Two documented causes explain a plant not flowering under lights. Your plant either gets too little total light each day, or your day runs too long for a species that sets buds only on short days. Both look the same from across the room. Green leaves, decent growth, no buds. You need a timer and the name of your species to tell them apart, because a stronger bulb fixes only one of the two.
My own Christmas cactus sat on the living room windowsill for three winters and never set a bud. A floor lamp a few feet away burned until eleven at night. University of Maryland Extension puts the trigger for poinsettia, kalanchoe and Christmas cactus at 11 hours of daylight or less. That lamp handed the plant a 16 hour day through November and December.
Start with the shortage. Flowering houseplants and succulents target 12 to 16 mol/m²/day. That number is a daily sum, not a brightness reading. So a dim fixture on a long run and a bright one on a short run can both land under it. Red wavelengths push flowering and fruiting, so check what your panel puts out before you blame your plant. Photoperiod houseplants stuck below the daily total keep making leaves and stop there.
The second failure has nothing to do with strength. A short day species reads the length of your night. It sets buds only once that dark stretch runs long enough. Give one a 14 hour day under a grow light and you get green, healthy leaves and nothing else for years. Tomatoes need about 6 hours of darkness to fruit. Peppers do not, so two of your plants on one timer can behave nothing alike.
You can tell the two apart without a meter. A plant short on total light stretches, and you see long gaps between the leaves with pale new growth leaning hard at your fixture. A plant on too long a day looks healthy and compact, with dark leaves and no buds at all. Check your stretched plants first, since more light is the faster test of the two.
Stray room light counts more than you expect. The signal that sets your plant's clock needs only about 10 foot-candles, no more than a lamp across the room throws. A fixture that actually feeds your plant runs 400 to 500 foot-candles. So a hallway bulb, a TV, or a porch light through thin curtains can cancel a long night.
Find out whether your species is photoperiod sensitive. Poinsettia, kalanchoe and Christmas cactus all are, and they will not bud on a long day no matter how bright your panel is.
Add up the natural daylight in that spot plus every hour your grow light runs. Your plant reads the total, not the part you control.
Hold the full window at 11 hours or less while buds set. An outlet timer does this better than you will by hand.
Move your plant away from lamps, screens and windows that catch outdoor light after dark. A spare closet works well during the weeks when buds set.
For plants that are not photoperiod sensitive, adjust the timer within 8 to 16 hours a day. Watch how growth responds before you change anything else.
In cold northern climates the supplemental light season runs October through March, the same stretch when short day plants want their long nights. Those two jobs pull against each other in one room. Run your strong light early in the day, then let your evening go dark and keep it dark. An outlet timer costs less than a new panel, and it fixes what a new panel cannot.
Read the full article: Grow Lights for Indoor Plants: What Really Works
