How far should a grow light be from plants?

There is a short answer and a better one. The short answer comes from Iowa State Extension: fluorescent fixtures should be 6 to 12 inches (15 to 30 cm) from the foliage, and LED fixtures can be 12 to 24 inches (30 to 60 cm) away in most cases. The better answer is to pick a target amount of light for your plant and move the light until a meter says you’ve hit it.

Starting points from Iowa State Extension

Iowa State’s setup guide gives a few concrete starting positions:

• A fixture sold as a grow light: install it as the package directions say and run it 14 hours a day.
• An ordinary 4 ft LED shop light with full-spectrum (“daylight”) output: about 18 in. (45 cm) from the foliage, at least 3,000 lumens, 14 hours a day.
• A 4 ft two-tube T5 high-output fluorescent shop fixture with full-spectrum tubes: about 9 in. (23 cm) from the foliage, 14 hours a day.

As plants grow, raise the light (or lower the plants) to keep the gap. Iowa State also warns that a fixture that runs hot can damage plants if it is moved too close, and that a closer light covers a smaller area. Some plant-specific advice exists too: Wisconsin Extension suggests lights 8 to 12 in. above African violets for 14 to 16 hours a day.

Why distance rules of thumb are only rough

Most people learned that light falls off with the square of distance: twice as far, a quarter of the light. That inverse-square law holds for a small, point-like source seen from far away. Grow lights are often neither small nor far away.

A physics-teaching paper by Stari and colleagues, a preprint posted on arXiv (not peer-reviewed), measured this with smartphone light sensors:

• Strip and tube lights (like LED strips): light fell in inverse proportion to distance (half the light at twice the distance), not with its square.
• Flat panels: light stayed nearly constant while the distance was smaller than the panel itself, then fell slowly.
• Ring lights: light peaked at an intermediate distance, then dropped quickly.

So if a flat LED panel is wider than the gap, raising it from 6 to 12 inches may change the light at the center far less than the inverse-square law predicts, while the edges of the lit area can still be much dimmer. Iowa State makes the same practical point: PPFD (the plant-usable light reaching a leaf, in µmol/m²/s) can drop significantly as you get further from the source.

Work backward from the light your plant needs

Plants respond to the total light they get per day, the daily light integral (DLI, in mol/m²/day). Iowa State Extension gives the formula:

DLI = PPFD × hours on × 0.0036

Its classes for indoor plants are low light 3 to 6 mol/m²/day, medium 6 to 10, and high 12 to 16; our plant pages use 3, 6, 10 and 12 as the growth levels for low, medium, bright indirect and direct-sun plants. Turning those into a PPFD target at leaf level:

Plant light level	Target DLI	12 h	14 h	16 h
Low	3	69	60	52
Medium	6	139	119	104
Bright indirect	10	231	198	174
Direct sun	12	278	238	208

(PPFD in µmol/m²/s, calculated from the formula above)

The table below runs the other way: the DLI a given PPFD delivers over 12, 14 and 16 hours. Iowa State suggests 12 to 14 hours of light for most plants, and no less than 10 or more than 16; Illinois Extension also says not to exceed 16 hours, because plants need a rest period. Some plants flower in response to day length, so check before running lights long.

If the light only tops up a window, you need less. The Window Light Planner estimates how far your window falls short each month, and the light converter turns that gap into a PPFD target.

Measure at leaf level

A quantum (PAR) meter reads PPFD directly. Most home growers have a lux meter or a phone app instead. Iowa State says smartphone light apps are accurate enough for the typical home gardener, while UF/IFAS cautions that app readings should be treated as approximations. Either way, measure at the top of the foliage, and if your meter has a light-source setting, set it to match.

To convert lux to PPFD, the factor depends on the light’s spectrum:

• Sunlight: 0.0185 µmol/m²/s per lux (Apogee Instruments, from Thimijan and Heins 1983).
• Cool white fluorescent: 0.0135 (Apogee Instruments).
• White LEDs: about 0.015, i.e. 1,000 lux ≈ 15 µmol/m²/s, from a 2018 bioRxiv preprint by Sharakshane (not peer-reviewed).

Example: 8,000 lux under a white LED is about 120 µmol/m²/s, which gives about 6 mol/m²/day over 14 hours, enough for a medium-light plant. Lux is based on what the human eye sees, which Iowa State notes favors yellow and green light, so for strongly colored red-and-blue lights a lux-based estimate is less trustworthy than a PAR reading.

Take several readings across the area, not just one in the middle; Iowa State suggests averaging multiple spots. With a light on a timer, a reading holds all day; Purdue Extension notes that with daylight, any single reading is only a snapshot.

Signs the light is too close or too far

Iowa State Extension’s troubleshooting table is the best final check:

• Too much light: leaf scorch, bleached leaves, leaf drop, excessive reddening of foliage, extremely compact growth. Raise the light, use a lower-output fixture, or remove one.
• Too little light: stretching, spindly growth, leaf drop, yellowing foliage, missing flowers. Lower the light, add a fixture, or use reflectors or side lights.

Clemson Extension similarly notes that leaves given too much light bleach or scald, and Wisconsin Extension describes a “washed out appearance” from too much light. Change one thing at a time and give plants a few weeks; Iowa State’s approach is to set up, watch the plants for several weeks, and fine-tune.

For when a grow light is worth adding in the first place, see how much light houseplants lose in winter.