Grow lights: choosing and setting them up

Daniel Platteau

From November to February, Belgium lives in the half-light. The sun rises late, sets before 5 p.m. in December and stays low on the horizon, often behind clouds. For us, it is a matter of mood. For a houseplant standing two metres from a window, or a tomato sowing started in February, it is a matter of survival: without enough light, it stretches, turns pale and wears itself out. A grow light solves the problem, provided you choose it on the right criteria. And those are not the ones printed in large type on the box.

In short: for a few plants, a white LED clip-on lamp is enough; for seedlings, light bars placed 10-15 cm above the plants. Choose white light rather than pinkish-purple, plug in a timer (12 to 14 hours a day for foliage plants, 16 hours for seedlings) and always leave a real night. Don't look for watts, look for PPFD. Expect a few euros to around thirty euros of electricity for a winter.

Why light your plants in winter?

Brussels lies at 50.8° north, further north than the whole of the United States outside Alaska. Yet even in the northernmost US states, Michigan State University Extension reports an outdoor daily light total in December of only 5 to 10 units ("moles" of light per square metre per day, see below). A houseplant receives only a fraction of that: the glass, the walls, the distance from the window and the net curtain reduce it further.

Most tropical houseplants do not die from this shortage: they slow down. But some cope badly, and all of them show it. The signs of too little light, according to University of Minnesota Extension:

• long, thin stems leaning towards the window (this is etiolation);
• longer gaps between leaves;
• pale, yellowing leaves, older leaves dropping;
• variegation fading;
• no flower buds.

With seedlings it is even clearer: a tomato sown in February on a windowsill often produces a thread-like stem that flops over. To recognise these signals and tell them apart from a deficiency, see also our article Reading your plants' signals.

The three terms that matter: PAR, PPFD and DLI

Plants do not "see" light the way we do. You need three notions, no more.

• PAR (photosynthetically active radiation): the part of light that plants use for photosynthesis, between 400 and 700 nanometres, from blue to red.
• PPFD (photosynthetic photon flux density): the amount of that useful light reaching one square metre of leaves every second, in micromoles per square metre per second (µmol/m²/s). It is the intensity measurement that matters to a plant. It depends on the lamp, but also on distance: the same lamp delivers far more at 15 cm than at 60 cm.
• DLI (daily light integral): the total received over a day, in moles per square metre per day (mol/m²/d). It is intensity multiplied by duration.

The calculation is simple: DLI = PPFD × number of hours × 0.0036. A plant receiving 150 µmol/m²/s for 14 hours therefore gets 150 × 14 × 0.0036 = 7.6 mol/m²/d.

Iowa State University Extension gives these orders of magnitude for daily needs:

The table says it all: a green houseplant makes do with little, a succulent or a pot of basil needs much more, and a tomato plant more still.

What about the lux on my phone?

Lux (and lumens) measure light as the human eye perceives it, which is very sensitive to green and yellow. A light-meter app on a smartphone gives a rough idea (brighter, darker), but not a reliable PPFD value: the ratio between lux and PPFD changes with the type of light. The only accurate instrument is a PAR sensor ("quantum meter"), mostly of interest to enthusiasts.

Watts, lumens, "450 W equivalent": why product listings mislead

On a shopping site, grow lights come with impressive figures. You need to know how to read them.

• Watts indicate electricity consumption, not the light produced. Two 20 W lamps can give very different amounts of light depending on the quality of the LEDs and the optics.
• "Equivalents" ("30 W, 450 W equivalent") compare the lamp with older, far less efficient technologies. It is a sales argument, not a measurement.
• Lumens describe what the human eye sees, not what the plant uses.
• The number of LEDs means nothing: 200 cheap small LEDs can produce less useful light than 30 good ones.

The right figure is PPFD at a given distance (for example "120 µmol/m²/s at 30 cm"), ideally with a map showing how it drops towards the edges of the lit area. Serious manufacturers publish it. A listing that gives only "equivalent" watts and a number of LEDs deserves suspicion.

White LEDs or pink-purple LEDs?

Plants mainly absorb blue and red. Hence the first horticultural lamps with red and blue LEDs, which give that characteristic pinkish-purple light. Is it better?

For home use, no. Bruce Bugbee, professor of plant physiology at Utah State University and a specialist in crop lighting, considers the effect of colour "greatly overrated". In his view, red and blue LEDs took over because, at first, they were the most energy-efficient; white LEDs have improved a great deal since. His trials show minimal differences between spectra, mostly affecting the shape of the plant: more blue gives more compact plants with thicker leaves, less blue gives taller plants.

Two very practical arguments in favour of white:

• You can see your plants. Under pink light, it is impossible to spot a yellowing leaf, a spot or insects. Under white light, you can.
• You live with it. A grow light in the living room also lights the room. Pinkish-purple light is tiring on the eyes; white light passes for ordinary lighting.

University of Minnesota Extension and Iowa State University Extension also recommend white "full-spectrum" light, or a mix of whites, for most plants at every stage. The Royal Horticultural Society, whose guide is still based on fluorescent tubes, advises combining "red" and "blue" tubes for flowering plants: with full-spectrum white LEDs, the question no longer arises.

Distance and duration: the settings

This is where everything is decided. A good lamp hung too high is useless; a lamp too close can bleach the leaves. Guidelines from University of Minnesota Extension:

These distances apply to medium-power LED or fluorescent lamps; a powerful lamp goes higher. The practical rule: start at the distance given by the manufacturer, then watch. Bleached, yellow-green leaves near the lamp signal too much light; stretching stems, not enough.

A timer, and a real night

Plug the lamp into a timer (a programmable socket costs around ten euros, and many lamps have one built in). Plants need a period of darkness: extending the lighting beyond the durations in the table brings nothing and can do harm. Preferably light in continuity with the day (for example from 7 a.m. to 9 p.m.), rather than in the middle of the night.

Watch out for plants that flower with long nights

Some plants only form buds when the nights are long enough: these are short-day plants. Minnesota Extension lists the poinsettia, Christmas and Thanksgiving cacti, and the chrysanthemum. Light them in the evening in autumn, even with an ordinary living-room lamp, and they may not flower. This is exactly the mechanism growers use to produce chrysanthemums all year round (see our article on chrysanthemums). African violets (Saintpaulia), on the other hand, are long-day plants that appreciate extended lighting.

Which lamp for which use?

For one or two plants, the clip-on lamp is the simplest solution:

Clip-on grow light with two adjustable heads
Give an orchid, a young avocado tree or a few succulents the light they lack in winter. Pleasant white light for the living room, flexible arms, control box on the cable. View on Amazon Affiliate link

For a seedling shelf, light bars are unbeatable: they hang under the shelf above and light the whole surface evenly.

LED light bars for a seedling shelf
Four 60 cm white-light bars, to fix under a shelf and link together. Control box with timer and dimmer: sturdy seedlings from February, without thinking about it. View on Amazon Affiliate link

Late-winter sowings under lights

This is where a lamp makes the biggest difference. Tomatoes, chillies, peppers and aubergines are sown indoors in February and March, at a time when natural light is still weak.

• To germinate, tomato, chilli and aubergine seeds mainly need warmth: they come up under a thin layer of compost, so in the dark. Install the lamp as soon as they emerge, without waiting: a seedling becomes leggy in its first few days.
• Place the light bars 10-15 cm above the seedlings and raise them as they grow.
• Light for 16 hours a day, with 8 hours of dark night.
• Aim for stocky plants, with thick stems and clear green leaves. A seedling that stretches needs more light (lamp closer); one with bleached leaves near the lamp is getting too much.

Purdue University reports that, for several bedding plants grown from seed in greenhouses (impatiens, salvia, marigold, viola), seedling quality improves up to a DLI of about 12 mol/m²/d. Fruiting vegetables are among the most light-hungry plants (see the DLI table): a Belgian windowsill in February is not enough for them.

Houseplants and plants to overwinter

For most green houseplants, a lamp is not essential: it is a comfort that keeps up their growth and colour. It becomes really useful in a few cases:

• a dark room or a plant far from the window;
• full-light plants brought in for the winter: citrus, potted olive trees, succulents, young avocado trees, which quickly become leggy behind glass in December;
• orchids: the RHS lists Phalaenopsis among the plants that adapt well to artificial lighting. Our 10 orchids to grow at home give their light needs;
• collector aroids and other tropical understorey plants (see our top 10 Araceae for the flat), which prefer moderate but steady light.

One exception: plants that rest in winter do not need a lamp. Caladium tubers, for example, spend the winter dry and dormant: a lamp does nothing for them.

For protecting plants that stay outdoors, see our article Overwintering garden plants.

How much does it cost in electricity?

An LED lamp uses little power, but it runs for a long time. The calculation: power (in kW) × hours per day × number of days × price per kWh. In Wallonia, the average all-in price (energy, network, taxes and VAT) was around €0.47 per kWh in September 2026 according to the comparison site Selectra; your actual price depends on your contract and municipality.

Read the power actually consumed on the listing (not the "equivalent"), and think of the dimmer: a dimmed lamp uses less (though not necessarily in exact proportion).

True or false: some common beliefs

"Any LED bulb will do." Partly false. An ordinary white LED bulb does emit light that plants can use, and placed very close to a small plant it helps a little. But the RHS advises against household bulbs: their intensity is too low and their light is spread across the whole room instead of being concentrated on the leaves.

"More light is always better." False. Too much light bleaches the foliage (yellow-green, scorched-looking leaves near the lamp) and slows growth, according to the RHS. Each plant has its range of needs (see the DLI table).

"LEDs don't get hot, so you can put them right against the leaves." False. LEDs run much cooler than older lamps, but a lamp that is too close delivers excessive intensity on a small area. Respect the distances, especially with a powerful lamp.

"You need to light 24 hours a day to make things grow faster." False. Recommended durations range from 12 to 18 hours depending on the plant. A dark period is part of the plant's normal cycle, and in short-day plants it is what triggers flowering.

"Pink lamps are more efficient than white ones." Mostly false today. Modern white LEDs are just as effective for growth, and spectral differences mainly affect the shape of the plants (see above).

"The more watts, the more powerful the lamp is for plants." False. Watts measure consumption. Only PPFD at a given distance tells you how much useful light reaches the leaves.

Test yourself

Read also

• Reading your plants' signals, to recognise a lack of light.
• Orchid myths, true and false about growing them.
• Amaryllis: plant it for Christmas, the other houseplant of winter.
• Peacock begonia (Begonia pavonina), an understorey plant that makes do with little light.

Sources

• Cannabis Business Times. (2016, October 6). Spotlight on LEDs: White light is all right [interview with B. Bugbee]. https://www.cannabisbusinesstimes.com/home/article/15699348/spotlight-on-leds-white-light-is-all-right
• Hortidaily. (2014, December 17). US: Low daily light integrals in northern latitudes. https://www.hortidaily.com/article/6013810/us-low-daily-light-integrals-in-northern-latitudes/
• Royal Horticultural Society. (n.d.). Artificial lighting for indoor plants. https://www.rhs.org.uk/plants/types/houseplants/artificial-lighting
• Selectra. (2026, September 16). Les prix du kWh en Belgique en 2026 : électricité et gaz. https://selectra.be/fr/energie/tarifs/kilowattheure
• Steil, A. (2023). Important considerations for providing supplemental light to indoor plants. Iowa State University Extension and Outreach. https://yardandgarden.extension.iastate.edu/how-to/growing-indoor-plants-under-supplemental-lights/important-considerations-providing-supplemental-light-indoor-plants
• Torres, A. P., & Lopez, R. G. (2010). Measuring daily light integral in a greenhouse (HO-238-W). Purdue Extension. https://www.extension.purdue.edu/extmedia/ho/ho-238-w.pdf
• Weisenhorn, J., & Hoidal, N. (2026). Lighting for indoor plants and starting seeds. University of Minnesota Extension. https://extension.umn.edu/planting-and-growing-guides/lighting-indoor-plants