High pressure sodium grow lights are the way to go once you are ready to force your plants into flowering. They are, in fact, just what the doctor ordered. On this page I will help you decide which light is right for you by explaining more about the different sizes, styles, and options for high pressure sodium grow lights. I also discuss some considerations that come with each of the features.
At this stage (flowering), your plants are craving the yellow and red light of Fall...and lots of it! High pressure sodium grow lights deliver both of these things to your plants. The light that they emit is more in the yellow/red spectrum, and so more of the light is in a usable wavelength that the plants are looking for. It is a WIN WIN for your plants. That is why, no matter what the rest of your garden setup may be, I always recommend a high pressure sodium grow light for the flowering stage of growth.
This depends on how much space you have to work with really. See the chart below for an idea of the coverage...
These are only general guidelines. Good flowering and vegetable production happens at around 40-80 watts per square foot. I usually shoot for 50 watts/sq.ft. when setting up an indoor garden.
If you have a 600 watt light and a closet with 2'x 3' of floor space (6 square feet), then simply divide 600 by 6 and you get 100 watts per square foot, in which case you should use a smaller light (or bigger closet) :) Using a dimmable digital ballast and running the light at 50% would be another great solution in this case.
During a several month stretch, I had two friends with gardens that were producing more than mine. I was using a 1000 watt light, and they both used 600 watt lights. How you ask? The 600 watt lights produced half as much heat as my 1000 watt light, so the lights could be placed much closer to the plants without heat damage.
Light looses its punch quickly as it moves away from the light source, indeed! In addition, not only were my friends using less electric for their lights, they were also using less air conditioning. Needless to say, I have now traded in my 1000 watt grow light for a 600 watt light. You can see all of my personal design and setup choices on my tips and tricks page.
The distance you keep your high pressure sodium grow light away from the tops of your plants (no matter how big the plants actually are) will be determined by three or four things. They are...
If you have a 400 watt light, keep it at least 12 inches away from your plant tops. A bigger light = bigger distance. I keep my 1000 watt light at least 30 inches from the tops of my plants. The best thing to do is buy a cheap digital indoor-outdoor thermometer, and place the "outdoor" probe in your garden's hot-spot.
If you have a light with a reflector that is air cooled, then you will be able to move your light a little bit closer to the tops of your plants....but only if you have the glass in the bottom of the reflector and are cooling it with an exhaust fan. The bigger the vents and the more direct the air flow through the reflector, the better the cooling benefit will be.
Your exhaust setup is critical. For complete exhaust setup and design advice check out my exhaust setup page.
Basically the bigger the fan, the better. The more efficient the fan,
the better. The shorter the exhaust run, the better. By the way, centrifugal fans are the most efficient way to move air in and out of your garden (in case you were wondering).
When all of your temperature control efforts fall short, it's time to crank up that A.C.! Sometimes (like all Summer) there's no way around it.
A strong high pressure sodium light requires a strong exhaust fan, and is a critical choice as the central component of your garden design. Your garden design should be focused on controlling heat, as well as providing for all the other needs of your plants. Any design that makes it easier or more efficient for you to provide for the needs of your plants is a good garden design.
The exhaust setup is the key to controlling high temperatures in your garden. Controlling high temperatures, in turn, is one of the major keys to indoor gardening success. Learn more about controlling the temperature in your grow room, and learn how to maintain your nutrient solution properly, and you are 90% of the way to having great success gardening indoors.