For most indoor crops the answer fits in one sentence: about 14 to 16 hours of light a day for herbs and salads, about 16 hours for microgreens and seedlings, and a dozen hours for ornamental foliage plants. In every case a real dark period is needed: a lamp left on continuously does not speed growth up. A timer settles the question once and for all.

What the number of hours really changes

A plant does not take in light the way a glass is filled with water: what counts is the total amount accumulated over a day. That amount depends on two things, the intensity of the lamp at leaf level and the number of hours it stays on.

This is why duration is as useful a lever as power. A modest lamp left on for a long time can deliver as much as a strong lamp on for a short time. It even has a practical household advantage: less instantaneous power, so less risk of scorching the foliage if the lamp is close.

The limit is that this trade-off is not endless. A lamp that is too weak or too far away delivers almost nothing at leaf level, and stretching the hours will not make up for a light fitting hung a metre above the plants. Duration sets the amount; distance and power set the intensity. Our guide grow lights for indoor growing covers that second half in detail.

The hours crop by crop

The figures below come from university extension services. They hold as a starting point, to be adjusted afterwards by watching the plants.

These ranges assume the lamp is the main source. In a room that already gets several hours of strong daylight, you can stay at the low end.

Microgreens are the exception at the start. During their first days they are deliberately kept in the dark under a lid, so that the seeds come up evenly. The lighting hours only begin when the tray is uncovered. The full sequence is in our microgreens guide.

Why the night is essential

It is tempting to assume that a lamp on twenty-four hours a day would grow plants twice as fast. That is not what happens.

• Plants respire continuously, in the dark as well, and part of their metabolism takes place during that phase. The day-night alternation is part of how they normally work.
• Many species set their flowering by day length. Very long lighting can push some plants to flower or bolt earlier, which is not what you want when you are growing for the leaves.
• Continuous light damages the foliage of some species, with spots or discoloration appearing after several days.
• Electricity counts. Going from 16 to 24 hours raises consumption by half for no gain at all, or even a loss.

The university figures cap out around 18 hours for seedlings in any case. Beyond that, you spend without gaining.

Setting the timer, once and for all

1. Pick a window that suits you. For 15 hours, a cycle running from 07:00 to 22:00 works well: the lamp is on while you are in the room, and off at night.
2. Programme the timer from day one, built into the lamp or as a plug-in timer. That is what prevents forgetting, in either direction.
3. Keep the same window every day. A steady duration is worth more than an average reached with days of 10 and 20 hours.
4. Write down the date you set it. When a crop misbehaves two weeks later, knowing what was set and when saves a great deal of time.
5. Check after a power cut. Some timers restart their cycle at the moment power returns, which shifts the whole schedule.

Adjusting for the season and the spot

In winter, daylight is weak and the days are short: the lamp supplies the bulk of it, and you should aim at the top of the range. It is also the season when seedlings grow leggiest for want of light; the subject is covered in our guide on leggy seedlings.

In spring and summer, a well-lit window can be enough for sturdy herbs. The lamp goes back to being a top-up, over shorter hours, or only on grey days.

Away from a window, in a kitchen or on a living-room shelf, the programmed hours are the only light received. Stay at the top of the range all year in that case, without going beyond 18 hours.

On several tiers, each shelf needs its own light: the shelf above lets nothing through to the one below. Strip lights or clips under each shelf save improvising.

Letting the plants correct the setting

Plants say soon enough whether the duration and the distance suit them.

• Stems stretching, leaves widely spaced, plants leaning over: the amount of light is not enough. Move the lamp closer first, then lengthen the hours if needed.
• Pale, yellowed or curled leaves at the top of the plant: the lamp is too close or too intense. Move it a few centimetres away before touching the hours.
• Slow but compact growth, dark green foliage: the plant is getting enough light but lacks something else, often warmth or nutrients. This is not a timer problem.
• A herb flowering early: pinch out the buds and, if it happens again, cut back by an hour or two.

Wait a week between two adjustments. One change a week, calmly observed, is worth more than three settings in three days, after which you will no longer know which one produced which effect.

What it costs to run

The sum is simple: multiply the power actually drawn by the number of hours. A 24 W lamp on for 16 hours draws 384 Wh a day, or about 0.38 kWh; over 30 days, that comes to about 11.5 kWh. Multiply that total by the price of a kWh on your tariff to get the monthly cost.

Two practical consequences. First, it is the power drawn that goes into the sum, never an "equivalent" wattage printed on the packaging. Second, one hour more or less a day has a price: on this lamp, an hour a day comes to about 0.7 kWh a month. That is no reason to trim useful hours, but it is a reason not to light for twenty hours without cause.

In a hydroponic system the lighting hours are set at the same time as the rest of the routine: our guide indoor hydroponics without the jargon brings light, nutrient solution and renewal together. Strip lights and mounts are in the grow lights section.

Sources

• A Primer to Supplemental Lighting for Indoor Seed Starting — University of Illinois Extension
• Growing Microgreens and Baby Greens Indoors — University of Maryland Extension
• Starting Vegetable Seeds Indoors: II. Materials — Utah State University Extension
• Introduction to Microgreen Production in Indoor Vertical Farms and Greenhouses — Virginia Cooperative Extension
• Hydroponics at Home — University of New Hampshire Extension