Depends on the wattage of the bulbs you get.. Home Depot has 100 watt for about $14.99 or so and that one stays about a foot away from my plants. Also depends on your fans. I keep my fans on high speed for cooling and also for a fat stalk. I water them only in the early morning hours before sunrise and again 15-20 minutes before sunset.. They seem to really love feeding and watering times like that and the smell just tells it all when they are so young and smelling so strong...