How To Provide Ideal Lighting For your Medical Marijuana Plants
I change out metal halide bulbs every 6 to 9 months and high pressure sodiums every 9 to 11 months. 
Many factors contribute to a flourishing indoor Medical Marijuana garden. Lighting, ventilation, nutrients and water are the “essential four” elements of success, and due to their unique complexities, mastering each one is an art of its own. The focus of this article is lighting, which can be best explained by examining three key principles: the point of diminishing returns, the diminishing of light energy itself, and the diminished output of your artificial light source.
POINT OF DIMINISHING RETURNS
The Point of Diminishing Returns is used to calculate the best amount of light energy to give your plants without wasting energy. It is calculated by the watts of light per square foot. It’s very important to remember when discussing garden square footage, that we’re talking about plant surface area, not the physical size of the room. Through my experiments, I’ve found the point of diminishing returns to be 40 watts per square foot of garden. In other words, a productive use of a 1000 watt HID would be 25 sq. ft or a 5 ft by 5 ft area of plant surface area.
1000 watts = 1000 watts = 40 Watts per Square Foot
5 ft. x 5 ft. 25 sq. ft.
The Point of Diminishing Returns is an efficiency point, not an absolute rule. I personally run about 50-60 watts per sq. ft. because I don’t mind paying a little extra on the energy bill for the yield increase. But be aware that if too much light is used, the light saturation point will be reached. At this point, the plant can’t process any more light, so added energy will result in no gain.
DIMINISHING OF LIGHT ENERGY
The inverse square law states that light energy diminishes exponentially. This fact is very important for the indoor gardener. The further the distance from the light source, the less energy there is for photosynthesis. For example, light energy two meters away from the bulb will be 1/4th the light energy at one meter away from the same bulb. The light energy at three meters will be 1/9th the light energy at one meter, and so on.
The optimal distance from bulb to plant canopy depends on many factors, but the two most pivotal are wattage of the bulb and the way a grower removes radiant heat. The rule of thumb is to place the bulb as close to the plant canopy as possible without burning the plants. This ensures your plants are receiving the maximum amount of light energy available.
DIMINISHED OUTPUT
Artificial light sources break down over time and their light energy output declines. Just because a bulb continues to produce light doesn’t mean its energy output is the same as the day it was purchased. HID bulbs for indoor gardening should be changed every year if not sooner. Metal halides tend to decline in light energy at a faster rate than high pressure sodiums, but both should be replaced regularly. Personally, I change out metal halide bulbs every 6 to 9 months and high pressure sodiums every 9 to 11 months.
By calculating the proper wattage for the plant surface area, creating the ideal distances between you plants and bulbs, and changing your bulbs regularly, you can optimize your return and allow your plants to bask in the brilliance of the light energy you pay for every month.
