GROWING GUIDE
Grow Lights for Indoor Plants
A bright winter window is enough when a low-light plant stays compact and is not stretching. Add a grow light when the room is dim, the plant wants high light, or new stems grow long and pale. Run most lamps 12 to 14 hours, then give a real night. A short-day bloomer still needs that long dark night.
Iowa State University Extension says nearly all indoor plants benefit from some supplemental light in winter, even though they do not need the same amount. A window that was enough in summer can fall short once the days shorten. University of Maryland Extension says to match the plant to the light you have, and to supplement when the room cannot supply it. Intensity falls quickly as a lamp moves away, and too much direct light can turn leaves pale or brown. For duration, Iowa State calls 12 to 14 hours a sound span for most species, and asks home setups to stay between 10 and 16 hours so the plant still gets a dark period. Maryland’s ceiling is 16 hours in total, daylight included. Christmas cactus, poinsettia, and kalanchoe are the exception: their buds form when the day is about 11 hours or shorter, so a lamp left on in the evening can block flowering. Measuring a lamp’s daily total is a different page.
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Is the window enough this winter?
Winter light: keep a compact low-light plant in a bright window. Add a grow light when the room is dim, the plant wants high light, or new growth stretches. Most lamps run 12 to 14 hours and then go dark. A labeled grow light stays at the distance on the box. Christmas cactus, poinsettia, and kalanchoe still need a long night, about 11 hours of day or less, or the buds will not set.
| What you have | Window or lamp | Duration | Distance | What you should see |
|---|---|---|---|---|
| Low-light plant, and a window where growth stays compact | The window is enough. Do not add a lamp yet. | Daylight only. | No lamp. Keep it out of harsh midday sun. | Compact growth. A pale stretch means the window is no longer enough. |
| A dim room, or a plant that is stretching now that the days are short | Add a light. | 12 to 14 hours, then dark. Stay between 10 and 16 hours. | Labeled grow light: the distance on the box. A shop LED example is about 18 inches. Fluorescent tubes are about 6 to 12 inches. | New growth shortens and holds color over the next few weeks. |
| A high-light plant, such as jade or indoor herbs, or no useful window | Add a light. A north window will not replace it in winter. | The same 12 to 14 hours. Maryland’s winter herb note uses 14 to 16 hours of supplemental light. | Same distance rules. Intensity falls quickly as the lamp moves away. | Leaves hold color. Scorch means the lamp is too close, or the sun through the glass is too hard. |
| A short-day bloomer: Christmas cactus, poinsettia, or kalanchoe | Use the window. Do not leave a lamp on in the evening. | Buds form when the day is about 11 hours or shorter. | Do not park a lamp on the plant through the night. | Buds form. If they drop, that is the bud guide, not more hours of light. |
| Too little light: long stems, a lean, faded leaves, or flowers that never come | Move the plant closer to the window, or add a fixture. | Do not fix stretch by leaving the lamp on all night. | Closer, or a second fixture. Iowa State lists those moves before a longer day. | Stretch eases. Color returns on new growth. |
| Too much light: pale, brown, bleached, or scorched leaves | Move the lamp farther away, or soften the window. | Do not add hours to a scorched plant. | Farther from the lamp. A sheer curtain for harsh sun. | The burn stops spreading on new leaves. |
This table is the winter choice. Indoor plant light is the other guide: it measures light at the leaves and can turn a steady lamp reading into a daily total. Use it when you already have that reading. It does not replace this decision, and this decision does not replace it.
Everyday light, water, and the pot still live on indoor plant care. A lamp does not tell you to pour. When to water plants is the habit, and the watering calculator sets the next check once you enter the season. A cool, dim week dries slowly.
A Christmas cactus that is setting buds needs the long night in the table, not a brighter evening. The plant plan is Christmas cactus. Buds that are already falling are Christmas cactus buds falling off.
Common questions
When is a window enough?+
When a low-light plant stays compact in that window. Stretching, a lean, or pale new growth means the window is no longer enough, which is common in winter.
How long should the lamp run?+
Iowa State says 12 to 14 hours for most plants, and not outside 10 to 16 hours. Maryland says no more than 16 hours total, daylight included. Short-day bloomers need a longer night.
How far should the light be?+
A labeled grow light uses the distance on its box. Iowa State’s shop LED example is about 18 inches, and fluorescent tubes are about 6 to 12 inches. Intensity falls quickly with distance.
How is this different from the light calculator?+
The indoor plant light guide measures light at the leaves and can turn a steady lamp reading into a daily total. This page chooses window or lamp before that measurement.
Sources & editorial notes
- Iowa State University Extension: supplemental lights for indoor plants
- University of Maryland Extension: lighting for indoor plants
- University of Maryland Extension: growing herbs indoors
This is a winter decision, not a daily-light measurement and not a second indoor-care hub. Sources checked November 9, 2026. Editorial responsibility.