Safer® Hydro FAQ: How Close Should Lights Be?
The sun is millions of miles from Earth, but its light is strong enough to provide steady energy to plants. Indoor grow lights need to be close to deliver their energy, but you must take care to be sure that they don't burn your plants.
At the same time, lights too far away from your plants causes them to become "leggy," with long spindly stems that are not sturdy enough to stand straight up.
For clones and seedlings, you can use fluorescent lights, which are relatively cool. Set them just 2 to 3 inches above the tops of the plants and raise them as the stems grow longer.
When plants are in the vegetative and blooming stage, they need much more light than fluorescents can provide.
LED bulbs are cooler than high-intensity grow lights but not as strong. They can be as close as a foot from the tops of your plants. When using more powerful high-intensity lights, keep them at least 36 inches from the tops of your plants and up to 2 feet above for the strongest (1000 watt) lights.