Finding the Sweet Spot for Artificial Lighting

September 18, 2020 steven

When I was a hydroponic store manager, growers repeatedly asked me how they could increase their indoor gardens’ overall performance. My answer was always the same: get the most out of your light source. A grower should think of his or her lighting system as the main energy source for the garden.

This energy is not infinite, which means only a certain amount is available. In other words, every indoor garden has a set amount of energy to work with and this energy should be used as efficiently as possible. An efficient use of the available energy will equate to the most productive garden possible. When it comes to indoor horticulture, maximizing the available light energy is the key to success.

There are various techniques and technologies available to help growers maximize light energy. Growers who implement these techniques and technologies will have not only more consistent results, but also a higher return on investment.

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Distance from the Light Source

The inverse square law states that light diminishes exponentially from its source. What this means to an indoor horticulturist is the farther the light source is from the plant canopy, the less usable energy that is available for plant growth. This is why indoor horticulturists should keep their lighting systems as close as possible to the plant canopy at all times.

A good rule of thumb is to have the lighting system around 12-24 inches above the plants. However, there are many factors that can contribute to how close a lighting system can be placed to the plants. For example, an HID lighting system, especially a DE lighting system, will be very hot and intense. Plants placed too close to these systems may be burned or over-saturated. Never position plants so close to the light source that the pigment of the plant is damaged. Put another way, if discoloration on the top of the plants is noticed, the light source is probably too close and should be adjusted.

Air- or water-cooled lighting systems will generally allow the grower to place the light source closer to the plants than non-cooled systems. However, plants can still be damaged by reaching the light saturation point (the point when the tops of the plants turn white). When in doubt, pay close attention to the very top of the plant canopy.

Other lighting technologies, such as LEDs, will rarely burn the plants due to a lower operating temperature; however, growers should still be on the lookout for discoloration on the upper plant canopy that can occur from over-saturation.

The Sweet Spot Defines the Space for the Most Usable Light Energy

The sweet spot of a lighting system refers to the area where the most usable light energy is available to the plants. This sweet spot for plants is generally found 12-24 inches below the light source or just below the section of light that is too intense for plant growth.

Once the sweet spot of a lighting system is determined, a grower should do everything he or she can to maximize the amount of vegetation in that area. By maximizing the amount of plant material which lies in the sweet spot, the grower automatically maximizes his or her use of light energy. The more efficiently the light energy is used, the more energy that is converted to fruit or flower production.

Plant Manipulation Techniques: Topping and Super Cropping

One relatively easy way to maximize the amount of vegetation in the lighting system’s sweet spot is to implement plant manipulation techniques. Plant manipulation is exactly what it sounds like: manipulating the plant’s normal growth to better serve the horticulturist’s desires. The two most common plant manipulation techniques used indoors are topping and super cropping.

Topping – Topping is a technique where the tops of the plant are pruned or cut. This is generally done at a node space (the place where horizontal branching occurs). At each node space there are two potential new shoots. The idea behind topping is that with every one top removed, two new shoots take its place; thereby increasing the number of shoots (and eventually fruits or flowers) that will develop. Growers who use the topping technique are able to better control the vertical growth of the plant and position more vegetation in the lighting system’s sweet spot.

Super Cropping – Super cropping is similar to topping because the purpose is to get the plant to grow new shoots from the node spaces. However, with supper cropping the tops are not cut or removed, but are bent to the point of damaging them. The best way to implement this is to squeeze the stem between the thumb and pointer finger until the stem folds like a drinking straw. The advantage of super cropping is that the tops are retained while still promoting new shoots from the node spaces. As with topping, plants that are super cropped will have more vegetation that can be placed in the sweet spot of the lighting system.

Trellis Netting and Tie Downs

Using trellis netting or physically tying down the plants are other common techniques used to maximize the amount of vegetation in the lighting’s sweet spot. Trellis netting is great because it can be stretched horizontally across the garden and positioned right in the lighting system’s sweet spot.

When done proactively, the trellis net would be set up prior to the plants’ full development and placed so the plants could grow into the netting. This allows the grower to weave, twist, and bend the plants into the trellis net, thus maximizing the vegetation in the sweet spot. Trellis netting has become my favorite technique for maximizing light energy. Once the trellis net is in place, it is really easy to keep an even plant canopy.

Some gardeners prefer physically tying down the plants to maximize efficiency. This can be done with various materials, but, essentially, the plant is bent over and tied down to the plant’s container or other fixed structure in the garden. Once a plant is bent over, it will quickly revert toward the light and grow upward. The tie down method can be especially advantageous with certain plant varieties that tend to stretch during the transition from the vegetative stage to the flowering stage.

Reflectors and Reflective Materials

When maximizing light energy is discussed, it is important to mention reflectors and reflective materials. For lighting systems that do not have built-in reflectors, it is wise for the horticulturist to purchase one that complements the lighting system. Unless the garden space is set up for vertical growing or another unusual set-up, a reflector complementing the lighting system is a valuable tool.

As mentioned earlier, the farther the light energy has to travel, the more it diminishes. A reflector can help direct a good portion of the light energy back toward the plant canopy instead of in all directions where it would be unusable by the plants.

Reflective materials on the walls in the growroom is another way for growers to maximize the available light energy. Reflective materials help direct some of the light, that would otherwise be lost, back toward the plants. Mylar, foylon, or pebble stone aluminum sheeting are all great reflective materials that can be used indoors.

High Efficiency Lighting Systems

Not all lighting systems are the same. There are a few different lighting technologies used by horticulturists. Growers should research the advantages and disadvantages of each. Most of the newer lighting technologies offer a higher amount of photosynthetically active radiation (PAR) per watt of energy consumed.

This means they are more effective at converting electricity into usable energy for the plants. The most efficient lighting systems will have the highest ratio of PAR to watt of energy consumed.

Lighting System Maintenance

Maintaining the lighting system is another way growers can get the most out of their lights. Regular bulb changes (on lighting systems that require it) will go a long way in maintaining a high level of usable light energy. All lighting systems should be cleaned routinely to remove unwanted filth from the lens or bulbs of the system.

Dust and dirt from the growroom can accumulate quickly on the surface of a lighting system and greatly reduce the light output. A regular cleaning of bulbs, ballasts and reflectors will help keep the available light energy at its highest possible level.

The available light energy of an indoor garden is the ultimate determining factor over plant growth and yield. By implementing various techniques, indoor horticulturists can be sure that the available light energy is being used as efficiently as possible. The more efficiently the light energy is used, the better the yield per watt of energy consumed. After all, when discussing indoor horticulture, that is where the return on investment comes into play: the yield per watt of energy consumed.

Every indoor garden has a certain amount of input energy. The job of the indoor horticulturalist is to get as much out of that energy as possible. However the grower chooses to do this, one thing remains the same: energy out cannot exceed the energy in. An indoor garden has a set amount of available input energy (the light energy). A horticulturist must decide how to use and how to capitalize on the available light energy to make the garden as profitable and rewarding as possible.

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