Grow Lights: What to Buy, Where to Put It, How Long to Run It

Human eyes adapt across an enormous range of brightness, which makes them useless for judging light for plants. A room that feels well lit under a ceiling fitting can be delivering a small fraction of what a plant a metre from a window receives, and a tiny fraction of outdoor shade.

The two numbers worth knowing are PPFD and DLI. PPFD is the intensity of usable light arriving at the leaf, measured in micromoles per square metre per second. DLI is the total delivered across a whole day, in moles per square metre — intensity multiplied by hours. DLI is what actually drives growth, which is why a moderate light running twelve hours can beat a brighter one running four.

As rough targets for houseplants: low-light foliage plants do reasonably at roughly 50–100 µmol/m²/s at the leaf; bright-indirect plants such as monstera, calathea and most aroids want somewhere around 100–300; and high-light plants — succulents, cacti, citrus — want 300 and upwards. A typical living room away from a window sits well below the bottom of that range.

You do not need a meter to use a grow light well, but a phone app that estimates light levels, or a cheap PAR meter, turns guesswork into a decision and is worth it if you are buying more than one fitting.

Buy a white LED. Full-spectrum white LEDs grow houseplants as well as the pink-purple fittings that dominated the market a decade ago, and they have a decisive practical advantage: you can see the true colour of your plants and spot problems. Pink light makes chlorosis, pests and burn almost impossible to notice.

Ignore the advertised wattage of cheap fittings, which is often the theoretical maximum of the chips rather than what the fitting draws. Look instead for actual power consumption, and for a manufacturer publishing a PPFD figure at a stated distance. A fitting that will not tell you what it delivers is usually not delivering much.

For a single plant or a small shelf, 10–25 W of real consumption is plenty. For a shelf a metre long, look at 30–60 W spread across a bar or panel rather than a single point source, which lights the middle and leaves the ends dark.

Beware of two categories. Screw-in bulbs in a standard lamp are convenient but generally weak and narrowly focused; they work for one small plant at close range and little else. And clip-on gooseneck lights with tiny heads look appealing but usually deliver useful light over only 10–15 cm.

Put it on a timer. This is not a refinement; it is the difference between a grow light working and a grow light being an inconsistent nuisance. A mechanical plug timer costs very little and removes the whole question from your daily attention.

Run it 10–14 hours a day for foliage plants and up to 14–16 for seedlings and succulents. Plants do need a dark period — respiration and several developmental processes happen at night — so continuous light is not better, and for a handful of species it actively prevents flowering.

Mount it above the plants and pointing down, not from the side. Side lighting produces the same lean and one-sided growth that a window produces, and it wastes most of the output on the wall.

Set the height by the target above, then adjust by watching the plant for a fortnight. Growth that is compact with slightly smaller leaves and good colour means it is about right. Bleached, yellowing or bronzed patches on the upper leaf surfaces mean it is too close. Continued stretching and pale growth means it is too far away or running too few hours.

Clean the fittings occasionally. Dust on a diffuser reduces output measurably over a season, as does dust on the leaves.

Grow lights attract more folklore than any other piece of houseplant equipment, and most of it is either outdated or invented for marketing.

Before buying, exhaust the free options, because they are often enough. Move the plant closer to the glass — a metre nearer a window is a large gain. Clean the windows and wipe the leaves. Remove or tie back anything shading the window from outside. Swap heavy curtains for a sheer blind. Move a plant from a north-facing room to an east or south-facing one.

A grow light earns its place in four situations: a genuinely dark room with no usable window; a north-facing home in a cloudy climate through winter; growing plants that want considerably more light than a home provides, such as succulents and citrus; and propagation, where consistent light on cuttings and seedlings makes a real difference.

It is also worth being realistic about running cost. A 30 W fitting for twelve hours a day uses roughly 11 kWh a month — the price of a couple of coffees in most places. The reason to hesitate is rarely the electricity.

Finally, expect a change in watering. A plant under a good light grows faster and uses more water, so the interval that suited it in a dim corner will be too long once the light goes in. Check the medium rather than keeping the old routine.