How to Set Up Indoor Grow Lights for Plants

Quick answer: For most houseplants, a full-spectrum LED 12 to 24 inches above the foliage, switched on 12 to 14 hours daily by a timer, does the job. Plants you want in bloom need more, so hang the light nearer and give them 14 to 16 hours.

In numbers, low light plants want roughly 50 to 150 PPFD at leaf level, medium light plants 150 to 250, and cacti, citrus and other high light plants 250 to 450. Then let the plant guide you. Stretching asks for more light, and bleached or burned leaves ask for less.

Does Your Plant Need One?

Light drives photosynthesis, the source of the oxygen in what plant produces the most oxygen. Full sun outdoors reaches close to 2,000 PPFD, the sunniest windowsill gets a small slice of that, and short, gray winter days shrink it further. Almost any houseplant gains from a winter boost.

A light earns its place when:

• the plant lives in a windowless room or far from the glass (an east or west window already gives only low light at 3 to 10 feet),
• you grow cacti, succulents, citrus or similar sun lovers without a south or southwest window,
• you want something to flower indoors, since most bloomers need high light, or
• the plant shows the low light symptoms further down this page.

Setting Up a Light, Step by Step

1. Pick a full-spectrum LED. See the buying notes below.
2. Hang it at the right height. Start 12 to 24 inches above the leaves for LED fixtures, or follow the distance on the box. Low-output LED strips and fluorescent tubes go 6 to 12 inches away. Flowering and high light plants sit at the near end of whatever range applies.
3. Make the height adjustable. An adjustable chain, or blocks under the pots, lets you keep the gap right as plants grow.
4. Plug it into a timer. Give foliage plants 12 to 14 hours and flowering plants 14 to 16, and stay between 10 and 16 hours in all cases.
5. Watch for several weeks. Change one thing at a time, using the symptom list below.

Nearer is not automatically better. Very intense light or a hot fixture can harm leaves, and a close light covers a smaller patch. A dimmer helps, since LED output tracks power, so half power gives roughly half the PPFD. For tall plants, add side light with vertical LED strips or reflective film.

Plants also need darkness, so leaving a light on day and night does not help. If you cannot bring the light closer, an extra hour raises the daily total. A plant near a window only needs its shortfall covered, and that gap is biggest in winter.

Picking an LED

LEDs are the sensible choice. They give more light per unit of energy, stay cooler than fluorescent tubes, last for years and lose output far more slowly. They do cost more up front, and their glare can annoy in a living room.

Plants absorb red and blue best but use green too. Blue keeps growth tight, red powers photosynthesis and flowering, and far-red, which incandescent bulbs give off in quantity, makes stems stretch.

• Red and blue lights put out mostly usable light but tint the room pink or purple.
• White lights, sold as full-spectrum or "daylight," suit most plants through every stage of growth and look far more natural in a room.
• Blue-leaning lights suit foliage plants, and red-leaning lights suit flowering ones.

Check the box for PPF. With only lumens listed, divide by 60 for a rough PPF (the true factor runs about 40 to 80). Avoid incandescent bulbs, which run hot and cause stretching. Fluorescent tubes are still usable when hung 6 to 12 inches away, but their output fades, so swap them every one to two years.

The Budget Option

An ordinary 4-foot LED shop light, full spectrum and rated 3,000 lumens or higher, hung 18 inches above the plants for 14 hours a day, is an inexpensive way to add to window light for medium and high light houseplants. Alone it delivers just 50 to 80 PPFD or so, a low light level by the targets below.

The Numbers Behind the Targets

Lumens, lux and foot-candles describe light as human eyes see it, overrating the yellow and green that look brightest to us and underrating the deep red and blue plants use best. Watts only show power draw.

PPFD (photosynthetic photon flux density) measures how much usable light hits the leaves every second, given in micromoles per square meter per second, and it drops off fast as the fixture moves away. PPF, often on the box, is everything usable the fixture emits, before that light spreads across an area. PPF efficacy (micromoles per joule) rates efficiency, not quantity of light.

DLI (daily light integral) is the usable light a plant collects across a full day, in moles per square meter. To work it out, multiply PPFD by hours on, then by 0.0036. At 150 PPFD for 14 hours, that comes to about 7.6 moles. A dimmer light run longer can equal a brighter one run for fewer hours. This Iowa State guide to supplemental light walks through the math.

The PPFD and DLI columns only line up loosely. Spread over 12 to 14 hours, the DLI bands work out to roughly 60 to 140 PPFD (low), 120 to 230 (medium) and 240 to 370 (high), so longer days let you sit lower in the PPFD range. Moth orchids fit the medium group, and citrus carrying fruit needs the top of the high band or beyond. For other plants, use these Minnesota Extension plant lists by light level.

Treat each low number as a minimum. Plants survive there, but fuller growth and flowers take more, and most houseplants accept a broad range.

Measuring What Reaches the Leaves

You can start without a meter, using the heights and hours above and watching the plant for several weeks. For readings, a phone light meter app is accurate enough at home. Hold it at leaf height, choose the setting for your light source and take readings in the middle and toward the edges. A quantum meter reads PPFD directly and costs more, but it pays off with rare, pricey plants where a mistake hurts.

Apps that show foot-candles or lux need a conversion. Under daylight, estimated PPFD is roughly foot-candles divided by 5, or lux divided by 54. LEDs vary, so use the app's LED mode. If a medium light plant's window spot measures far below 150 PPFD around noon, it is a strong candidate for one. Window rules of thumb don't convert cleanly, so measure if unsure.

Reading Your Plant

• Long, thin stems with big gaps between leaves, leaning to the light. Too little light. Lower the fixture, lengthen the day or put in a second light.
• Pale or yellow leaves, or variegated leaves going plain green. Too little light. Move the light in or lengthen the day.
• No buds on a plant that should flower. Not enough light, or day length is off. Increase intensity and check the timer.
• Scorched, bleached or strongly reddened leaves, or oddly compact growth. Too much light. Lift the fixture, dim it or shorten the hours.

Dropping leaves can mean either too much or too little light, so weigh it with other signs and allow several weeks per change. Yellowing can also come from watering or feeding, covered in our houseplant care guide.

Holiday Plants Are the Exception

Christmas cactus, Thanksgiving cactus and poinsettia need long nights to form buds, and the two cacti also respond to cool nights near 55°F. Long light hours, or a lamp or streetlight interrupting their darkness, can stop them from blooming.

Common Questions

Can a Grow Light Stay On All Day and Night?

No. Houseplants need darkness, so keep the light to 16 hours or less and let a timer handle it.

Do Succulents Need a Grow Light Indoors?

Most do, unless a south or southwest window is available. As high light plants, cacti and succulents do best around 250 to 450 PPFD.

What DLI Do Foliage Houseplants Need?

Aim for 3 to 6 moles daily if the plant is a low light type, or 6 to 10 if it is a medium light type.