Grow Lights

The Problem With Grow Lights: One Size Does Not Fit All

Bill Lermer discusses various grow light technologies and how to evaluate them for your specific situation.

So, you have an empty room that you want to turn into a grow room. Now, definitions of "grow room" differ from grower to grower so let me be explicit: I'm talking about a garden lit entirely by grow lights with no natural sunlight—for intensive indoor production. The most common questions at this early stage concern lighting and growing methods. However, there is no one type of grow light that performs best in all situations—otherwise there wouldn't be such a choice on the market—similarly there one ideal growing method that kicks ass in all situations for all crops. 

What's Your Limiting Factor? Area, Wattage, or Dollars?

It's important to consider your limiting factors. Yes, it would be nice to live (and grow) in a world without limits but most of us are constrained by one or more factors. What will give YOU the best results is determined primarily by the size of your room, the amount of wattage you're prepared to pull from the grid and, of course, your budget. 


Let us say the area available is large, but the wattage and budget available are small. Assuming the main objective is to get the highest total yield possible: in such a situation, the grower should consider something like this: grow plants tall in rows of two abreast, and track bare lamps in the aisle space between them. That's right, I'm talking bulbs without any reflectors at all, hanging vertically between the rows, trackling back and forth along the rows. 

Of course, you waste half of your potential floor space in order to provide sufficient spacing between the  light bulb and the plant. But, in this scenario at least, space isn't an issue. Tracking the light will reduce the average intensity, enabling more plants to be covered but resulting in less yield per plant. To avoid increasing the plant count and lowering of average light intensity, the lights could be stationary. However this would reduce the number of angles of light, worsening distribution of light. Probably the best route would be to shorten the length of the track, getting the advantage of more angles of light, without significantly reducing average intensity or plant count. 


In this case, you might try the vented tubes, hung vertically with vertical bulbs inside and no reflector. This can waste less floor space  (because the plants can be closer to the light source); but the closer the plants are, the less plants can be illuminated. And the vented tubes are difficult or impossible to move on a track (think of all that ducting!) and without tracking, there would be fewer angles of light (and fewer plants). 


Perhaps a more typical example is where the space available is the limiting factor. How can the area be fully covered with the power and budget available. In this case, mostly overhead horizontal lights would be most efficient.

Supplemental side lighting is beneficial for plants over 1' in height (about 1/3 of a meter). This can be done with side-lights from the corners, because the floor space would be fully utilized. Corners are the best place for side lights (because corners are the furthest distance from the center of the room). I use 70w Elite cmh or 150w cmh for side lighting, because they take up less floor space. These require a LF e-ballast. Higher wattages could be enclosed in a vent tube, enabling the plants to be closer and wasting less floor space.

Induction lights are also good for side lights, but tend to cost more. 150w induction lights can be used for side lighting, they have cooler surface temperature so can be used in the midst of a cluster of plants without losing a lot of floor space. 

1000 vs. 600 vs. 400 Watts

This open-ended Adjust-a-Wing reflector houses a high pressure sodium lamp, producing a light that is rich is the orange / red part of the light spectrumIn a test of 400w vs. 600w (with the total wattage, floor space, e-ballast and bulbs were identical) the 400w room yielded 30% more, because there were more points of light, resulting in more angles of light and fewer shadows on leaf surfaces. While the amount of light per watt is slightly less on 400w, the small loss is more than made up for by better distribution of light. 

Although 1000w produce the greatest total amount of light and greater light intensity, the distribution of light is worse because there are fewer points/angles of light. 

Increasing the wattage of the bulb from 400w to 1000w does not necessarily increase the vertical penetration through the canopy because at a distance of 12"-14" (depending on hood type), the foot candles are optimum for 400w but higher than optimum for 600w and 1000w. So the higher wattage bulbs will need more distance from the top leaves, resulting in the same intensity as 400w.  Vertical penetration is actually less with higher wattages, because the angles of light are fewer and fewer will be able to find a path to the lower leaves. 

The optimum intensity is about 5500 foot candles. You will tend to get more growth up to about 10,000 foot candles during the rapid veg phase, but this amount of intensity could burn the leaves once growth slows down in bloom. The greatest yield per watt is with 5500 foot candles. 

I believe that this intensity is best achieved from multiple angles of light / multiple smaller wattage lights. Lifting a light up higher does reduce  intensity at any given point, but the total amount of light is virtually the same, it is simply spread out over a larger area. So if there is a cluster of lights, and all are raised, each will cover a bigger area, and the light footprints from all will overlap. This results in better light distribution due to multiple angles. Leaf surfaces are not smooth, but have bumps which create shadows if there were just one source/angle of light. The light meter can be angled differently to measure intensity from different sources. 

What do you think of Bill's ideas? Don't forget to comment below!