The Right Height for Your Light

One thing you want to do is make sure the light you use is at the right height.  The light source is the single most important factor in good growing.  For fluorescent lights with big bulbs (T-8 or T-12) you want the plants as close as possible, within a cm (1/2 inch).  These are good lights to start seedlings under or to start growing indoors with but you can't really grow plants over one foot tall with these lights. When you move up to a high output fluorescent T-5 you can have the plants a bit further (several inches) from the light.  With LED or HID you can have the light even further from your plants as the light intensity is much greater.  The further from your plants the light is the larger an area you can grow in.  However, the energy of light decreases over space so if you have the light too far from the plants you will get tall lanky plants that are trying to grow up to get closer to the light source.  You need to find the best height for your plants and your grow environment.  I blow air cleaned by a carbon filter right at the reflector and bulb of my 400W HPS and this allows me to keep the HPS light only about 1 foot above plants.  With HID's the limiting factor as to how close to have the light to the plants is usually heat, these bulbs emit a lot of heat and you must keep them 1-2 feet above your plants or the heat will harm your plants.  You can look for brown leaf tips as a sign of high heat stress.  I sometimes put a 600W HPS several feet above a lot of plants that get some secondary light through a window.  This light and the hour of sunlight on sunny days is all I need to have happy healthy plants in my solarium.  LED's do get hot too, but not as much as an HID.  WIth LED you can have  them two feet or more above the plants, but the closer the light is to your plants the more intense the light.  More intense light will speed up the growth rate, and the flowering rate, so long as the lights don't burn your plants

If your plants do grow too close to the light you will have tissue damage, with low energy fluorescent you will get yellowing with T-5 and HID you will get wrinkled leaves or  brown leaves and branches (DEAD) on the parts of the plant that are close to the light.