In order to grow the shortest plants, keep the lights as close as possible. With fluorescent lights, you can allow the plants to be extremely close without harming them, just dont grow them into the plant. If using metal halide or HPS lights, the plants need to be farther away. I use a 400W metal halide and keep the plants 8 inches away. If your hand feels too hot when placed on top of the plant for 30 seconds, its too close. You must raise the lights about every day.

If your lights are too far away, the plants will stretch.

Peace.

joe