A grow light is an electric light designed to give plants the light they need when daylight is not enough — in winter, in a dark room, or for seedlings started indoors in early spring. Used well, a grow light can keep herbs alive through winter, stop seedlings going leggy, and help houseplants through the darkest months. Used badly, it wastes electricity, scorches leaves, or does so little that the plants still stretch towards the window. This guide explains how grow lights work in general terms, which types suit which jobs, and how to set one up properly. No prices and no invented energy figures are given here: costs and consumption vary by product and by electricity supplier, so check the label on the product you buy and your supplier's rates.
The first thing to understand is that plants need light for photosynthesis, and the amount and duration of light matter more than the exact colour. A plant in a dark room will stretch towards the nearest window, grow pale and weak, and eventually fail, however much you water and feed it. A grow light does not replace daylight entirely for most plants — it works best as a supplement for a bright windowsill, or as the main light for a small indoor setup such as a shelf of seedlings or herbs. If you are growing on a windowsill, see our windowsill gardening guide first, because a brighter sill often solves the problem without any equipment.
Types of grow light
The most common types are fluorescent tubes, LED panels and LED bulbs. Fluorescent tubes have been used for seedlings for years: they give off little heat, they are inexpensive, and they work well hung close above young plants. LED grow lights are now the most common choice for home use: they use less electricity than older types for the same light, they last a long time, and they give off little heat, so they can be hung close to plants without scorching them. Some LED lights are full-spectrum, giving a white light that looks natural and suits living rooms; others give a pink or purple light, which looks odd in a home but is designed to give plants the wavelengths they use most. For most home gardeners, a full-spectrum white LED is the most practical choice, because it lights the room normally and suits seedlings, herbs and houseplants alike.
Older types, such as high-pressure sodium and metal halide lamps, are mainly used by commercial growers: they are powerful, but they give off a lot of heat, use more electricity, and are not practical for a home windowsill or shelf. Avoid them for home use. When you buy a light, check that it is sold as a grow light or a plant light, not just as an ordinary household bulb, and check the manufacturer's guidance on how high to hang it and how large an area it covers. The grow light calculator can help you work out coverage for your space.
How to set up a grow light
Hang or mount the light above the plants, not to the side, because plants grow towards light and a side light makes them lean. The distance matters: too far away and the light is too weak to help; too close and it can scorch leaves or overheat them. As a general rule, LED lights are hung closer than older hot lights, but the right distance depends on the product — follow the manufacturer's guidance for your light, and watch the plants: if they stretch and go pale, the light is too far away or on for too short a time; if leaves bleach, curl or scorch, it is too close. Raise the light as seedlings grow, so the distance stays roughly the same.
Give plants a day and a night. Most seedlings, herbs and houseplants do well with roughly 12 to 16 hours of light a day, followed by a period of darkness, because plants need darkness as part of their normal cycle. Do not leave a grow light on around the clock: it wastes electricity and can stress plants. A simple plug-in timer is the easiest way to manage this, and it costs little. Set the timer so the light comes on in the morning and goes off in the evening, matching a natural day as closely as you can. Keep the light and its cable away from water, and never use a light with a damaged cable.
Which plants benefit most
Seedlings benefit most of all. Tomatoes, peppers, chillies and flowers started indoors in early spring often go leggy on a windowsill, because the days are short and the light is weak. A grow light hung above them keeps them short, sturdy and green until they can go outside. Herbs such as basil, parsley and chives also benefit, especially in winter — see our indoor herb garden guide. Houseplants that are struggling in a dark room may perk up with a grow light, but first check whether the real problem is water, temperature or a pot that is too small, because a light will not fix those. Tropical houseplants such as anthuriums and calatheas, which are covered in our houseplant guides, generally prefer bright indirect light rather than a strong light hung close above them.
Using grow lights safely
Treat a grow light like any other electrical device. Keep it dry, keep cables tidy and away from water, and do not overload sockets. Check that the light is suitable for indoor use and that it has no damaged parts. Do not look directly into a bright grow light for long periods, and avoid pink or purple lights in a room where you spend a lot of time if you find them uncomfortable. Keep the light out of reach of children, and teach children that it is not a toy. If you are unsure about the electrical setup, ask a qualified electrician rather than improvising. Check your local extension service if you want advice on whether a grow light is the right solution for your plants, or whether a brighter position would do the job better.
Common mistakes
The most common grow light mistakes are hanging the light too far away, so it does too little; hanging it too close, so it scorches leaves; leaving it on all day and night; buying a light that is too small for the area it needs to cover; and using a grow light to fix a problem that is really about water, temperature or soil. Another is expecting a grow light to turn a dark cupboard into a greenhouse: it can help a bright sill or a small shelf, but it cannot replace daylight for a large plant in a dark room. Start with the brightest natural position you have, add a light as a supplement, follow the manufacturer's guidance on distance and hours, and watch the plants — they will tell you whether the setup is working.
Frequently asked questions
Do I need a grow light for seedlings?
Not always. Seedlings sown in late spring on a bright windowsill often do well without one. Seedlings sown early, when days are short and light is weak, benefit greatly, because a grow light keeps them short and sturdy instead of tall and pale. If your seedlings keep going leggy, a grow light is the most reliable fix.
How many hours a day should a grow light be on?
Most seedlings, herbs and houseplants do well with roughly 12 to 16 hours of light a day, followed by darkness. Use a plug-in timer to switch it on in the morning and off in the evening. Do not leave it on around the clock, because plants need a dark period and it wastes electricity.
How far above the plants should a grow light hang?
It depends on the light, so follow the manufacturer's guidance for your product. As a general rule, LED lights can hang closer than older hot lights, but if leaves bleach, curl or scorch, the light is too close, and if plants stretch and go pale, it is too far away. Raise the light as seedlings grow.
Are LED grow lights better than fluorescent tubes?
For most home gardeners, yes. LED lights generally use less electricity for the same light, last longer, and give off little heat, so they can hang close to plants. Fluorescent tubes still work well for seedlings and are often cheaper to buy, so either can work — the key is to match the light to the area you need to cover.
Can a grow light replace sunlight completely?
For small setups such as a shelf of seedlings or herbs, a good grow light can be the main light source. For most plants, though, a grow light works best as a supplement to daylight on a bright windowsill. A large plant in a dark room will not thrive on a small grow light alone, whatever the packaging says.