For LED grow lights, you need to work out how much light your plants are receiving and compare that number with the light they should be receiving.
Most manufacturers provide light intensity measurements at several relevant heights above the plant. If you can’t find light intensity measurements from these manufacturers, you can buy a plant spectrum tester and measure it yourself.
Unlike the plant spectrum tester, the Lux meter is very cheap, and in general these values are positively correlated and can be used as a reference. Buying a light meter and measuring how much lux your plants are receiving at their current distance from the grow lights can be an important way to adjust the hanging height.
How many micromoles of light do cannabis plants need at each growth stage?
Plants in the seedling stage require between 200-300 uMol/sec;
Vegetative plants receiving 18 hours of light should receive light between 306 and 617 uMol/sec;
Flowering plants receiving 12 hours of light should receive light between 460 and 925 uMol/sec.
If the light intensity your plants are currently receiving is within the above range, there is no need to adjust the height of the grow lights, below this range will require moving the grow lights closer to the plants, above this range will require moving the grow lights further away from the plants place.
Remember, higher light intensity equals higher yield. Each grow light varies in intensity at a given height, which is why these numbers need to be roughly determined by looking at the chart provided by the manufacturer or using a light meter.
In addition to the above, there are two other issues that need to be clarified to determine the height of hanging grow lights.
Is this grow light dimmable? If your grow lights are dimmable, you can dim the lights (adjust the power) without having to adjust the distance of the lights from the plants. Again, the most critical aspect of the entire process is having a light meter handy so you can measure, convert and compare light levels to the actual light levels your plants need to grow.
How high should led grow lights hang when growing cannabis? The proper grow light height depends on the wattage of the grow light. Low-power grow lights need to be closer to the plant than high-power lights, and the growth stage of the plant is also a factor. Generally speaking, LED grow lights can be hung between 16 and 24 inches. HPS lights need to hang at 36 or even 48 inches, depending on the power output.
Do you need to adjust the height of your grow lights as your plants grow? Yes, the plant will receive more intense light when it is close to the light fixture, and unless you adjust the height of the grow light, you may get the plant burnt.