International Carnivorous Plant Society

LED Plant Lighting

LEDs are cool. They didn't burn this Heliamphora flower.

 

LED lighting is an emerging technology. Unlike the mature technology of fluorescent lighting where significant changes are not expected in the future, LED lighting technology is constantly evolving now. The goals of innovative major LED lighting companies are to make lighting more energy efficient and pleasant for humans. Lighting for plants is too small a niche to be on their radar. There are small manufacturers that make specialized components for plant lights and cater to the exploding cannabis industry.

It is almost impossible to keep up with new LED plant lighting options and to see beyond marketing hype, some of which is very misleading. I am sorry I can't tell you what to buy for your plants because I don't believe most of what companies say about their their lights. Even if there was a product that was great, it could be gone tomorrow and a whole new crop of lights will be on the market. I hope an understanding of lighting technology will help with choices. 

You can generally assume lights marketed to the cannabis industry may or may not be optimal for the carnivorous plants you want to grow. There is no way to tell whether a light will work for your plants until you try to grow them under it. For instance, the typical blue-biased cannabis lights are almost magical for growing Sarracenia seedlings and other full-sun plants but some of the carnivores I grow do not like that light. This article is intended to explain how LED lighting works and how that relates to growing plants under LEDs. 

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 very visible to humans and is not in one of the efficiency sweet spots for LEDs so they are not common.

 

LEDs

A deep red LED chip mounted on a ceramic base with a plastic bubble over it. Notice the wires bringing power to the chip. This is "old" technology and you are not likely to see similar parts in new light fixtures. New fixtures have miniature, much much less expensive versions of this part. (This may actually be a chip with royal blue light emitting diodes heavily coated with a deep red phosphor.) 

LEDs 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.

 

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 efficiently 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.

 

LED Modules

LED light modules removed from failed LED bulbs.  The bulbs failed because of improper assembly causing the modules to overheat. Individual LEDs do get hot and will fail if they get too hot.  I have also seen LED bulbs fail from bad solder joints.

The power supply and LED module on the left is from a white LED bulb. An AC to DC power supply is necessary for efficient LED lights. The six white and yellow objects mounted on the white circuit board to the right of the power supply are the LEDs. The yellow color is from the phosphors in a resin over the LED chips which converts most of the blue light the LED chips generate into the spectrum of colors that appears white to humans. The module on the right is from a purple LED plant light bulb. It was higher wattage and uses 20 LEDs. The phosphors have a red color because the inherently blue LEDs just happen to be exactly the color plants need for blue light while the red phosphor is needed to convert some of the blue light to the deep red light plants also need.

Strip LED lighting

White LED strip lights. The strip is 10mm wide with three type 2030 LEDs per 25mm. It takes 12 feet of these strips to replace one T5-HO fluorescent light bulb. The strips are very expensive but useful in situations where other available LED lights will not work.

 

For more information on the technical aspects of how LED lights work please see the Wikipedia Light-emitting diode and Wikipedia LED lamp articles.

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, brown, or purple. The plants look those colors 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 90+ CRI bulbs have their peak light output near the red peak absorption of chlorophyll but the bulbs cost more than 80 CRI bulbs. 90+ CRI bulbs are becoming more common with some states mandating these bulbs in new installations.

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 be designed 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.

 

LED dispersion

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.

 

Colored LED lighting.

For growing plants commercially or in sterile culture, the color the plants 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 high color rendering index lights and adjust them accordingly because we already know how much of that light the plants need. With the blue and deep red LED lights the Lux meter will not give a meaningful measure of plant-available light. It is not a coincidence that human vision is tuned to colors reflected and not used by plants. Lux meters are tuned to give human visible light measurement. There are meters with a flat response of a wider than human range. They are also worthless for our purposes because they do not exclude light the plants do not use.

The common 460 nm royal blue LEDs are almost perfect for plants. The problem is plants really need 660 nm deep red light in photosynthesis. They convert 460 nm blue light to 660 nm red light in their cells. Pure 660 nm deep red LED bulbs are available and do work for plant lighting. I mix them in with the overly blue cannabis bulbs because I like how my plants grow under those light conditions. 

If you do use lights with pure purple (i.e. red and blue) it is best to add in some full spectrum white light.  It is possible the plants use other colors of light but the main reason is human eyes perceive the deep purple lights as being dim.  They are not dim!  They are extremely bright.  Adding white light will help your eyes adjust for the light intensity and you can see your plants better.  

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.

 

LEDs in tissue culture

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
March 2014
Latest update January 2020

For more information please see:

The information in the tables and figures is based on Bridgelux, Philips, Osram Sylvania, Feit, Flexfire, and Sunlight Supply documents unless otherwise specified.

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 emerging topic of LED plant lighting. LEDs are a better choice for plant lighting than fluorescent lighting.

 

Chlorophyll Absorption

Chlorophyl Spectrum

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.

95 CRI 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.

80 CRI 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.

T5-HO 3000K

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.

Bulb CRI Watts Lumens* Lumens*
per Watt
CFL 1 82 26 1365 53
T5-HO 2 85 54 4650 88
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.

LED and Fluorescent dispersion

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.

LED Lux by distance

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.)

Under-bench supplemental lighting in a greenhouse.

Greenhouse LED lighting

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.

Greenhouse LED lighting

Photo © Andreas Wistuba.