LED Grow Lights

LED Grow Lights: Ready for Prime Time?

Let’s face it: Everyone that uses high-intensity discharge (HID) garden lights has a love/hate relationship with them.

We love how Metal Halide (MH) or High Pressure Sodium (HPS) lights flood our garden’s canopy with intense light, providing lots of energy for photosynthesis and keeping internodal distances short. We love how we can easily switch between them to match or manipulate our plants’ growth cycle. We love how MH and HPS lights are widely available at reasonable prices.

But we hate how much energy these lights consume: in our area, it costs at least $35 per month to run a 600 watt MH or HPS light for 12 hours per day, more in the summer due to higher seasonal electric rates. HID lights are one of our most “un-green” garden tools, with their high energy consumption, low efficiency ratings and mercury-containing bulbs. Critics rightly view them as inconsistent with earth-friendly gardening methods.

A big part of the problem is that HID lights are inefficient. A 400 watt HPS light is only about 35% efficient, which means that only 35% of the electrical energy delivered to the bulb is converted into light, with 65% converted into heat. Lighting ballasts are hot, too, contributing to high levels of excess heat in most HID-illuminated garden rooms.

This waste heat has to go somewhere, so we consume even more energy to cool and vent gardens that use these lights. Air conditioning is often required, and chillers may be needed to keep nutrient solutions from getting too hot.

HID waste heat increases evaporation from our reservoirs and growing media, and the transpiration rate of our plants. Nutrient and water consumption go up, as do the required flow rates and reliability of our irrigation systems. Just about every garden system is significantly impacted by the waste heat from HID lights.

What to do? It’s not like we can just turn off our grow lights and wait for something better. T5 fluorescent grow lights are energy efficient and cool running, but their lower light output makes them appropriate for plants that need less light, such as leafy greens and foliage plants. At present, only an HID garden light can put out enough light to provide a bountiful harvest in plants that produce large fruit and flowers.

We’ve been waiting a long time for garden lights based on light-emitting diodes (LEDs) to make it to “prime time.” NASA and others have been experimenting with LED grow lights since the mid-80’s, and we are finally starting to see products reaching the market that reflect their findings. These lights represent a major step forward.

LED grow lights offer the dual benefit of low energy consumption and low heat generation. Some LED grow lights use so little electricity that they can be powered by a small solar panel. According to their manufacturers, today’s LED grow lights consume between 2% and 50% of the electricity used by the HID grow lights they reportedly replace. We’ve yet to see in real-world application whether these lights will perform as well in the garden as those “comparable” HIDs, particularly for high-light crops.

LEDs, short for Light Emitting Diodes, are tiny semiconductor chips that generate light when electrified. It takes a small amount of electricity to make an LED glow (emit electrons), and the elements that the diode is made from determine the light spectrum it emits. An LED is backed by an internal reflector and encased in an epoxy body with an integrated lens. Together, these components determine the angle of the light emitted by the diode, which is shaped as a downward-facing cone. Individual LEDs produce relatively small light cones, so they are clustered into arrays so the light cones overlap, increasing light intensity and coverage area.

LED grow lights are designed to stimulate photosynthesis by providing light in the frequencies that plants primarily use for this critical biological process. Individual LEDs may contain one of 29 known combinations of elements that emit light in different colors when excited by electrons. Grow light manufacturers emphasize blue and red LEDs in their fixtures, sometimes with other colors, which gives many LED grow lights their distinctive purplish-red color. Optimizing the light spectrum helps in two ways: it enhances photosynthesis and saves energy by not generating light in colors that plants do not use.

LED grow lights have a longer useful life than MH or HPS lights, as long as 15 years for some fixtures, and thus a lower “total cost of ownership.” The total cost of ownership includes the cost to buy the light, use it, and replace expired bulbs. One manufacturer estimates that the total cost of owning an LED grow light over five years is about half the cost of a comparable HPS light, based on an analysis that balances the upfront cost to purchase an LED grow light against lower electrical costs and eliminating the need to periodically replace bulbs. Their analysis used an electric rate of $0.07 per kilowatt hour – here in Southern California where electric rates are twice as high, the total cost difference between an LED grow light and a MH or HPS light would be even greater.

You may also be able to save space, or move into a smaller space, with LED grow lights. There is no need for bulky reflectors, and ventilation/cooling equipment such as fans, air conditioners and ductwork may be eliminated or scaled back. The room will also be significantly quieter with fewer noisy fans, ballasts, and chillers.

Thus, the promise of LED grow lights has five aspects: lower energy consumption, lower heat emissions, customizable photosynthetic spectrum, lower total cost of ownership, and smaller garden space requirements. Can today’s LED grow lights actually deliver these benefits? The answer to that question depends on who you ask. Before we get to those questions, however, let’s take a step back and examine light and photosynthesis a little more closely.

Electromagnetic radiation spans a broad range of wavelengths, which includes what we recognize as “visible light.” At the one end of the electromagnetic spectrum are gamma rays, which have a wavelength of 5-10 nanometers (nm). A nanometer is one-billionth of a meter, a very small measure indeed. At the other end of the electromagnetic spectrum are radio waves with a wavelength of 1,012 nm. Smack in the middle are the wavelengths that can be seen by the human eye, that fall between 380 and 750 nm. This part of the electromagnetic spectrum is what we call “visible light.”

Plants use visible light differently than we do. Their leaves contain chlorophyll, a compound vital for photosynthesis, the process by which plants convert light into fuel. Chlorophyll molecules are highly selective about the light they absorb: blue light primarily in the 460-480 nm range, and red light around 640-680 nm. Chlorophyll, for the most part, reflects instead of absorbs most green and yellow light, which is why plants appear green to our eyes.

Limiting any grow light to just red and blue light would lead to poor results, since plants also use most of rest of the visible spectrum, though in lower amounts. NASA’s Biological Sciences team at the Kennedy Space Center has done extensive research on this subject: a 2006 paper published by the International Society of Horticultural Science documented improvements in lettuce yields when 24% green light was added to LED grow lights that otherwise produced only red and blue light. Including light in the yellow-green spectrum also helps to visually restore green color to plants, making them look more “normal,” and makes it easier to identify garden problems such as nutrient deficiencies or pest attacks. This combination also makes it easier for gardeners to work in their gardens.

Increasing the amount of red and blue light reaching the garden isn’t an idea limited to LED grow lights. Enhanced-spectrum MH and HPS bulbs, such as Hortilux and SolarMax lamps, provide more blue and red light than standard bulbs. So water-cooled HID lights, according to their manufacturers. Enhanced-spectrum and water-cooled HID lights still put out a lot of light in yellow-green wavelengths, too, that look bright to the human eye but don’t do much for our plants. An LED grow light designer isn’t trying to reproduce sunlight or make up for the spectral shortcomings of a light originally designed for another purpose. Instead, they are trying to create optimal light output for photosynthesis.

So do today’s LED grow lights live up to expectations?

Energy consumption is significantly lower for LED lights vs. “equivalent” HID lights. Most grow light manufacturers are working to replace the light output of a 400-500 watt HID light with between 30 and 300 watts of power consumed, and some go quite a lot lower. The jury’s still out on how these lights truly compare to their HID cousins on garden performance and yield, but on the energy promise they seem to perform well.

The lights fare well on the heat promise, too. Most LED grow lights put out very little heat, so do not have any specific cooling requirements. A light’s heat output is affected by its design: some lights cluster LEDs in round or square “pods” or “rosettes” that are spaced along horizontal bars. These designs help to limit heat accumulation by spacing the LEDs farther apart. Other lights bunch the LEDs close together in a single fixture to provide a greater light intensity directly below the light. These designs can accumulate heat, so generally incorporate heat management features such as heat shielding, fans for air cooling, or water cooling.

The third promise, customized light spectrum, is a topic where debates swirl. Skeptics do not believe LEDs can ever produce light that is intense enough to be effective for photosynthesis, no matter what color it is. These concerns are based on earlier generation LEDs that did not put out enough energy to work well as grow lights. Today’s “third generation” LEDs reportedly solve this problem by putting out up to three watts of light energy per LED, a 20-50 fold improvement over second-generation LEDs. New-generation LEDs also have a wider “beam angle” that permits an LED to project light across a 120 degree arc instead of mostly just straight down (“pin point” lighting). This means wider light cones, improving light intensity and coverage at the garden canopy.

Passionate debates about LED grow light design and effectiveness are waged on a regular basis in online gardening communities. Some posts describe LED grow light trials people are conducting in their homes. While these posts are interesting reading, and there’s a lot to be learned from them, they should also be taken with a grain of salt. Many hobbyists want to debate whether LED grow lights are or can be effective. The academic and scientific communities, satisfied for some time that LED grow lights are effective, have moved on to figuring out “how.”

One point that LED grow light manufacturers emphasize is that we must update our gardening methods to use LED grow lights to their best advantage. Using an LED grow light without changing gardening methods will likely result in a disappointing outcome. The first caution is to not overwater. You’ve eliminated one of the largest sources of heat in your garden: your HID garden lights.

With less heat, your plants consume less nutrient solution, and less of it evaporates, thus less solution is needed. One lighting manufacturer also suggests using a weaker nutrient solution to match lower plant transpiration rates. Less solution and less nutrients used: this should be good news, right? Yes and no. Reservoirs can be smaller, saving money and space. Fewer expensive bottles of hydroponic nutrient solutions are needed.

The dark side is subtle, but compelling: the most frequent cause of indoor plant death is overwatering. It’s just as true for plants in a fully automated indoor garden as it is for an African violet on the kitchen windowsill. Those accustomed to gardening under HID lights will find that it’s extremely easy to overwater under LED grow lights. Your plants simply need less water when there is less heat.

Overwatering can lead to root rot or drowning if it’s not caught and corrected right away. When using LED grow lights, be sure to select a porous growing media to provide adequate drainage and plenty of air space in the root zone. Also, let your plants almost dry more between feedings. Overwatering can also increase garden humidity levels, which can promote the development of fungus and other garden ills. Keep a close watch on growing media moisture levels and garden relative humidity when using LED grow lights. An LED grow light that emits some UV-A spectrum (invisible, but felt as heat) may help to kill or retard mildew. Maintaining constant air movement within the garden, using oscillating or fixed fans, will also help to keep mold and mildew in check.

Grow light manufacturers also recommend supplementing the garden atmosphere with carbon dioxide (CO2). While supplemental CO2 will improve the yield of any garden when properly applied, there are two particular benefits when using CO2 with LED grow lights. First, even though an LED grow light may look dim to the human eye, it is putting out a lot of light energy in the spectrum needed for photosynthesis – ideally, more than the plants would receive under “comparable” MH or HPS lights. Plants need higher CO2 levels in order to turn this additional light into fuel. The second benefit is that with lower heat output, there is less need to vent or air condition the garden space. It’s easier to seal the garden environment for CO2 when it’s not as hot.

We asked several LED lighting manufacturers about the concerns customers express when considering an LED grow light. Two primary concerns emerged.

The first is the price for LED grow lights. LED grow lights can be purchased on eBay for as little as $9 for a screw-in replacement grow bulb to as much as $1,500 for a water-cooled garden light fixture reported to be equivalent to a 400- to 500 watt HPS light. Don’t be misled by the low price on some of the LED lights on eBay – they often include lower-output, inferior LEDs, and some of them are recycled traffic signal lights. A large number of these inexpensive LED “grow lights” may be needed to provide illumination for a standard-sized hobby garden: buyer beware. A home gardener should expect to spend between $500 and $1,500 for LED grow lights to illuminate a 3’ x 3’ growing space. The “total cost of ownership” models discussed earlier are one way to understand the price issue: LED grow lights require a larger up-front investment which should be more than offset by reduced operating costs going forward.

The second concern is that nobody wants to go first. More people are interested in LED grow lights than are ready to start using them. Gardeners do not want their crop to wind up as the “guinea pig” in a failed garden lighting experiment. A lot of trials and research have been conducted in research laboratories that show LED grow lights to be effective and cost-efficient. But those trials were done in a laboratory setting. What’s missing are verifiable and repeatable consumer-oriented trials that put LED grow lights up against their HID counterparts, in conditions that resemble a hobbyist indoor garden room. Until that happens, individual gardeners can experiment with LED grow lights on their own, either on a small scale or by segregating a small number of plants within their garden for trials.

What is the future of LED grow lights? Near-term, we should expect to see a wider variety of LED grow lights coming onto the market. These lights will increasingly include higher output LEDs in a variety of light colors and configurations. We expect to see combination lights hitting the market, using LED plus T5 fluorescent or HID garden lights to provide a more balanced spectrum than available from these garden lights used alone.

We are starting to see LED grow lights designed to support different plant growth stages, such as grow and bloom, and should expect lights to be introduced for specific plants such as tomatoes, herbs, or orchids. We will see continued improvements in light output, particularly as fourth-generation LEDs continue their migration from the lab to the market. Prices will begin to come down when sales volume increases.

In the long run, LED lighting could obsolete almost every other type of lighting. Retrofit LED lamps that fit into standard fluorescent lighting fixtures are already available, and LED fixtures are in the works that will replace street lights, stadium lights and other interior and exterior lights. Already, Amsterdam and Rotterdam in the Netherlands are testing LED street lighting, and press reports indicate that other major cities are considering similar trials.

LED grow lights will be swept ahead with the tide of mostly government-funded research and development currently underway. Research dollars invested by NASA and other government agencies around the globe are fueling tremendous advances in LED lighting technologies. The energy saving potential of LED lights is something these entities cannot ignore: if we can find a way to consume less energy to light up public and private spaces, we may be able to postpone or scratch plans to build new power plants, or buy time until we are able to commercialize sustainable methods to generate and transport electricity.

There is a lot of promise associated with LED grow lights, but there are also unanswered questions and unresolved debates. The products that are available today should produce an acceptable result so long as the gardener makes appropriate changes to their gardening methods. We’re looking forward to further articles and reports that describe how LED grow lights perform against standard MH and HPS grow lights. We’re hopeful about how LED grow lights will fare in such comparisons.