Maximum Area for HPS Grow Lights

by Ray

I have a 9'x 5' garden area and 2- 600 watt HPS grow lights, and I want to cover the whole area. Is this a good idea? If so, what would the proper spacing of the lights be?

Answer: Ray- most garden plants require bright light, similar to sunlight. For most plants, this means 40 watts per square foot or more (up to 80 watts per square foot, but this is extreme). This is a very general guideline....once I had a 4' x 8' garden illuminated by one 1000 watt HPS light on an 18 inch light mover. If you do the math that is only 31.25 watts per square foot, and yet THAT garden grew very well (one of my better ones, in fact).

Because of this experience, I often recommend lighting a 4' x 8' garden with one 1000 watt light on a short light mover, or else two 600 watt stationary lights. Two 600 watt lights give a total of 1200 watts of light....over a 4' x 8' garden area (32 square feet), that comes to 37.5 watts per square foot. This is still a tiny bit short of the 40 watts/sq.ft. that I try to shoot for, but as I mentioned above, I HAVE HAD really good results with less.

Using two 600 watt lights to light a 5' x 9' garden area may be spreading the light just a bit too thin. 1200 watts/45 sq.ft. gives you only 26.67 watts/sq.ft. You may want to consider reducing the size of your garden to 4' x 8', or you may decide to give it a try and see what kind of results you get. The plants in the center of the garden will probably do fine, but the plants at the edges of the garden may struggle and will certainly yield less than the other plants.

When lighting a rectangular garden area with two 600 watt lights simply divide the space in half evenly, then hang one light in the center of each half. It is also important to use reflective material around the sides of the garden to reflect light back into the garden. This allows the plants to actually use every watt of light that your lights are putting out (instead of letting it "leak" out of the garden area unused). Without reflective material, your watts/sq.ft. will be even lower (considerably.

The reflective material is usually hung straight up and down (vertical), and is placed one or two inches away from the plants (to allow a little air circulation). The material needs to go as low as the lowest leaves on your plants, and needs to come up as high as the bottom of your HPS grow lights. Reflective material is often placed around three sides of the garden, leaving the fourth side open for an oscillating fan. The side left open for the oscillating fan should be one of the LONG sides, so the fan can provide air circulation to the greatest number of plants possible.

I hope this helps. If you decide to go with the 5'x 9' garden, I would like to know how things turn out....until then, Happy Growing!

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