Keep in mind the light fixture height, (including hangars) and the buffer zone between plant and bulb, (heat) and the depth of the pot(s). What remains of the 3.5 feet (42 inches) is viable headroom for the plant. Let's say you have a 1 ft tall pot, 6" deep CFL light fixture and hangars, 2" buffer zone for heat. Tallest plant possible is about 22" under these parameters. Substitute an HID light, and you can grow 12" plants. (more buffer space necessary) Unless you use a vented hood. Anyway...I'm sure you get the idea. :thumbsup: