Of the many variables which affect plant growth in an indoor garden or greenhouse, none are as influential as the artificial light source. The artificial lighting system is literally the sole energy source for an indoor garden and without light energy photosynthesis is impossible. It is important for horticulturists to remember that each indoor garden has a limited amount of light energy. To maximize his or her return on investment and to make an indoor garden as efficient as possible, an indoor horticulturist must make the most of his or her given light energy.
When I was a hydroponic retail store manager, growers constantly asked me how they could increase their yields. My answer was always “maximize the efficiency of your light source”. In other words, to get the most out of an indoor garden, a grower must know the basics of light energy, how it relates to indoor horticulture, and how to maximize its efficiency.
The inverse square law states that light energy diminishes exponentially as it travels away from its source. Light energy is emitted evenly and in all directions from one point. As the light travels and spreads, the intensity diminishes. The most important detail for indoor growers to remember is that light intensity is directly related to the distance from the light source. Plants grown outdoors under the sun are seemingly unaffected by this phenomenon because, after traveling all the way from the sun to the earth, the distance from the top of the plant to the bottom is quite insignificant.
Plants grown indoors, however, are extremely susceptible to the inverse square law because the light energy is only traveling a few feet in total. This means the distance from the top of the plant to the bottom is very significant in relation to the total distance traveled by the light. Any indoor horticulturist who has experienced large fruit or flowers on the top portion of a plant and small, undeveloped fruit or flowers on the lower section of a plant has directly observed the inverse square law. The lower section of the plant received less light energy and, therefore, could not develop like the upper portion of the plant, where there was more light energy available.
Finding a lighting system’s “sweet spot” is crucial to maximizing its light energy. The “sweet spot” refers to the area just below the light source where the available light energy is strongest. For most growers, this area is 8-20 inches below the light source. Please be aware that if the plants are placed too close to the light source, they can become damaged and/or stressed. When the top portion of a plant appears bleached or a whitening effect begins to show, a gardener is seeing tell-tale signs of over-saturation. If this is observed, a horticulturist should raise the lighting system to reduce any further stress to the plants. All in all, finding the “sweet spot” is a key to unlocking a grow room’s full potential.
The position of the “sweet spot” will vary from grow room to grow room, but will revolve around two factors: the wattage, or energy intensity of the light source, and how excess radiant heat is removed. For example, a grower can place plants closer to a 400 watt light source than a 1000 watt light source. Also, an air- or water-cooled light source, where radiant heat is being removed, can be placed much closer to the plants than a light source that is not cooled. A good rule of thumb is to start the plants 8-16 inches under a 400 watt system, 10-20 inches under a 600 watt system and 12-24 inches under a 1000 watt system. From there, a gardener can make observations and slight adjustments to find the lighting system’s “sweet spot”. Powerful grow lights and double-ended lights, however, might have manufacturer recommendations of multiple feet above the canopy, which can lead to a demonstrable diminishing effect when it comes to inverse square law. To understand how this works, grow lights that are positioned five feet above the canopy are only 50% as powerful as they could be. But, we can get around this and get increased efficiency out of our light.
Aside from finding a lighting system’s “sweet spot”, an indoor grower can implement various tools to help maximize the efficiency of the lighting systems in his or her garden. One of those tools is a light mover. Light movers help increase efficiency for a few different reasons. First, when compared with a stationary light, a light on a light mover will generate less heat, at least in one given area. Although a lighting system on a light rail will generate the same amount of heat as a stationary lighting system, the heat is distributed over the entire distance the light system travels.
Lighting systems on light movers can be placed closer to the plant canopy since their heat will be evenly distributed throughout the garden. Plants under a light mover system will also grow more uniformly because of the more evenly distributed light energy. This is a huge advantage for horticulturists who have to continuously rotate their plants in order to achieve uniform growth rates. In most indoor gardens, growing plants that are uniform in size automatically increases the efficiency of the lighting system. It is much easier for a horizontally-orientated lighting system to evenly distribute light to similarly sized plants than to plants of varying heights.
Another unique advantage of using a light mover is how it mimics nature. Indoor gardens and greenhouses have increased control advantages over outdoor gardening, but the overall goal is still to mimic nature and, in nature, the sun moves throughout the day. Leaves on the upper section of a plant will create shadows on the lower sections. As the sun moves, light energy reaches the other areas of the plant that were previously shaded. In nature, most of these shadowed sections would, at some point during the day, receive some light. Like the sun, light movers provide light energy to once shaded areas as they move across the garden. Although this does not replicate the sun’s movements exactly, it still increases the amount of leaves that are receiving light throughout the light cycle.
How this works is light movers get a stronger, more intense indoor plant light to each leaf area and at the correct amount of time for each of those surfaces. Leaf area index explains it scientifically but again, observing how the sun works demonstrates this principle in a way we can all understand. When all the leaves start working for the good of the plant, we get better results, and the light mover is the best way to accomplishes this. Stationary grow lights can create extreme hot spots and shadows. But, light movers light up all the mid and lower leaves, making the grow lamps much more efficient. More leaves working for the good of the plant gives leads to better results. It’s a two part effect of a closer, more intense light together with that better light getting to more leaves and at the correct time. In other words, we are now maximizing the light to our plants in two distinct ways that happen simultaneously. Light equals yield and better light gives us those positive results of faster growth and significant yield increases.
Reflective materials can be utilized by an indoor gardener to better maximize light energy efficiency. In fact, some gardeners will see a yield increase of 10-15% just by adding a reflective material to the grow room. Mylar, foylon, dimpled aluminum sheeting, white poly plastic, or flat white paint can all be used to cover the walls and ceiling of an indoor garden. A highly reflective grow room minimizes wasted light by redirecting the light energy back toward the plants. This, in turn, will increase efficiency and overall yield. The side walls and ceiling should be lined with reflective material, but reflective material on the floor should be avoided. A leaf is designed to take in light on the top side only; upward reflected light can actually cause leaves to twist and would be counterproductive.
Another important contributor to the available light energy of an indoor garden is light system maintenance. Some light technologies require annual bulb replacement to ensure high levels of light energy are available to the plants. Indoor horticulturists who wait for their lights to “burn out” will see a continued decrease in yields due to the declining available light energy emitted by the lighting system. In order to maintain the highest leve of usable light energy, most HID lights (with the exception of some of the newer duel ended bulbs), will need replacement bulbs every 8-12 months. Cleaning or wiping off reflectors, reflective materials, and/or the glass in air-cooled reflectors to remove dust, dirt, or other debris will go a long way in maximizing light energy. Newer technologies, such as LEDs and induction fluorescents, will not require bulb changes, but should still be wiped down to remove dust or dirt that could otherwise reduce the system’s light energy output. Something as insignificant as cleaning a lighting system once a week can make a huge difference in the lighting system’s efficiency. Dust or dirt left on the surface of a lighting system can reduce the light energy by 10% or more. In other words, do not discount the significance of cleaning or maintaining a lighting system.
Experienced indoor horticulturists understand the importance of maximizing their given light energy. By placing the plants as close as possible (without causing damage) to the light source, a grower can capture the full power of the lighting system’s “sweet spot”. The more plant material a grower can get in the lighting system’s “sweet spot”, the higher the yield will be. Gardeners who make use of the tools capable of increasing light energy efficiency, such as light movers and/or reflective materials, will also be rewarded with larger fruits and flowers. Finally, horticulturists who take the time to maintain their lighting systems by regularly changing bulbs and cleaning will get the most bang for their bucks out of their indoor gardening equipment.
Since the sun cannot be brought indoors, the world of indoor horticulture must revolve around the artificial lighting source. A gardener that makes the most efficient use of their artificial lighting system, by understanding the inverse square law and implementing the various tools at his or her disposal, will be able to truly maximize the efficiency of his or her garden’s lighting system and will be rewarded come harvest time.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse senior editor and can be contacted at Ehop@GardenAndGreenhouse.net.