Plant lights should be on long enough to make sturdy growth, then off so plants can rest. In a UK home that usually means somewhere between about 8 and 16 hours, not 24. The right number depends on what you are growing and how much real daylight the room already provides. A cheap plug-in timer does the remembering for you.
Guidance from the RHS, Dobbies and Which? all point the same way: long enough day lengths for growth, with seedlings often needing the longest days when natural light is weak.
Quick hour ranges for home use
| What you are growing | Typical day length on the timer | Notes |
|---|---|---|
| Houseplants topping up a bright room | About 8 to 12 hours | Shorter if the window is already strong |
| Herbs and leafy salad indoors | About 12 to 14 hours | Steady daily rhythm beats occasional blasts |
| Sun-loving seedlings (e.g. tomatoes) | About 12 to 16 hours | Which? notes sun-lovers often need at least 12 hours |
| Seedlings with little or no daylight | Around 16 hours | Matches RHS/Dobbies-style seedling guidance |
| Any plant | Avoid 24 hours | Give a proper dark night |
These are home starting points, not laboratory rules. Adjust after a week based on how the plants look.
Why plants still need darkness
Growth is not only “more light equals better”. Plants use the dark period for rest and normal development. Leaving lights on all night wastes electricity and can stress soft growth. Set the timer for a clear on block and a clear off block every day.
Daylight plus plant light
If your herbs already sit on a bright sill, the plant light is a top-up. You might run it for the dull middle of winter days, or for a fixed 10 to 12 hours that overlaps morning and evening. If the stand is in a dark corner with no useful window light, treat the timer as the whole day and aim toward the longer end of the ranges above.
How to tell the hours are wrong
Too little light time (or light too far away): stems stretch, leaves pale, seedlings lean and flop. Add hours or lower the light a little.
Too much intensity or duration: leaves bleach, look washed out, or crisp at the tips nearest the bars. Raise the light, shorten the day, or both. Dobbies notes pale leaves can mean too much light.
Change one setting at a time and wait a few days.
Timers make this easy
Plug the light or stand into a 24-hour mechanical timer (or a simple digital one). Set the pins or programme for your chosen day length. You will not forget to switch off when you leave the house.
Many lit stands at The Indoor Garden Centre run from a single UK plug, so one timer controls every shelf. See plant stands with lights and Plant Lighting.
Houseplants, herbs and seedlings are not the same job
A snake plant in a bright lounge may only need a short top-up on the dullest weeks. Basil on a lit kitchen stand is working harder and usually wants a fuller day. Tomato and pepper seedlings aimed at a spring planting are hungrier still.
If one stand holds mixed crops, set a compromise day length toward the neediest shelf, keep that shelf closest to the lights, and put easier houseplants on a lower or edge position. Or run a separate clip bar on its own timer for the seedlings.
Consistency beats perfection. Plants respond better to 13 hours every day than to 16 hours some days and nothing on others.
Electricity: a worked example
Which? reports grow lights can cost less than 2p an hour. Example only: a 54W stand on for 14 hours at 25p/kWh ≈ 54 × 14 ÷ 1000 × 0.25 ≈ 19p a day. Your tariff may differ. Shorter hours cost less; longer seedling days cost a little more.
FAQ
Can I run plant lights at night only?
You can, if that suits your routine, as long as plants still get a dark rest afterward. Many people prefer daytime hours so the white light doubles as pleasant room light.
Do different shelves need different hours?
Usually one timer for the whole stand is fine. If one shelf holds seedlings and another holds tough houseplants, put the hungrier crop on the shelf closest to the lights and keep a compromise day length, or run a separate bar on its own timer.
Should hours change in summer?
Often yes. In bright months you may shorten or pause supplemental light for plants that sit near a good window. Seedlings in a dark room may still need a full artificial day.
Is 18 hours better than 16 for seedlings?
Not automatically. Stick near published home guidance (around 16 hours when daylight is poor), watch for bleaching, and favour consistency every day over occasional extreme runs.
Where do I read about distance from the leaves?
See our guide on how far a plant light should sit from the leaves (in this Care and Guides series), and the product instructions for your model.
What to read next
Soft product pick
Pair any light with a timer. For a full setup that already uses one plug, try the Spider Farmer slim herb stand or Mars Hydro small 54W stand and set a steady daily schedule from day one.
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