Using grow lights effectively means matching light intensity and duration to each plant’s needs, keeping LEDs 6–18 inches above the canopy, and running them 12–16 hours a day on a timer rather than continuously.
Starting seeds in February or keeping a fiddle-leaf fig happy through a dark winter comes down to one thing: giving plants enough usable light. Natural sunlight is best, but when windows fall short, a grow light fills the gap—if you get the setup right. The wrong distance, the wrong schedule, or skipping the timer turns a helpful tool into a plant-killer. Here is exactly what works, measured in inches and hours.
Pick the Right Type of Grow Light
LEDs dominate for a reason: they run cool, last for years, and produce a full spectrum that plants actually use. Fluorescent T5 HO tubes are a solid second choice, especially for flats of seedlings, and the Royal Horticultural Society recommends a basic setup with two 60 cm T5 tubes suspended about 60 cm above a small group of houseplants. If you need something rugged for outdoor potting areas or greenhouse benches, our tested exterior grow light picks cover weatherproof options that hold up to damp conditions.
How Far Should You Place the Light?
Distance determines whether a plant gets fed or scorched. LEDs can sit as close as 6 inches from the top leaves without burning, though 10–12 inches is a safe starting point for most setups. Fluorescent and HID lights need at least 12 inches of clearance; incandescent bulbs should stay two feet or further. The critical rule is simple: the light must never touch the leaves. Mount the fixture directly above the canopy for even coverage, and if you use multiple fixtures, angle them slightly to reduce shaded spots. As seedlings grow or mature plants stretch, adjust the light up to maintain that working distance—check height every week or two.
How Many Hours Per Day?
Plants need darkness for normal metabolic processes, so 24-hour lighting does more harm than good. The safe range for most indoor plants and seedlings is 12–16 hours on and at least 8 hours off. Low-light plants like pothos or snake plants do fine near the lower end of that range, while seedlings and vegetative leafy greens perform best at 16–18 hours. Use a timer—it is the single cheapest way to guarantee consistency. Without a timer, you either forget to turn the light off (stressing the plant) or forget to turn it on (starving it).
Watch Your Plant for Signs You Got It Wrong
The plant tells you what it needs if you pay attention. Leaves that look bleached, curl at the edges, or develop brown scorched patches mean the light is too close or running too long. Stems that stretch thin and lean toward the fixture mean the light is too far away or not bright enough. Rotate pots every few days if your light comes from one side, so every leaf gets its turn under the beam. If natural light is already decent and you are just supplementing, run the grow light fewer hours than you would for a fully artificial setup—observe how the plant responds and adjust.
| Plant Type | Light Hours Per Day | Best Starting Distance (LED) |
|---|---|---|
| Low-light houseplants (pothos, snake plant, ZZ) | 10–12 | 12–18 inches |
| Herbs and leafy greens (basil, lettuce, mint) | 14–16 | 10–12 inches |
| Seedlings and vegetative growth | 16–18 | 6–10 inches |
| Flowering plants and succulents | 12–16 | 8–12 inches |
The table above is a starting point, not a rule etched in stone. If leaves bleach at 10 inches in your setup, move the light up by two inches and see if the plant settles. If seedlings look leggy at 14 hours, push to 16 and lower the light by an inch. Small adjustments over a week tell you more than any generic chart.
FAQs
Can I use a regular household LED bulb as a grow light?
Regular “daylight” LEDs around 5,000K work for low-light houseplants and seedlings in a pinch, but they lack the full spectrum and intensity of bulbs designed for plant growth. For consistent results with herbs, succulents, or flowering plants, use a dedicated full-spectrum grow light.
What happens if I leave grow lights on 24 hours?
Plants stop growing properly without a dark period—they need darkness for respiration and to process the energy they captured during the light cycle. Continuous light stresses the plant, can stunt root development, and sometimes triggers leaf drop or discoloration within a few weeks.
Do grow lights work for starting seeds without any sunlight?
Yes. A full-spectrum grow light placed 6–10 inches above the seed tray, running 14–16 hours a day, produces stocky, healthy seedlings without any natural light at all. The key is keeping the light close enough to prevent leggy growth and raising the fixture as the seedlings gain height.
References & Sources
- University of Minnesota Extension. “Lighting for Indoor Plants.” Recommends timer use and covers light types, hours, and distance for seed starting and houseplants.
- Royal Horticultural Society. “Artificial Lighting for Houseplants.” Describes T5 fluorescent setup with two 60 cm tubes at 60 cm height for 16 hours.