Most indoor plants do best with the lights on 12 to 14 hours a day. Iowa State University Extension puts the home range at no less than 10 hours and no more than 16. Set your grow light hours per day inside that window and almost anything on a shelf does fine. Short day plants are the exception. They set buds on long nights, so piling on run time works against them.
The fix cost nothing and took ten seconds. I pulled the plug-in timer off the shelf and moved the pins from a 10 hour run to a 14 hour run, then set the fixture back at the same height. The tray under it collected far more light each day. Longer run time is one of the four ways to raise a plant's daily light integral, and it is the only one that does not mean a brighter lamp or a lower shelf. A cheap mechanical grow light timer handles the whole thing. Mine clicks over on the hour and I can hear it from the next room.
That daily total is what a plant answers to, not the number printed on the box. Brightness is how hard the light falls and hours are how long it falls, and the plant only cares how much it adds up to by dark. So the two trade against each other. A fixture putting out 200 micromoles per square meter per second reaches a daily total of 8 mol/m²/day in about 11 hours. Cut that output in half and the same target takes twice as long.
The ceiling counts your windows too. Keep the total at or under 16 hours a day once you add daylight to lamp light, because most plants need a dark period, and too much light does as much harm as too little. Tomatoes want about 6 hours of darkness before they will set fruit. Peppers do not.
Stretching the schedule is no help at all for short day plants. They read the length of the night, so extra run hours can keep them from budding even while the leaves look fine.
The floor deserves the same respect as the ceiling. 10 hours is the low end of home guidance, and the one reason to go under it is a deliberate test on a plant that refuses to bloom. The published troubleshooting range bottoms out at 8 hours, which is short enough that you want a clear reason before you try it.
Start at 14 hours and then stop fiddling with it. That is the general starting point, and a timer holds it steadier than you will, because the days you forget are the days it counts. Change the schedule when a plant tells you something is off, and stay inside 8 to 16 hours while you test. Give any change a week or two before you judge it. One shift at a time, or you will never know which one did the work.
So how long to leave grow lights on in your own room? Count the daylight the plant already gets, then fill the gap up to 14 hours. A window with no direct sun in midwinter gives so little that the lamp does the whole shift. In June a bright one may leave you an hour or two to add. That is it. My own shelf runs a long schedule from November through March and a short one for summer, and the timer is the only thing I touch. Flowering trouble under lights runs on its own rules, so keep that apart from the clock.
Read the full article: Grow Lights for Indoor Plants: What Really Works
