LEDs are cool. They didn't burn this Heliamphora flower.
LED lighting is an emerging technology. Unlike the mature technology of fluorescent lighting where we do not expect significant changes in the future, LED lighting technology is constantly changing. The goals of LED lighting companies are to make lighting more energy efficient and pleasant for humans. Lighting for plants is not on their radar although some major manufacturers will do custom components for plant lights, especially for the indoor cannabis industry.
There are mainly two types of LEDs manufactured today for lighting. The technology being phased out for general human lighting uses discrete single colored LEDs. To get white light these lights have a mix of blue, green, and orange LEDs producing a spectrum similar to the newest technology fluorescent lights. Single colored LEDs are still made for special applications such as traffic lights and architectural and mood lighting. Blue architectural lights are appropriate for plant lighting but the red architectural LEDs are too orange. The deep red wavelength needed for plants is not in one of efficiency sweet spots for LEDs so they are less common.
The dominant type of LED for lighting manufactured today is the white LED. These LEDs work like fluorescent lights. Fluorescent lights use high voltage and mercury vapor to produce UV light which is converted to visible light by phosphors deposited on the inside of a glass tube. White LEDs work the same way but instead of producing UV light they produce intense blue light via semiconductor light emitting diodes. The blue light is passed through material coated with or embedded with phosphors to convert most of the blue light to longer wavelength colors. You can tell white LEDs by the fact that when they are off you can see the yellow-orange phosphors.
White LED chips. LED bulb manufacturers mount these chips in bulbs also consisting of a power supply, massive heat sink, diffusers, and connectors. The bulbs are expensive because of the cost of all the components unique to LED bulbs. Images from the Bridgelux RS Array Series, Product Data Sheet DS25 and the Philips LUXEON K Datasheet DS102.
White LED strip lights. The strip is 10mm wide with three type 2030 LEDs per 25mm. It takes three of these strips to replace one T5-HO fluorescent light bulb.
When deciding on the type of lighting for plants you need to first ask yourself, why are you growing the plants. If you are growing the plants because you enjoy seeing them then you would want to use lighting technologies designed for humans. If you do not care what the plants look like, then there are very efficient plant lights that make the plants look black or brown. The plants look this color because they are absorbing and using all the light. You can also go somewhere in between and use both kinds of lighting with the plants being somewhat off in color. Maybe you like purple plants.
White LED lighting.
For plants you want to appear in their natural colors, the best LED lighting is the kind designed for home use. The white LEDs with phosphors are being made to mimic halogen lighting. That is, unlike with fluorescent lights, LED lights have broad spectrum phosphors giving a more natural light.
An issue with LED bulbs is the "inexpensive" ones generally available have a color rendering index (CRI) of less than 85. What is missing with these bulbs is the deep red that the plants appreciate. The 95 CRI bulbs have their peak light output near the red peak absorption of chlorophyll but the bulbs cost twice as much as 80 CRI bulbs. The 95 CRI bulbs are considered specialized products and are not as easy to find as the consumer grade 80 CRI bulbs.
The best feature of LED lighting is it is inherently directional. You can choose bulbs appropriate for your needs without having to resort to fixtures with fancy reflectors and mirrors. LED spot lights do work best for plants that hold out their leaves more-or-less horizontally. You could use an LED spot light above a Nepenthes plant, a large pot of Drosera or Dionaea, or as lighting for a hexagon terrarium. However, the lack of light dispersion can also be a disadvantage. The beam of light is so narrow it would not work well with pitcher plants such as Heliamphora even if the plants were surrounded by mirrors.
Another good feature of LED lighting is the light and the lamps are cooler than fluorescent lighting. Fluorescent lights are more efficient at 35°C air temperature and the fixtures tend to trap heat which in turn heats the plants below. LEDs do generate heat and start degrading when they get hot. Care must be taken to make sure they get enough air circulation to keep them cool. Under ideal conditions, LEDs will normally lose 30% of their brightness after they have been in use 16 hours a day for 4 to 8 years depending on the exact lamps used. If the LEDs get too hot they will lose their brightness much quicker.
A problem with white LED lighting is the bulbs are expensive and the lumens per watt is low compared to the most efficient fluorescent lighting. For home lighting the story is different because they are very cost effective compared to incandescent and halogen lighting. The more you pay for electricity the more you need to use LED lighting for your home. But this is not the case as a replacement for the best fluorescent lighting for plants. T8 form factor LEDs are too dim, have too low a color rendering index, and cost too much to be considered for plant lighting.
LED strip lighting is an interesting new LED product. You can buy adhesive-backed LED strip lights, stick them to the bottom of a cabinet or on anything else, hook them up to a 12 volt power supply and you have a nice source of light. In your house you can have supplemental or task lighting in places and ways that are not easy with other lighting technologies. Water-proof LED strip lighting can even be used in water features and other wet locations. Unfortunately, for plants good quality white LED strip lighting is only cost effective in situations where you cannot use T5 or T8 fluorescent lighting and it may not even be more cost effective compared to compact fluorescents unless the CFL's are generating too much heat for your plants. Depending on the strip lights you choose it takes 2 to 8 feet of strip lighting to replace one 26 W CFL or to 8 to 30 feet to replace one 4 foot T5-HO bulb.
Light intensity from a Feit PAR30, 15W, LED bulb at 8 inches (20 cm), 10 inches (25 cm), and 12 inches (30 cm) from the bulb.
The 10000 to 20000 lux working distance for this LED spot light is about 11 inches (28 cm) above plants such as Dionaea and Drosera. At that distance the useful growing area is 8 inches (20 cm) in diameter. This bulb has a rated beam angle of 38°. This is considered wide for LEDs. Be sure to check the beam angle of any LED spot/flood lights you get.
The bottom line on white LED lighting is it is not as good as T5-HO lighting in situations where you can use T5-HO. However in situations were T5-HO lighting would not work or work well there are lots of creative things you can do with LED lighting.
Colored LED lighting.
For growing plants commercially or in sterile culture, the color they appear to be under grow lights is not a factor. All a commercial grower is concerned about is raising the best plants for the least amount of money. Commercial growers can cut their electric bills for plant lighting roughly in half by using colored LED lighting instead of fluorescent lighting because they essentially pay only for the light the plants need. For a sterile culture nursery that can amount to a savings of several thousand dollars or euros per year.
The trouble is figuring out how much and what mix of light is the best for the plants. With white human-oriented lighting you can just shove a Lux meter under the lights and adjust them accordingly. With the blue and deep red LEDs the Lux meter will not give a meaningful measure of plant-available light.
Another factor is 660 nm deep red LED lights are not readily available. They are a custom item because they are too dark for human use. The 620 nm red LEDs that are commonly available are too orange to be fully effective for plants. The common 460 nm blue LEDs are almost perfect for plants.
Unfortunately we cannot turn to our indoor farming friends for help on how much and what ratio of colors of LED lighting are best for plants. They find their plants grow better under T5-HO lighting than they do under LED lighting at the same power usage per unit of growing area. This is because (I am told) they find their plants also need white lighting to grow the way they want them to grow. Using white LEDs blows any cost advantage of LEDs over fluorescent lighting.
Because the cost of electricity in Europe is more than twice the typical cost within North America, European commercial growers have been doing a lot of experimentation with LEDs. Andreas Wistuba uses custom-made water-proof LED strip lights at various ratios of LED colors. He has found there is no one ratio of colors that work for all his plants—some kinds like extra blue light, others hate it. He has also struggled with variable quality of the strip lights he has received from his suppliers. It is only worth while because using colored strip lighting saves him thousands of euros per year in electricity costs.
As LED lighting technology matures it looks like a mix of blue, white, and deep red LEDs will work very well as general plant lights. The ratio of white LEDs can also be adjusted for the level light required for humans to see the plants more or less in natural coloration. It only takes 10 to 15% white LEDs in a mix of blue and deep red LEDs for the plants to have almost a natural look.
Colored LED lighting for sterile culture plants.
The use of LEDs for sterile culture plants can actually be better than fluorescent lighting if the ratio of deep red, blue, and white LEDs are what the plants appreciate. The ratio that works best for one genus and even species may not be good for another. To inspect the plants it is necessary to have a source of white light such as an LED flash light, head lamp or the old fluorescents.
Notice the light dispersion and intensity distribution on the wall at the end of the benches. For LED lighting the shelves need to be farther apart than with fluorescents to get even lighting for the plants. Photo © Andreas Wistuba.
-- John Brittnacher
For more information please see:
Bridgelux (2011) Bridgelux Decor Series LED Arrays, Product Data Sheet DS26.
Bridgelux (2012) Bridgelux RS Array Series, Product Data Sheet DS25.
Philips Lumileds Lighting Company (2013) LUXEON K Datasheet DS102.
Philips Lumileds Lighting Company (2013) LUXEON Rebel ES Datasheet DS61.
For definitions and an introduction to plant lighting, please see the Indoor Plant Lighting page.
This page covers the special topic of when LEDs are a better choice for plant lighting than fluorescent lighting.
Relative spectral response of chlorophyll. Chlorophyll absorbs mainly blue and red light. Plants appear green because they reflect green light.
Spectral Power of light from a 95 color rendering index, 3000 K, white chip LED.
This high CRI LED provides a wide spectrum of light with peaks at the preferred wavelengths of chlorophyll.
Spectral Power of light from an 80 color rendering index, 3000 K, white chip LED.
A 5000 K LED would have a higher 465 nm peak and more green light.
Spectral Power of light from a 3000 K T5-HO fluorescent bulb with an 85 color rendering index.
Spectrum from an 85 CRI, 800 series phosphor T5-HO bulb. Note that the vertical axis a relative scale.
Comparison of the efficiency of fluorescent and LED lighting technologies.
|PAR30 LED 3||95||15||880||59|
|PAR30 LED 4||80||15||790||53|
|PAR AgroLED 5,10||n/a||12||850||71|
|T8 LED 6||83||17||1700||100|
|T8 AgroLED 7,10||n/a||22||1248||57|
|LED Strip Light 8||>80||3/ft||152/ft||52|
|LED Strip Light 9||>80||7.5/ft||575/ft||75|
* Initial lumens for LEDs, mean lumens for fluorescent bulbs. LED bulbs do decrease in intensity as they age but not as quickly as fluorescent bulbs. Expect a 30% decrease in lumens after 4 to 8 years of use at 16 hours a day.
1 GE FLE26HT3/2/841
2 Sylvania FP54/841/HO/ECO
3 Sylvania LED15PAR30/DIM/P/930/Fl40 (41 lm/w after 4 years)
4 Feit 15PAR30L/LEDG5 (37 lm/w after 5 years)
5 Sunlight Supply 901430 (7 red/3 blue/2 white LEDs)
6 Feit T4817/LEDCL/41K
7 Sunlight Supply 901422 (red LEDs)
8 Flexfire Colorbright Warm White LED strip light
9 Flexfire Industrial Ultra Bright™ Warm White LED Strip Light (requires a heat sink)
10 Not full spectrum so lumen rating may not be comparable to other bulbs listed.
Comparison of the dispersion of light from a PAR30 LED compared to a two-bulb fluorescent fixture.
The light intensity distribution was measured 12.5 inches (32 cm) from the LED bulb and 8 inches (20 cm) from the fluorescent bulbs. At those distances the fluorescent fixture illuminated a growing area 11 inches (28 cm) wide and as long as the bulbs while the LED illuminated a growing area 7 inches (18 cm) in diameter. It would take eight 15 watt LED spot lights using a total of 120 watts to provide as much light as two T5-HO bulbs using 108 watts.
The fluorescent fixture is a four bulb T5-HO Tek 44 using only the inner two bulbs. (Data were measured with a Lux meter, averaged and smoothed.)
Peak lighting from a PAR30 LED at various distances as compared with a two-bulb fluorescent fixture.
Distances are in inches. Remember this is peak light level. The fluorescent fixture is a T5-HO Sun Blaze 42. (Data were measured with a Lux meter, averaged and smoothed.)
Discrete LEDs in an LED plant light fixture.
The LED on the left is a white LED with phosphors. The LED on the right is a colored LED. This is "old" LED technology. The LED chips are mounted on a ceramic holder under a plastic bubble.
Discrete LED light fixture turned on.
This fixture has 50% deep red, 20% blue, 10% orange, 10% red, and 10% white LEDs. It is a meter long and uses 156 watts. To a human eye the plants have a color close to natural but to a digital camera the plants are purple. The manufacturer claims 80% of light emitted by the fixture is potentially absorbed by plants.
Under-bench supplemental lighting in a greenhouse.
Plant recently out of sterile culture can hardened off under greenhouse benches with supplemental lighting. Photo © Andreas Wistuba.
The under-bench greenhouse plants at night.
Photo © Andreas Wistuba.