Some links on this page are to Amazon UK. If you buy through them, Plant Light Guide may earn a commission. This does not change the price you pay. Affiliate disclosure.

Watering plants under grow lights

Adding a lamp changes more than the light. A plant that was ticking over on a dull sill starts to grow, and growing plants use water. At the same time you have put something electrical on the shelf where the watering can goes. Both deserve a little thought before the first winter under the lamp.

A lamp changes how fast compost dries

Under a lamp, especially in a warm room, a plant is more active, and the leaves give off more water. The lamp and its driver also add a little warmth to the air right around the pot. The result is usually compost that dries faster than it did on the same shelf without a light.

That is not a rule to water more. It is a reason to check more often. A schedule that worked last winter, before the lamp, may now leave the compost dry for days. Or it may still be right, if the room is cool and the plant is slow. The only reliable test is the compost:

• Push a finger in to the first knuckle. Dry at that depth is the usual cue for most foliage houseplants.
• Lift the pot. A dry pot is noticeably lighter than one you have just watered, and you learn the difference quickly.
• Look at the plant, not the calendar. Drooping in damp compost is a different problem from drooping in dry compost.

Succulents and cacti are the exception that catches people out. A lamp may make them grow, but they still rot in wet compost, especially in a cool room. The succulent guide covers that balance.

Hours matter too. A plant on a long lamp day uses more water than one on a short top-up. If you lengthen the photoperiod, expect to check the compost more often. The ranges are in hours and a dark period, and the dark period still applies, whatever the watering does.

Keep water off the lamp, the lead and the timer

Most houseplant grow lights are not sold as waterproof. A bar under a shelf, a bulb in a table lamp and a clip gooseneck are all indoor fittings meant to stay dry. Water reaches them in a few predictable ways:

• Pouring from a can held under a low bar, so the spout or the splash hits the fitting.
• Misting leaves directly under a lamp. Mist drifts, and the lamp is the nearest thing above the plant.
• Overflow from a saucer running along the shelf to where the lead or the plug sits.
• A plug-in timer on a socket next to the sink or a draining tray.

The simplest habit is to take the pot off the lit shelf to water it. Water it in the sink or over a tray, let it drain fully, wipe the base, and only then put it back under the lamp. Switch the lamp off first if you water in place. If you mist, mist away from the lamp, or skip misting for plants that live under one.

Keep the lead, the plug and the timer away from where water runs. Route the cable along the back or above the shelf, not along the front edge where drips land, and keep the timer on a socket away from the tray. Cable routing and timers are in timers, cables and shelf layout.

If water does get into a fitting, a lead joint or a plug, unplug it at the wall if you can do so without touching wet parts, and stop using it until it is dry and you are sure it is undamaged. If in doubt, replace it. This site does not give repair instructions. We do not give wiring instructions. Anything to do with fixed wiring, a new circuit, or a fitting in a bathroom or kitchen belongs with a qualified electrician working to Part P.

Do not leave pots sitting in water

Saucers stop water reaching the shelf. They should not hold water for long. A pot standing in a full saucer keeps its lower compost waterlogged, and roots in waterlogged compost cannot get air. Under a lamp the top can look dry while the bottom stays wet, which tempts you to water again.

Empty the saucer a little while after watering. If you use a tray of gravel or clay pebbles for humidity, keep the water level below the base of the pot so the pot sits above it. On a lit shelf, standing water is also one more thing that can splash or overflow towards the lamp.

Cachepots, the decorative outer pots without holes, are where this goes wrong most often, because you cannot see the water collecting at the bottom. Lift the inner pot out and tip the outer one after watering.

Herbs and kitchens

Kitchen herbs are usually the thirstiest plants under a lamp, and the kitchen is the room with the most water nearby. Basil in a small pot under a bar can dry out quickly in a warm kitchen, while parsley on a cool sill may not. The herb guide covers splash from kettles and sinks, and how to keep the fitting back from them: grow lights for kitchen herbs.

Closed cabinets

A glass cabinet with a lamp holds moisture as well as warmth. Water less often than you think you need to, vent the door, and watch for condensation on the glass and mould on the compost. Fitting bars inside cabinets, and keeping the lead clear of the door, is in grow lights for shelves and glass cabinets. Heat inside a closed cabinet has its own page: heat and ventilation on a plant shelf.

Questions

Should I water more often when I start using a grow light?

Check more often, rather than water more by default. A lamp often makes compost dry faster, but the room temperature and the plant matter as much. Water when the compost tells you it is dry.

Is it safe to mist plants under a grow light?

Not directly under the fitting. Most houseplant grow lights are not sold as waterproof. Mist away from the lamp with it switched off, or skip misting for plants that live under one.

Can I water plants without moving them off the lit shelf?

You can if you switch the lamp off, pour slowly at the base of the plant, and keep the spout and any splash away from the fitting, lead and timer. Taking the pot to the sink is easier and drains it properly.