Our team has compiled responses to some frequently asked questions (FAQs) about LEDs for plant applications.
1. Why use LED lighting in greenhouses?
Recent developments in LED technology create new opportunities for low- and high-intensity lighting of greenhouse crops. Possible advantages of LEDs compared to conventional high-intensity discharge (HID) lamps, such as high-pressure sodium (HPS) and metal halide (MH), include the following:
- Longer lamp life
- Potentially smaller fixture size
- Lower lamp wattage per light-generating unit (photons), which allows for placement close to the plants due to less emitted heat
- Flexible design options for horizontal (top-down) lighting or vertical lighting (also called intra-canopy lighting, side-lighting, and inter-lighting)
- Light spectrum can be tailored by selecting specific colors to induce targeted plant responses
- Prospect of higher quantum efficiency of LEDs (number of photons of photosynthetically active radiation emitted per unit of energy input)
Potential challenges include the following:
- Less overall heat generation compared to conventional lighting systems, which can result in a cooler crop temperature
- Lack of information about plant responses to timing, duration and composition of specific light spectra
- Higher unit purchasing price
Some, all, or none of these features may play a role when considering commercially marketed LED systems for plant applications and, therefore, LED systems need to be carefully scrutinized before purchase.
2. Can LED lighting replace traditional HID supplemental or sole-source lighting?
LEDs appear to be a suitable light source to supplement sunlight in some cases. Small–scale, scientific and field-tested trials need to be done before we can recommend to growers to utilize LED lighting. For example, red, far-red, and blue color spectra all have different effects on flowering, root formation, and plant growth. However, appropriate color ratios, lighting periods, and lighting intervals have yet to be determined. As academics with responsibility to the horticulture industry, we need to make sure that plant responses are not adversely affected by the light quality and light-delivery method of LEDs. Our industry-academic collaboration is specifically working on the plant responses to LED lighting for a variety of crops, conditions, and stages of plant growth. Mitigation of risks may favor a hybrid approach as the science develops. We will periodically update the outcomes of our research on this website.
3. In what types of plant applications are LED lighting most appropriate?
LEDs can be applied as traditional overhead lighting, or as intra-canopy (within the canopy) lighting for more “vertical” crops such as high-wire tomato. LEDs can be applied at high intensity to increase photosynthesis as well as at a low intensity to induce morphological or developmental changes, such as increased stem length or accelerated flowering. Thus, lighting can have different applications, depending on the response desired by the grower. The following terms relate to horticultural lighting applications:
- Photosynthetic: high-intensity lighting to increase photosynthesis and thus plant growth
- Photoperiodic: low-intensity lighting to control flowering of plants sensitive to day length
- Photomorphogenic: low- or high-intensity lighting to manipulate the shape and characteristics of a plant, such as leaf size, branching, and stem length.
4. What color spectrum should be used and should it be adjustable?
Prior scientific research has suggested that we focus on the following colors of LEDs: 1) red (600 – 700 nm), 2) blue (400 – 500 nm), and 3) far red (700 – 800 nm). For photosynthetic lighting, red is the main color used because it is considered to be the most efficient for driving photosynthesis. A small percentage (5% to 20%) of blue light is usually included because research has shown that some crops grew abnormally under 100% red light. For photomorphogenic and photoperiodic lighting, red and far-red are the main colors that elicit photoperiodic and photomorphogenic responses.
5. How should LED lighting be mounted for maximum efficiency (multilayer, interlighting, ceiling)?
The answer to this question depends on the crop, the growing structure, and customer demands. All of these applications can work technically with (currently) varying economic efficiency. For low-stature crops spaced closely together, traditional overhead placement of arrays can be most efficient. If the application is for supplemental rather than sole-source lighting, using LED luminaires (fixtures) that block a minimal amount of sunlight in the greenhouse is highly desired. If the application is for sole-source lighting, such as in a warehouse or closed building, overhead fixtures could be solid or open. A potential advantage of an open design is that it could allow for excellent air circulation and temperature uniformity around plants. Intra-canopy arrangement of LEDs is well suited for production of tall or trellised crops in cloudy climates or during low-light periods of the year.
With overhead lighting from point sources, such as from vertical HPS lamps, light intensity drops off rapidly with increasing distance from the lamps unless a luminaire is used that channels light without substantial beam spread. In addition, high-powered lamps can not be placed very close to the canopy due to the large amount of radiative heat generated, which can over-heat plants. Therefore, multi-layered LED growing systems may be appropriate for plant factories or vertical farms where low-profile crops or transplants are being grown. Open-bar LED arrays may also be a good idea for multi-layered crop growth to encourage vertical air movement and to preserve temperature uniformity within the growing environment.
6. What is the effect of LED lighting on plant development such as stem growth, flowering, and plant quality?
A light quality rich in red light relative to far-red light can suppress elongation growth, making many plants shorter; a light quality relatively rich in far-red light promotes extension growth, making plants taller. Flowering responses are complicated, and research is being conducted (e.g., at Michigan State University) to determine how different light qualities influence flowering of day-length-sensitive species. We will update the research outcomes on this project website periodically.
7. How much energy can be saved using supplemental LED lighting?
It is hard to definitively estimate the potential savings because systems and applications vary. Simply from an electrical efficiency standpoint, we know that LEDs convert up to 45% (depending on color) of electrical energy to light while HPS convert about 30% of energy to light. In comparison, incandescent lamps convert only about 6% of energy to light. The ballasts necessary for HPS lighting have a loss factor from heat generation of approximately 7% to 15%. This means that LEDs can be 66% more efficient (without consideration of controllers, cooling systems, or other system components). Additionally, while the heat generated by either system may help reduce heating costs, generation of heat via electricity costs about $17.50 per million BTU (assuming an industrial price of $.06/KWH), while natural gas presently costs only about $2.25 per million BTU.
Other possible energy savings can be from: 1) reducing losses due to misdirection and internal reflection (traditional lamps lose light due to reflector design), 2) selecting most energy-use efficient wavelengths for targeted plant responses (photosynthetic, photoperiodic, or photomorphogenic), and 3) design to deliver the light more efficiently to the plant canopy. For example, intracanopy LED lighting of greenhouse tomatoes saves energy by eliminating the exponential drop-off of intensity with incremental distance between traditional lamps and the canopy, and by converting electricity only into the light spectra used most efficiently by plants for photosynthesis. Intracanopy applications are possible because, although LEDs emit heat and most applications require active cooling (fans or water), there is less heat emitted than with traditional lamps. Additionally, the heat developed can be distributed more evenly in most designs and allows the light to be placed much closer to the plant without the danger of thermal distress.
Current research is replacing 60-watt incandescent lamps with 10 watt (2x5 watt) LED bulbs for photoperiodic applications. So far, we are encouraged by the results as these systems use a conventional E27 light bulb base and represent lower economic and operational risks for producers.
8. What maintenance is required for LED systems versus traditional systems?
Specific maintenance requirements are to be determined because newer, more efficient LED systems have not yet been tested over long periods of time. While LEDs can have a life expectancy of at least 50,000 hours, systems using high-intensity LEDs that require active cooling are relatively new and need to be tested for robustness. The cooling is required because the temperature of the LED impacts both the lifetime and LED efficiency over time (HID systems also degrade in efficiency over time). It is anticipated that the mean time between failures will increase as the technology is developed to be more robust. Mechanical failure of cooling fans, for example, can typically be predicted, but not enough testing has been done yet to determine LED system failure rates.
9. What is the advantage of LEDs running cooler than traditional systems?
There are several additional benefits of LEDs running cooler than traditional systems. The first is the electrical efficiency as stated in question 7. More energy is converted into usable light for photosynthesis or photomorphogenic effects. This represents potential for savings and an increase to the bottom line for growers. The second is that the light sources can be placed closer to the areas that need light without thermal distress. For example, LEDs can be placed closer to plants to avoid lighting non-productive areas such as aisles or unfilled beds, and to miminize light scattering. Third, the very nature of their size and efficiency lends the LEDs to new methods of lighting such as intracanopy lighting. Overhead lighting produces a gradient where the bottom leaves of a plant do not get the same light intensity as leaves higher in the plant. Experiments where LEDs are added in vertical or horizontal arrays show that you can attain much more uniformity in lighting with intracanopy sources. Another example of LEDs allowing new methods would be in small bedding or plug applications that could be vertically layered. This could allow growers a more efficient use of floor space or alternative uses of facilities.
10. How long will LED systems last and what needs to be replaced, if anything?
A properly designed LED system can have a very long lifetime before anything needs to be replaced. LEDs generally do not catastrophically “burn out” like other light sources, but their light output does gradually decline over time. The most commonly quoted “lifetime” metric for LEDs is 50,000 hours before they dim to 70% of their original intensity. However, it must be noted that many factors influence this number. Drive current and operating temperature are the two most important factors determining LED lifetime, both of which can be controlled by the design of the LED system. Increasing the LED drive current produces more light from a given LED, but also reduces the lifetime of the device (and reduces efficiency). However, operating at a reduced drive current may require more LEDs to reach a desired light output, therefore increasing the price of the system. A similar tradeoff exists with operating temperature. LEDs last longer the cooler they are operated, but this may mean a more expensive cooling system is needed (liquid cooling vs. fans, for example). Depending on the price/performance requirements, LED systems can last anywhere from 10,000 hours to 100,000 hours or greater, with the 50,000 hour target being a very common design goal. Because of this it is anticipated that the weak link in many LED systems will be active cooling components such as fans or pumps. This must be considered in the design phase of the systems to plan for replacement of these commonly lower-cost components.
11. What types of LEDs should be used?
An ideal LED system consists of highly efficient/effective units with colors or color-combinations that elicit the desired plant responses at economically competitive investment costs. Make sure the units can operate in growing environments (such as in high humidity) and that the manufacturer is reputable, provides an adequate warranty, and stands by any claims regarding system longevity. Make sure the installer is familiar with greenhouse lighting systems and is licensed appropriately.
12. When selecting an LED system, what are the most important things to be considered?
- Efficient use of conversion of energy to useable light from a plant (not human) perspective. For photosynthetic lighting, consider how many photons of light within the photosynthetic waveband (400 to 700 nm) are emitted per watt of energy input. Measurements made in lumens, lux, and footcandles are not appropriate for plants, since they are based on the response of the human eye to light. The preferred unit is µmol·m-2·s-1.
- Return on investment. How much do the lamps cost, and how many hours (or months or years) do the lamps need to operate before any energy savings or yield increases pay for the initial investment?
- Small profile. In greenhouses, lamp infrastructure should be small so that they don’t cast a large shadow on plants below.
- Robust. Lamps should be made so that they can tolerate greenhouse conditions, such as high humidity.
- As mentioned previously, purchase from reputable manufacturers who offer a reasonable warranty.
13. What is the approximate light saturation for photosynthesis?
Leaf photosynthesis of most high-light greenhouse crops saturates around instantaneous light levels of 500 – 750 µmol·m-2·s-1 within the 400 to 700 nm waveband. For closed canopies of plants, the saturation point is higher. In species with photosynthetic systems adapted to high-light environments, the rate of photosynthesis saturates at a much higher irradiance, even as high as 2000 µmol·m-2·s-1.
For greenhouse plant growth, the total quantity of light each day (the daily light integral, or DLI) in the unit of mol·m-2·d-1 is an effective way to measure light quantity to predict canopy photosynthesis and plant growth. A common minimum value for acceptable crop quality of ornamentals is ~ 8-10 mol·m-2·d-1, but it is much higher for most vegetable crops. As a rule of thumb, a 1% increase in the DLI increases growth (fruit size, mass, and/or number) by 1%.
14. Where can I find out more?
There are a number of ways to learn more about LEDs, including lighting and LED manufacturers’ websites, trade publications, and trade shows and other sources on the internet. Recently, we published a short review on LEDs in horticulture, which can be found under the “Publications” tab of this website. Additional sources can be found on the Links tab, which include upcoming scientific and grower meetings. Our intent is to continue to inform the trade of the state of the science as this project develops. The science is still evolving, and as it does, we will try to keep this site reflective of the advances.