Grow lights explained
Which kind to buy, how close to hang it and how long to run it
When a grow light helps
A grow light fills in where windows fall short: dark corners, windowless rooms and the short, cloudy days of winter. Many houseplants benefit from extra light in winter, and some all year round.
Seedlings need one most of all. Light from a window is rarely enough for strong, sturdy seedlings.
Which kind
Two kinds work well at home.
- LED: efficient, long-lasting and cooler than other lights. They can cost more up front but use less power.
- Fluorescent tubes: still useful, especially slim T5 tubes. Replace them every year or two as they dim.
- Skip ordinary incandescent bulbs. They run hot, waste energy and make plants stretch.
What color
Plants use mostly red and blue light, which is why many LED grow lights glow pink or purple. Full-spectrum or daylight lights, or a mix of warm-white and cool-white tubes, work well too, and your plants still look natural under them. For most houseplants, special plant bulbs aren't worth paying extra for.
How close and how long
Distance depends on the light. As a guide, hang fluorescent tubes 6–12 inches (15–30 cm) above houseplants and LED fixtures 12–24 inches (30–60 cm) above. For seedlings, ordinary tubes go just 2–4 inches (5–10 cm) above the tops. High-output lights sit farther away, so follow the maker's guide.
Most plants do best with 12 to 14 hours of light a day, and never more than 16. Plants need darkness too, so put the lights on a timer.
Reading the signs
Bleached, scorched or reddened leaves, or unusually squat growth, mean too much light, so raise it. Stretched, spindly growth, yellowing, dropped leaves or no flowers mean too little, so lower it or add more light.
Stay safe
Keep plugs, cords and timers away from water and off the floor where you water, and don't overload outlets. Give fixtures space so heat can escape. The very hot lamps used in greenhouses need ventilation and aren't practical at home.
Related plant problems
Sources
- Iowa State University Extension: Sources of Supplemental Light for Indoor Plants
- Iowa State University Extension: How to Set Up Supplemental Lights for Indoor Plants
- RHS: Artificial lighting for indoor plants
- Clemson HGIC: Starting Seeds Indoors
General guidance, not a diagnosis. Written by PlantWiz in our own words from the sources above.