To use a grow light, hang it at the height its maker recommends, put it on an outlet timer, and adjust as needed. For vegetable seedlings, University of Maryland Extension says to keep fluorescent and LED fixtures 2 to 6 inches above the plants and lit 14 to 16 hours a day, adjusting as needed to keep plants stocky rather than stretched and leggy. For other fixtures and plants, follow the light maker’s hanging guidance.
Below we walk through each step in detail so you can dial in distance, timing, and spectrum for your specific plants, whether you’re starting seeds, growing herbs, or keeping houseplants thriving through winter.
Step 1: Choose a Grow Light That Matches Your Plants and Space
Before worrying about settings, choose a fixture that fits your plants and space; where the maker lists a coverage area, use that.
- Clip-on or strip LED — a small fixture that clips on or runs as a strip.
- T5/T8 bar lights — slim tube-style bars that mount over a shelf or tray.
- LED panel (quantum board) — a flat board of LEDs that hangs above the plants.
- Full-spectrum pendant — a hanging, lamp-style fixture.
If you haven’t chosen a light yet, our grow light recommendations guide covers current picks across budgets and grow spaces.
Step 2: Position Your Light at the Correct Distance
Distance changes how much light reaches the plants: Iowa State University Extension says PPFD can drop significantly as you get further from the light source. It lists leaf scorch and bleached leaves among the signs of too much light, and excessive elongation or stretching among the signs of too little.
For vegetable seedlings, University of Maryland Extension says 2 to 6 inches for fluorescent and LED fixtures; for other fixtures and plants, use the hanging height in the light maker’s instructions.
For seedlings, University of Maryland Extension says to adjust as needed to keep plants stocky rather than stretched and leggy.
Step 3: Set the Right Timer and Light Schedule
The right schedule depends on what you’re growing. For vegetable seedlings, University of Maryland Extension recommends 14 to 16 hours a day. We give an hours figure only where a named extension source does; for everything else, check the seed packet, the plant’s care guidance or the light maker’s instructions.
| Plant Type | Daily Light Schedule | Notes |
|---|---|---|
| Vegetable seedlings | 14 to 16 hours (University of Maryland Extension) | Use an outlet timer and adjust to keep seedlings stocky, not stretched and leggy (University of Maryland Extension) |
| Herbs (basil, mint, parsley, cilantro) | See the seed packet or care guidance | Check the seed packet or the light maker’s instructions for hours |
| Leafy greens (lettuce, spinach, microgreens) | See the seed packet or care guidance | Check the seed packet or the light maker’s instructions for hours |
| Fruiting plants (tomatoes, peppers) | See the seed packet or care guidance | Check the seed packet or the light maker’s instructions for hours |
| Low-light houseplants | See the seed packet or care guidance | Supplemental use; check the plant’s care guidance |
Timer tip: A basic plug-in timer keeps the schedule consistent.
Step 4: Monitor Your Plants and Adjust
Watch how your plants respond and adjust the light. Iowa State University Extension lists these signs:
Too much light: leaf scorch, bleached leaves, leaf drop, excessive reddening of foliage, extremely compact growth.
Too little light: excessive elongation or stretching, leaf drop, yellowing foliage, absent flowers (if there should be flowers).
Among Iowa State’s suggested changes: move the fixture closer to the plant for too little light, or further from it for too much.
Common Grow Light Setup Mistakes
Running lights without a schedule. Set a timer.
Placing the light too far away because you’re worried about burning. For seedlings, University of Maryland Extension says to adjust as needed to keep plants stocky rather than stretched and leggy.
Buying on wattage claim alone. PPF (µmol/s), where a maker publishes it, is the amount of plant-usable light the fixture puts out (Iowa State University Extension).
Frequently Asked Questions
How far should a grow light be from plants?
For vegetable seedlings, University of Maryland Extension says to keep fluorescent and LED fixtures 2 to 6 inches above the plants. For other fixtures and plants, follow the hanging height in the light maker’s instructions and watch how the plants respond (see Step 4).
How many hours a day should a grow light be on?
For vegetable seedlings, University of Maryland Extension recommends keeping fixtures lit 14 to 16 hours a day on an outlet timer. We don’t give hour ranges for other plant types here, so check the seed packet, the plant’s care guidance or the light maker’s instructions. A plug-in timer keeps the schedule consistent.
Can I leave a grow light on 24 hours?
We don’t recommend it. For vegetable seedlings, University of Maryland Extension recommends 14 to 16 hours a day.
Why are my plants stretching toward the grow light?
University of Maryland Extension says seedlings in window light usually stretch and lean toward the light, which can result in weak plants, and Iowa State University Extension lists excessive elongation or stretching as a sign of too little light. For seedlings, University of Maryland Extension says to adjust as needed to keep plants stocky; for other plants, check the light maker’s hanging guidance.
Will a regular LED bulb work instead of a grow light?
University of Illinois Extension says that in a pinch pretty much any type of light fixture can be used, as long as incandescent bulbs are swapped for LED bulbs.
Do I need a special timer for a grow light?
No. A plug-in outlet timer can switch the light on and off on a schedule; University of Maryland Extension recommends an outlet timer for seedling lights. Check the timer’s and the light’s instructions to confirm they work together.
Ready to Pick a Grow Light?
Now that you know how to set up and use a grow light, the next step is choosing the right fixture. Our grow light recommendations guide covers current picks across budgets and grow spaces. If you’d rather skip assembling a light and reservoir separately, our guide to the best smart and hydroponic indoor gardening systems covers all-in-one countertop and vertical setups instead.
This guide links to our gear recommendations pages where affiliate links and disclosures appear. General growing information is provided for educational purposes.