PlantWhy

Grow lights for indoor plants: choosing and using them

A window rarely gives indoor plants as much light as they would get outside, especially in winter. A grow light fills that gap. This guide explains when you actually need one, which type to buy, what the numbers on the box mean, and how far away and how long to run it for different plants.

Grow lights for indoor plants: choosing and using them
Photo: David J. Stang, CC BY-SA 4.0 · Wikimedia Commons
Difficulty
Easy
Cost
Low–moderate; a small LED panel or bulb is enough to start
Daily on-time
10–16 hours depending on the plant (estimated)
Typical power
10–50 W for a shelf or a few pots (estimated)

When do you need a grow light?

Plants tell you when light is short. Consider a lamp if you see any of these:

  • New stems are long and thin, with wide gaps between leaves, and the plant leans hard toward the window.
  • Variegated leaves turn plain green, or new leaves come in smaller.
  • Flowering plants keep their leaves but stop blooming.
  • You are starting seeds or growing herbs and salad leaves indoors, especially from autumn to early spring.
  • The room has only a north-facing window, or the plant sits more than 6–10 ft from the glass (estimated).

A lamp is a supplement, not magic: it will not fix overwatering or a pot without drainage.

Foliage plants like calathea want bright, indirect light; in a dim room a lamp keeps their patterns sharp.
Foliage plants like calathea want bright, indirect light; in a dim room a lamp keeps their patterns sharp. Photo: Katherine Wagner-Reiss, CC BY-SA 4.0 · Wikimedia Commons

Types of grow light

Almost every good home grow light today is LED. The real choice is which kind of LED, or whether an old fluorescent tube will do.

Full-spectrum LED

  • Looks white or warm white and covers the whole visible range.
  • Suits nearly every plant, from seedlings to flowering ones.
  • Pleasant to live with in a living room.
  • Most efficient choice per watt; long life.

Plain white LED (household bulb or strip)

  • Cheap and easy to find.
  • Fine for foliage houseplants and as a top-up to a window.
  • Light output is usually low, so it has to sit close.
  • Check the lumen or PPFD figure; many are too weak for fruiting plants.

Purple (red-blue) LED

  • Gives the red and blue the leaves use most, efficiently.
  • Grows plants well, but the pink-purple glow is tiring to the eyes.
  • Makes it hard to spot pests or leaf colour changes.
  • Best in a cupboard, tent or spare room.

Fluorescent (T5 tube, CFL)

  • The traditional choice for seed trays; still works.
  • Cool to the touch, so it can sit very close to seedlings.
  • Uses more power than LED for the same light; tubes fade with age.
  • Contains mercury: recycle old tubes properly.

⚠️ Ordinary incandescent and halogen bulbs are a poor choice: most of their energy becomes heat, not usable light. To be bright enough they have to be close, and at that distance they scorch leaves.

The numbers on the box, in plain words

  • Lumen (lm): how bright the light looks to the human eye. Useful for comparing two white bulbs, but plants do not see light the way we do.
  • PAR and PPFD: PAR is the part of light plants use for photosynthesis. PPFD (µmol/m²/s) is how much of it lands on a spot. It is the most honest number, but it depends on distance, so look for a chart showing PPFD at several heights. Rough targets: foliage houseplants 50–150, herbs and lettuce 150–300, seedlings 100–250, succulents and flowering plants 200–400 (estimated).
  • Colour temperature (K): 2700–3000 K looks warm and yellowish, 4000 K neutral, 5000–6500 K cool white. Plants grow under all of these; 4000–6500 K is common for leafy growth (estimated). Pick the one you like to look at.
  • Watt (W): how much electricity the lamp uses, not how bright it is. Beware of "equivalent" watts on cheap lamps; use the real draw for cost. As a rough guide, 10–20 W covers one to three pots, 20–40 W a small shelf (estimated).

Distance and hours by plant group

Start with the figures below for a typical LED, then adjust using the signs in the next sections. Stronger lamps go further away; weaker ones closer.

Lettuce and other salad leaves grow well under a lamp and need no fruiting-level light.
Lettuce and other salad leaves grow well under a lamp and need no fruiting-level light. Photo: Geographer, CC BY 1.0 · Wikimedia Commons
Seedlings4–8 in above the tops, 14–16 hours a day. Raise the lamp as they grow (estimated).
Herbs and lettuce6–12 in, 12–16 hours. Basil likes the upper end of the range (estimated).
Succulents and cacti6–12 in, 12–14 hours. They take strong light but need a dark night (estimated).
Foliage houseplants12–24 in, 10–12 hours. Often a top-up to a window is enough (estimated).
Flowering plants (e.g. African violet)8–12 in, 12–14 hours. Some plants only bloom with long nights, such as Christmas cactus in autumn (estimated).

Use a timer

Plants need a regular rhythm of light and darkness; darkness is when they rest and use the sugars they made. Leaving the lamp on 24 hours is not better, and for many plants it is harmful.

  • A simple plug-in mechanical or digital timer costs little and removes the risk of forgetting.
  • Keep the same start time every day, e.g. 7:00 to 21:00 for a 14-hour day.
  • Many lamps now have a built-in timer or dimmer; that is enough.
  • Give at least 8 hours of darkness for most plants (estimated).
African violets bloom well under a lamp on a steady 12–14 hour cycle.
African violets bloom well under a lamp on a steady 12–14 hour cycle. Photo: RobertoMM, CC BY-SA 3.0 · Wikimedia Commons

Step-by-step setup

  1. Decide what you are lighting: one pot, a shelf or a seed tray. Measure the area.
  2. Choose a full-spectrum LED sized for that area; check the PPFD chart if there is one.
  3. Pick a mounting method: clip-on arm, hanging chain, or a shelf with the light fixed underneath. Adjustable height is very useful.
  4. Hang the lamp at the starting distance for your plant group from the table above.
  5. Plug it into a timer, and the timer into a grounded socket away from water.
  6. Set the daily hours and check that the lamp switches on and off on time.
  7. Watch the plants for 1–2 weeks. Adjust height first, then hours.
  8. Clean dust off the lamp every few months; dust cuts the light.

Too much or too little light

Echeveria stays tight and colourful under strong light; it stretches when light is weak.
Echeveria stays tight and colourful under strong light; it stretches when light is weak. Photo: EriaWei, CC BY-SA 4.0 · Wikimedia Commons
Long, thin stems; pale new leaves; leaning toward the lampToo little light. Lower the lamp or add an hour or two.
Leaves bleached, yellow-white or with brown dry patches on topToo much or too close. Raise the lamp or shorten the day.
Leaf edges curl upward, soil dries very fastHeat or light stress. Raise the lamp and check that the soil is not drying out.
Succulents turning red or purple, still compactUsually a harmless stress colour from strong light. Only act if spots appear.
Flowering plant healthy but not bloomingLight may be too weak, or the plant needs long nights. Check the variety's needs.

Electrical safety

  • Keep plugs, timers and power strips higher than the pots and away from where water drips or splashes.
  • Leave a drip loop: let the cable dip below the socket before rising to it, so water runs off instead of into the plug.
  • Do not mist plants under a lamp that is switched on, and never touch fittings with wet hands.
  • Use lamps rated for damp locations near humidifiers or in grow tents.
  • Avoid chaining extension leads. Use one good power strip and do not load it beyond its rating.
  • Check cables for cracks and heat; replace anything that feels hot or smells.
  • Keep cables out of reach of pets that chew.

⚠️ Plug rules, socket types and wiring standards differ by country. Use lamps certified for your country, and leave any hard-wiring to a qualified electrician.

Estimating electricity use

The formula is simple: watts × hours per day ÷ 1000 = kWh per day. Multiply by 30 for a month, then by your electricity price.

Example: a 20 W LED panel running 14 hours a day uses 20 × 14 = 280 Wh, or 0.28 kWh a day. Over 30 days that is about 8.4 kWh. At 17 cents per kWh that comes to roughly $1.40 a month (estimated). Check the real price on your bill.

Use the lamp's real draw printed on the label, not the "equivalent" figure. A cheap plug-in power meter shows the exact consumption.

Frequently asked questions

Can I use a normal LED bulb?

Yes, for foliage houseplants or as a top-up to a window. It needs to be quite close, and it is usually too weak for seedlings, herbs or flowering plants.

Should I run the light 24 hours a day?

No. Most plants need a dark period every day. A timer set to 10–16 hours, depending on the plant, works better (estimated).

Can a grow light burn leaves?

LEDs run cool, but strong ones too close can bleach leaves. Fluorescent tubes rarely do; incandescent bulbs easily do.

Is the purple light harmful to my eyes?

Long exposure to very bright light of any colour is tiring. Don't stare into the lamp, and choose white full-spectrum light for living spaces.

Do I still need a window?

Not strictly, but natural light plus a lamp is the cheapest combination. A lamp then only needs to cover the dim hours.

Plants that do well under grow lights

BasilLettuceParsleyEcheveriaHaworthiaCalatheaAfrican VioletPeperomia

Sources

Last updated: 2 November 2026