Every garden is different, but the laws of physics apply equally to them all…

The horticultural lighting industry is in a state of constant evolution involving pure science, technology and plant physiology as indoor growers strive to recreate the sun within their indoor gardens. Many factors are at play when choosing the proper lighting system—energy consumption, physical space and budget constraints will all influence your final decision. Because indoor gardens are nearly as diverse as their operators it isn’t feasible to deal with every possible scenario for horticultural lighting applications in a single article, but we are able to break down any discussion on horticultural lighting into the ways it is affected by the laws of physics. These physical laws apply equally to all indoor horticultural lighting applications and are an invaluable source of reference when designing or assessing an indoor garden.

Diminishing Light Energy

The inverse square law states that light energy diminishes exponentially. This physical law is very important for the indoor gardener because it means that the farther a plant is from a light source the less energy there is available for photosynthesis. For example, the light energy hitting a leaf seven-feet away from a light source will only be one quarter the amount of light energy hitting that leaf at 3.5-feet away, the light energy at 10-feet will be 1/9th the light energy at 3.5-feet and so on. The optimal distance from light source to plant canopy depends on many factors but the two most pivotal are the wattage of the bulb and the way a grower chooses to remove radiant heat. A good rule of thumb is to place the light source as close to the plant canopy as possible without burning the leaves, which should ensure your plants are receiving the maximum amount of light energy available without subjecting them to stress.

Point of Diminishing Returns

The point of diminishing returns is used to calculate the point of maximum efficiency in terms of light energy. We use this formula to find the maximum amount of light energy that can be applied to the garden’s physical space without wasting energy—in other words, we are looking for the point at which adding additional light energy is no longer validated by any increase in yield. We usually discuss the point of diminishing returns in terms of watts per square foot. It’s very important to remember when discussing a garden’s square footage in a lighting context that we’re talking about the plant surface area, though—not the physical size of the room.

Through my experience with growing everything from sprouts to squash indoors I have found it is easier to separate vegetable crops into two general categories: leafy greens and fast-growing annuals. Lettuces, basil, sprouts and almost all of the culinary herbs are categorized as leafy greens, while tomatoes, strawberries, cucumbers, peppers, squash and most vegetables bearing fruit fall into the fast-growing annual category.

I found the point of diminishing returns for leafy greens to be around 25-watts per square foot and for most fast-growing annuals it seems to be around 40-watts per square foot of garden. For example, a productive use of a 1,000-watt HID bulb for leafy greens would be a 40-square-foot plant surface area, which would take up an eight-foot by five-foot space. A productive use of a 1,000-watt HID bulb for fast-growing annuals would be a 25-square-foot plant surface area or a five-foot by five-foot space.

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These examples are quite generalized and only represent two lighting footprint options for a 1,000-watt light. The point of diminishing returns will vary greatly depending on the crop that is being grown and is somewhat subjective—depending on the crop’s market value and what I like to call its ‘sentimental value.’ Sentimental value is unique to the individual grower because it is not based on purely financial considerations like overhead costs or profit margins and a person who really enjoys growing their own food indoors and values knowing where it came from probably doesn’t mind paying a little extra for the end results. For some fast-growing annual crops I will actually deploy more watts per square foot (50- to 60-watts), because paying a little extra on the energy bill is justified by the resulting increase in yield—this is especially applicable when the market value of a particular crop is higher.

No matter the market value, there is a point at which adding more light becomes useless. When more light energy is added to a given area than a plant can possibly process the light saturation point will be reached and at this point adding additional light energy will result in no further gain.

Diminished Output

All artificial light sources break down over time and their light energy output declines—just because a bulb continues to produce light doesn’t mean its energy output for horticultural purposes remains the same as the day it was purchased! High pressure sodium and fluorescent bulbs for indoor gardening should be changed every year, if not sooner. Metal halides tend to decline in light energy at a faster rate than high pressure sodiums and should therefore be replaced more often. I replace metal halide bulbs every six to nine months and high pressure sodium and fluorescents every nine to 11 months.

The light energy in an indoor garden is the driving force for plant photosynthesis. Wasting light energy or not providing enough to effectively cover your square footage is essentially depriving plants of the chance of performing up to their potential. Placing your light source as close as possible to the plant canopy, calculating the proper wattage for the plant surface area and changing your bulbs regularly will help maximize your return on investment and optimize the efficiency of your indoor garden.