How Far Should Your Grow Light Be From Your Plants?

Why

No single height works for every setup. When I measured mine, I ended up nearly twelve inches higher than the manufacturer and the online "experts" recommended. I'll show you how to check yours without buying a separate meter.

The reading you want is PPFD. It measures how much plant-useful light reaches an area each second. Your light, dimmer setting, and coverage all affect it. Too little can limit growth. Too much can stress your plants.

Spectrum matters too. Grow lights combine wavelengths such as blue, green, and red. PPFD tells you how much light arrives, but not the proportions of those colors. Research has found different yields under different spectra, even at the same PPFD. If your light offers seedling, veg, or flower spectrum settings, select the recommended setting for your stage before measuring PPFD.

A dedicated PAR meter measures PPFD. With the light warmed up, hold its sensor level at plant tops, facing upward. Keep your shadow off it. Check the center and several spots toward the edges.

For healthy photoperiod cannabis, the kind you switch to twelve-hour days to flower, practical starting points are about 200 PPFD for seedlings and 400 in veg under eighteen hours of light. In flower, about 700 under twelve hours. Longer days give plants more daily light at the same reading.

Start with the manufacturer's suggested settings, then measure. If readings are above your target throughout, dim the light or raise the fixture. If below it throughout, increase brightness or lower the fixture. Keep within the manufacturer's required clearances. Raising the fixture may also help even out a bright center and dim edges.

Increase light gradually. Change one thing, check the same spots again, and watch plant response and canopy temperature.

Here's what I use: Photone on my phone, with printer paper as a diffuser.

Follow the app's setup for your phone and select PPFD. If it needs paper, use twenty-two-pound white printer paper and select paper diffuser. Match the app's light-source setting to your type of grow light. Some settings cost a few dollars. Then check those same spots with your phone, keeping it level at plant height and your shadow off it.

I compared mine with a friend's dedicated PAR meter. Under my light, the readings were almost identical. Results depend on your phone and setup.

So use the recommended height as a starting point, then measure what your plants are actually getting. Adjust one thing, check again, and recheck as they grow.