When deciding on the type of lighting for plants, first you need to 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 are tuned to the light spectrum most appreciated by plants. That spectrum has the side effect that the plants can appear black, brown, or purple. Maybe you like purple plants.

The basics of LED lighting.

The dominant type of 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. For white LEDs, instead of producing UV light with high voltage, they efficiently produce intense deep blue light via semiconductor light emitting diodes at low voltage. The deep blue light is passed through a coating of phosphors to convert most of the blue light to longer wavelength colors.

The LED chips are mounted in packages. The package on the left contains a deep blue LED chip mounted in the center while the chip on the right has a deep red LED chip. The deep red LED chip has a phosphor coating over the chip to convert the inherent deep blue light to deep red. These chips are covered with a clear resin. They are part of a blue/red panel light intended for plant lighting. The 225 LED panels with blue and red LEDs produce intense to very intense lighting for plants depending on the wattage and distance from the plants.

For white and purple LEDs, manufacturers can adjust the phosphors to produce any color spectrum desired for lighting. White lighting products will contain LED chips in packages with a yellow-orange phosphor coating to convert the blue light to white light. Purple plant lighting LEDs would use a different mix of phosphors that would look red when the power is off.

This strip light is sold for use as accent and under-cabinet lighting but is useful for plant lighting under conditions where other available products will not work. These lighting strips can be very expensive to use for plant lighting. It takes about 200 of these chips per square foot (30 cm²) to provide a moderate amount of light for plants.

LED spotlights also work well for plant lighting. A power supply and LED module on the left are from a white LED spotlight. The AC to DC power supply is necessary for efficient LED lights. The LED module on the right is from a purple LED plant light bulb. It is higher wattage and uses 20 LEDs. The phosphor coating has a red color from phosphors that produce the deep red light plants appreciate.

The LED light modules were removed from failed LED bulbs. The bulbs failed because of improper assembly in a factory causing the modules inside the bulbs to overheat. Notice the scorching on the right module circuit board. Individual LEDs do get hot and will fail if they get too hot.

Care must be taken to make sure LED bulbs and fixtures get enough air circulation to keep them from getting too hot. The useful lifetime of individual LED chips is determined by how hot they get. The hotter they get, the quicker they dim over time. If they get too hot, they will fail. 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 fixtures used and how hot they get.

If you use spotlights over 10 watts, they should not be in any sort of enclosure like a desk lamp. Most purple LED plant spot lights are 18 to 35 watts. If you use strip lights, they must be adhesive-backed and stuck to aluminum heat sinks. If you use panel lights, there must be air circulation around them. This is especially important with 30 watt and higher panel lights. 35 watt panel lights can be 18°C (30°F) higher than ambient temperature. A small fan may be necessary if the light fixtures are in enclosed spaces. A fan is not necessary if air can naturally circulate around the light fixture.

The weak link in many LED fixtures is the power supply. The parts can degrade over time from the heat produced by the power supply itself and the LEDs. This is another reason not to enclose LED bulbs and fixtures.

Poorly designed power supplies can produce radio and TV interference. Ferrite core RFI/EMI suppressor cable clips may help with minor interference. Unfortunately, fluorescent tube to LED conversion kits may produce more interference than can be blocked by ferrite cores. There are much better choices than converting fluorescent fixtures to use tubes with LEDs mounted inside.

When shopping for LED fixtures and bulbs, you need to be careful about hyped or misleading claims. It is common to state the watt-equivalents of a fixture or bulb. Equivalent to what? Did they actually do experiments to determine the claims? The light spectrums are very different for other light technologies so there can be no easy conversion number. Instead, carefully look for the actual wattage of the device. This is what matters. It may be in the fine print. And for your particular situation, higher wattage may not be better because higher wattage means more heat produced. Also ignore many other claims such as light dispersion angle and area illuminated. Properly specified, light dispersion angle should include percent attenuation of light at that angle. Quite often they use the 50% attenuation level. Likewise, area illuminated should have distance from the light source and attenuation at the edges of the area. These issues will be discussed below.

Most LED plant lights available are designed for growing Cannabis in a basement or closet. The lights are hung from a ceiling on cables that can be adjusted as the plants grow. Mounting these lights can be problematic under shelves or over desks and tables. ALWAYS, when setting up lighting for plants, remember safety is the number one concern. Make sure the light fixtures are mounted securely. Make sure air is able to circulate around the bulb or fixture to help cool it. Make sure cords and wires are secured. Make sure plugs are plugged in all the way. Use timers instead of constantly plugging and unplugging power cords.

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

What kind of light do carnivorous plants need?

When it comes to plants, humans, and light, there is a conflict of interest between plants and humans over light. Human eyes are tuned to colors reflected by plant leaves. The plants reflect those colors of light because they have little use for it. If human eyes had exactly the same light sensitivity spectrum as the absorbance spectrum of plants, the plants would look black to us. That would not be very useful to humans. This means we have to make choices about how we grow plants under lights. It uses less energy to grow plants under the light spectrum that is most efficient for them. But we cannot see those colors of light very well. You think the plants are in the dark, but they are not in terms of light energy.

The peak light absorption of chlorophyll in plants is at the blue and red tails of the human light sensitivity spectrum. The precise spectrum the plants use is not known because it is difficult to measure light usage in live plants. Most graphs of light absorbance of chlorophyll and other light sensitive chemicals are after the chemical is extracted from leaves. Some experimentation may be necessary to find the right light spectrum for your plants.

Plant lights consisting of just deep blue and deep red LEDs that match the color of the chlorophyll peak absorbance points work well for most plants. The ratio of blue to red light appears not to be a major consideration for some plants while the ratio can be a life or death issue for other plants. One reason for this variation in light color sensitivity is, in the mid and high latitudes, sunlight is bluer during summer than it is in winter because of the way light interacts with the atmosphere. Temperate plants can use this seasonal variation in sunlight properties to determine when to grow and when to bloom and go dormant or expect to die. Equatorial plants would not see this variation but may use sunlight properties to determine if they are in full sun or in shade.

To confuse the issue even more, before the advent of LED lighting, fluorescent lighting was the best source for plant lighting. Under most conditions, fluorescent grow lights were not much better than white fluorescent tubes. The color spectrum of light from the white fluorescent tubes did not match chlorophyll spectrum very well but the plants grew spectacularly if they could handle the heat.

Plants can also use changes in day length to determine life stages. When growing plants that require seasonal light cues under artificial light, a timer that is capable of changing the on and off times daily according to the date and latitude is critical. This helps prevent the plants from getting stuck in a particular season. This is especially bad if the season they get stuck in is the dormant season.

How much light do carnivorous plants need?

In their native habitats, many carnivorous plants live with full exposure to the sun. This is about 100,000 lux of light at noon. To grow these plants indoors requires a lot of light, not 100,000 lux, but still a lot. Even plants that normally live in somewhat shaded conditions still require more light than a lighting designer would suggest for indoor lighting for humans. Bright shade is about 15,000 lux; typical office lighting is 1000 lux.

We are using lux here to indicate the amount of light the plants need. A lux meter will measure the light intensity with the same sensitivity spectrum as the human eye. Unfortunately there is no definitive way to accurately measure the amount of light from a fixture that is useful for plants. Most of the time we have to resort to trial and error.

Lighting requirements for carnivorous plants.

The plants will survive with less light but not grow as well. 

Light levels.

Lighting levels need to be determined by trial and error because everyone has a unique situation. It helps to have a Lux meter when using white lighting. There are meters that are marketed for use in determining plant-relevant lighting levels. They will give you a number that could be of little value.

White LED lighting for plants (and humans).

LEDs manufactured for in-home human lighting typically come in various color temperatures and color rendering indexes (CRI). LEDs with color temperatures of 5000K mimic full sun light color and may appear a high energy blue in homes. LEDs with color temperatures of 3000K mimic shade and appear a calming yellow in homes. 80 CRI LEDs are less expensive than 90 CRI LEDs and are slightly brighter per unit of power. However the colors of objects under 80 CRI will not match what you would see under sunlight or incandescent lighting. 90 CRI LEDs use more phosphors to tune the light for better color rendition. For plants sensitive to blue/red ratio, 5000K and 80 CRI LEDs have a relatively higher amount of deep blue light and less deep red than 3000K and 90 CRI LEDs. For some plants, the choice of color temperature and color rendering index will make a difference.

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.

Light panels are another choice for plant lighting. The light panels have arrays of LED chips mounted on a circuit board. The panels will give more even light than a spotlight but it is not as even as one may expect at first glance. There are significant edge effects. If you put a 30 cm square light panel 38 cm over two side-by-side 1020 (26 x 52 cm) plant trays, the plants on the edges will get 45% less light than the plants in the center. If your plants want high intensity light, it may be best to use two lower watt panels per tray or one panel per 30 cm² (1 sq ft). Make sure you check the wattage of the panels to get the amount of light the plants appreciate.

An interesting concept is panel lights that use mostly white LEDs with some deep red and deep blue LEDs. The light from these panels appears "normal" to human eyes. However, the plants may or may not like the light from the available panels. The white/red lights are problematic for many plants. Some growers swear by the white/blue panels. The panel above has blue, red, orange, and white LEDs. It has enough white to see the plants well. How well these panels work for some plants hinges on the ratio of colors the plants use in nature.

LED lighting designed for plants.

With the dramatic rise in home farming of Cannabis, many new plant light bulbs and fixtures have become widely available and inexpensive. These bulbs and fixtures can be great for situations where plants are being propagated or otherwise not part of a display. It is a general principle in horticulture that if you want a nice display of plants, unless you constantly want to buy new plants, you need an additional grow area with at least twice the space for plants to be rotated into the display. This can mean a lot of electricity for plants you will not be seeing all the time.

The lights tuned for plants produce light that plants require and do not "waste" energy on light that plants do not need. The more you pay for electricity, or the more you care for your carbon footprint, the more you need to consider plant-specific lighting for your non-display plants. Generally, you can figure the electricity usage will decrease at least 50% compared to white LEDs. The electricity usage could be even 70% lower if it was possible to measure the amount of plant-active light and use the minimum necessary. However, there is a tendency to think maybe the plants would appreciate more light and, of course, you decide why not.

The most basic of panel plant lights have a combination of just deep blue and deep red LEDs, usually at a ratio of about one blue to two red. They produce light that appears dim and purple to humans. For purple plant spotlights it is difficult to determine the blue/red ratio unless they are constructed with separate color LEDs and you can see them without breaking the bulb. The kind in the image of a burned out module above seems redder than a typical blue/red panel.

Most full-sun plants love this purple light. However, humans cannot judge how bright the light is. Without some white light human eyes react as if it is dark when in fact the light can be extremely bright. It is best when viewing or working with the plants under purple plant lighting that there also be white lights on to help your eyes adjust.

As with these Cephalotus plants, it is difficult to determine how well the plants are doing or if they need feeding while under pure deep blue/deep red light. To judge how they are doing requires seeing how green the new leaves are. The lighter green the leaves are, the more they need to be fed. This is another reason to have white lights as part of a growing arrangement. And ideal plant light or fixture will have some white light LEDs or you can use some standard white LED bulbs that can be turned on as needed.

Basement grow room with a mix of deep blue/deep red LED panels and deep blue/white panels. This part of it is mostly Sarracenia. Photo by Carson Trexler.

For some plants, the typical Cannabis "grow" ratio of one deep blue to two deep red LED chips is too blue while the Cannabis "bloom" ratio of one deep red to three white LED chips is too red. This is especially a problem with many Drosera species. It may be difficult to find a panel plant light with the best ratio of deep blue, deep red, and white LED chips in a regular pattern that works well for most Drosera. Temperate species such as Drosera intermedia, Drosera hybrida, and others will go dormant prematurely and remain dormant for nine months if the color is not to their liking. Winter growing tuberous and pygmy Drosera can have problems with premature dormancy. Using a combination of purple plant spotlights, pure deep red spotlights, and white spotlights does work under conditions where spotlights are feasible but is not optimal. Many growers use panels with white and deep blue LEDs. It is unclear at this point why Drosera are so finicky and how to deal with that.

Quick changes in lighting type may induce plants to do things they may not do under the same lighting once they are used to it. When doing "experiments" with light fixtures, unless the plants really get upset with the lights, it can take a year to determine whether a setup works or not. If all else fails, use white light.

Which plants need a sunrise/sunset celestial timer that adjusts light on/off times daily based on date and latitude?

Display terrarium with LED strip lighting using 180 high color rendering index LEDs per square foot (30 cm²) rated at a total 2000 lumens. The LEDs plus power supply use 30 watts per square foot (30 cm²). The LED strips are stuck to aluminum heat sinks. The terrarium has mirrors on three sides. The LEDs are a mix of 4000K and 3000K color temperature.

Initially the LEDs provided about 15,000 lux of white light 10 inches (25 cm) from the LEDs. After 6 years (32,000 hours) of use, the LEDs are providing about 10,000 lux of light at the same wattage. Except for Cephalotus 'OG Black', the Cephalotus plants are paler colored with the lower light levels but grow just fine. At some point the LEDs will become too dim and will need to be replaced.

Propagation terrarium lit by 1400 lumen, 23 watt, 2700K color temperature, high color rendering index LED lighting fixtures. The fixtures are spaced to use 30 watts per square foot (30 cm²) of growing space. The growing space has mirrors on three sides. Initially the plants were getting 25,000 lux of light 9 inches (23 cm) from the light fixtures. After 3¹/₂ years (18,000 hours) of use, the fixtures are providing 20,000 lux of light.

Heliamphora minor var. pilosa under white LED panels. The panels are 22 watt, 7700K, white LEDs. With 22 watt panels, it is best to use one panel per one square foot (30 cm²) of growing area. In practice this works out much better than using higher wattage panels farther from the plants. Photo by Mike Wilder.

Heliamphora tatei "Cerro Huachamacari" (left) and Heliamphora tatei "Cerro Duida" (right) under white LED panels. Photo by Mike Wilder.

Mixing in LED panels with some white LEDs with blue/red LED panels will provide enough white light to see the plants well. Of course it results in areas under the lights with stronger deep blue and deep red than other areas. These are the same Cephalotus plants as shown under pure blue/red panels. It is much easier to monitor the health of the plants with the white light.

Red/Blue and Blue/Red/Orange/White panels over Sarracenia. Photo by Carson Trexler.

Photo by Carson Trexler.

Sarracenia like deep blue and deep red panels. Photo by Carson Trexler.

Pinguicula and Utricularia can be grown under purple light as long as the light is not too bright. Photo by Carson Trexler.

Drosera capensis growing under a blue/white panel light. Photo by Scott Straus.

Drosera petiolaris complex plants growing under a blue/white panel light. Photo by Scott Straus.

Plants in sterile culture do not need as much light as plants potted in soil. These blue/red strip lights have a ratio of 1 blue to 3 red LEDs. Andreas Wistuba finds that some of the plants he propagates dislike the blue/red light strips which is why there are white/red light strips in the adjacent shelving. Photo by Andreas Wistuba.