Grow Lights for Indoor Plants and Seedlings: Which and How Close

A grow light is a lamp or LED fixture made to give plants the light they grow by, at the strength and for the hours you choose. This guide explains whether they work, why seedlings go leggy, which of ours suits a windowsill, shelf, cupboard or greenhouse, and how close and how long to run it. The LED fixtures, HID lamps and timers are in our grow lights section, and the 12 W strips, T5 lights and PL lamp for seedlings under propagation lighting.

Do grow lights work?

Yes, when the light is bright enough, close enough and on for long enough. Between October and March a UK window gives short days and a low sun, so seedlings stretch towards the glass and herbs on a sill slow down or stop; a houseplant on a dark shelf has the same problem. A grow light replaces or tops up that daylight, so seedlings can stay compact, cuttings can root in a cupboard, and salad leaves and herbs can keep cropping through winter (see our winter herbs, salad and microgreens guide).

Leggy seedlings: too little light

Leggy seedlings are tall, thin and pale, with long bare stems, and they flop over: they have stretched for light, on a windowsill or under a light hung too high. The cure is more light, closer: get the light on as soon as the first seedling shows, as our sowing seeds indoors guide says. The Gorilla Propagation LED 12W and the Maxibright LED T5 Propagation Lights run cool and can sit close to young plants, their pages say.

Lamp types at a glance

Kelvin (K) is colour temperature: the higher the number, the cooler and bluer the white.

Lamp types with an example from our range, as their product pages describe them.
Type	Example we sell	Colour or spectrum	For
Propagation LED strip	Gorilla Propagation LED 12W	Balanced blue	Roots and compact early growth
LED T5 propagation light	Maxibright LED T5 Propagation Lights 60cm	Not given	Seedlings, cuttings and young plants
PL lamp	LUMii 55W EnviroGro PL Lamp	6500K cool white	Cuttings, seedlings, vegetative stage
Full-spectrum LED	LUMii Switch Blade 150W	Full spectrum	Seedling to harvest
Dimmable full-spectrum LED	Maxibright Daylight 200W PRO	White LEDs plus red and far red	Whole life; run lower for young plants
Metal halide (MH)	Sunmaster 600W Metal Halide Lamp	7200K cool white	Vegetative stage
HPS	Sunmaster 600W Super HPS	Warm red-orange; 95,000 lumens	Flowering and fruiting
Dual spectrum HID	Maxibright Lamps	Blue and red in one lamp	Growth through flowering; red Super Bloom for flowering

Full spectrum, as our LED wattage guide puts it, means white LEDs plus extra red and far red, so one light covers growth and flowering. The HPS and metal halide lamps need a ballast and reflector and run hot: for a windowsill or a propagator, choose an LED. The PL lamp is a lamp only, for an E40 fitting: check the fitting on your fixture before you order, or ask us, its page says. LED or HPS for a tent? Most growers run LED grow lights, and some add an HPS for flowering: read why the best growers use both.

How close, and how many hours

Hours. Our advice is 18 hours of light and 6 of dark for seedlings, cuttings and plants in leaf, the usual 18/6 schedule in our growers' glossary. Put the light on a timer: the Time-R Digital Timer (24-hour on/off mode) or the Optimum Timer (15-minute segments). Check the load, both pages say; an HPS or metal halide goes on a contactor or a lighting controller such as the Maxiswitch Quad Timer, not straight into a plug-in timer, as our LED or HPS guide says.

Height. Hang a Gorilla strip or a Maxibright T5 just above the propagator lid or the seedlings, our advice is, and raise it as they grow, sooner if they look stressed, as the Gorilla page says. A full-size LED at full power usually hangs 30 to 45 cm above the canopy, higher over seedlings or when dimmed; seedlings and cuttings want a PPFD of about 100 to 300, plants in leaf 300 to 600, both from our LED wattage guide. Easy-Rolls raise and lower the light by hand through 1 m of cord, and a Maxibright PAR Meter reads the light at the tops of the plants.

Windowsill, shelf, cupboard, greenhouse or tent: what to do

Seedlings on a windowsill: a Gorilla 12 W strip or the 60 cm T5 (2 or 4 tubes) over a Jiffy Botanian Mini Greenhouse Kit or a Probox Propagator Small, sized for windowsills and small greenhouses, its page says.
A shelf or rack: the Maxibright LED T5 Propagation Light 120cm, 4-tube for one row of propagators or 8-tube for a wider area, or Gorilla strips chained up to 10 from one power lead (sold separately).
A houseplant on a dark shelf: our advice is the same lights, hung a little higher than over seedlings.
A cupboard or small tent: the Cloning & Propagation Tent Kit - Small puts a 12 W Gorilla strip and a Garland propagator in a light-tight DP60 tent, 60 x 40 x 60 cm. For herbs or chillies kept over winter, our suggestion is the LUMii 55 W PL in an E40 fixture: it suits tents and cupboards, its page says. For more room, the Switch Blade 150 W or a dimmed Daylight 200 W PRO, which covers 0.6 to 0.8 m, its page says. See growing chillies indoors.
A greenhouse: for seedlings on the bench, the Maxibright LED T5s, whose pages name greenhouses among their uses. To light a crop, the AC Infinity Ionframe LED Growlights are IP65 water resistant and suit greenhouse use, their page says; the Philips Greenpower Lamps page lists greenhouse propagation and supplementary lighting among its uses, 400 W or 600 W to match a ballast and reflector from our HID lighting section.
A grow tent for flowering and fruiting: size the LED with our LED wattage guide and the tent with our what size grow tent guide.

Quick answers

Do grow lights work for indoor plants?

Yes, if the light is bright enough, hung close enough and run long enough: our advice is 18 hours a day on a timer, with a propagation light just above the seedlings.

Why are my tomato seedlings leggy, and can I save them?

Usually, yes. They stretched for light. Pot them on deeper, up to the seed leaves (tomatoes root along a buried stem, as our sowing guide says), then put them under a grow light, close, for 18 hours a day.

6500K or full spectrum for seedlings?

Either. The LUMii PL's 6500K cool white suits cuttings, seedlings and the vegetative stage, its page says; full spectrum covers flowering and fruiting as well, so choose it if the same light will take chillies or tomatoes to harvest.

Do plants need UV light?

Not to grow: PAR, the light plants use, is roughly the visible spectrum, our LED wattage guide says. We sell no separate UV lamp. The Ionframe page lists added UV and far red diodes, and the Daylight 200 W PRO and Maxibright Daylight 300W PRO pages list red and far red; none says what the UV or far red does on its own. Growers who add an HPS for flowering put part of the effect down to its UV and radiant heat: our LED or HPS guide gives that view, with the hedging it needs.

More Growers' Guides

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.