow light if the greenhouse gets daylight?

Not always. Plants need enough light for photosynthesis, and a greenhouse already passes daylight through the glazing. A grow light is supplemental: it adds light when daylight is short or shaded. Watch the plants for signs of too little light, such as long, thin stems reaching toward the window.

What does coverage mean on a grow light listing?

Coverage is the footprint, in square feet, that the maker names for one light. It is often tied to a tent name such as 2x4 or 4x4. Read it as the maker's stated area, not as a measured guarantee.

What is full spectrum on a grow light listing?

Full spectrum is a listing word that means the light covers the visible range rather than a single narrow band. It does not replace a measured spectrum chart; the maker's page for the model is where such a chart, if present, is read.

Can any grow light be used in a humid greenhouse?

A listing may state an IP code, two digits for protection from solids and moisture. Read that code on the listing or the maker's page, and follow the manual for placement. The manual and local electrical rules come first.

Where do I find the light intensity for a model?

Intensity figures such as PAR, PPFD and DLI, when a maker states them, appear on the maker's manual and page for the model, not in the title. This article reads the listed wattage and coverage only.

Do I need a timer for a greenhouse grow light?

A timer is a control listed on the light. It automates on and off cycles, which helps when a greenhouse is not visited daily. Read whether the light has a built-in timer on the listing.