what kind of light are you using? If it is HPS, do you have glass cover on your reflector?
18 inches to grow and bud a plant... wow... that's tight... that will be a full time job... you will need to check your plants at least twice a day if you are using hps... plants can grow several inches very very quickly.... you'll need to train... train... train..

send pics if you decide to pull this off... we'd love to see this experiment : )
good luck