Figuring out what type of light carnivorous plants need to grow well indoors can be a daunting task. This is partially due to the needs of the plant, the configuration of the growing space, and the number of lighting options available. It’s also largely due to the fact that light itself can be complicated to measure.

To help simplify things, we’re going to break this topic down into several parts:

 

Light color (quality)

Light is made up of many colors. These colors are produced by different wavelengths of energy. The size of the wavelengths is measured in nanometers (nm). Plants use the area on the light spectrum between 400 and 700 nm to conduct photosynthesis. This is known as the Photosynthetically Active Radiation (PAR) region.PAR Region of the Electromagnetic Spectrum

 

Photosynthetically Active Radiation (PAR) – The area on the light spectrum between 400 and 700 nm that plants can use to conduct photosynthesis.

Plants use different PAR regions for a variety of processes:

Violet, Blue ( 400-520 nm) – Important for leafy growth and conducting photosynthesis.

Green, Yellow, Orange (520-600 nm) – Used by carotenoids and other pigments to conduct a variety of processes including energy transfer for photosynthesis and protection from over-exposure.

Red (600-700 nm ) – Necessary for photosynthesis and promoting fruit and flower production.

The amount of light needed in each color range may differ from one species to the next and depends on what stage of growth a plant is in. However, it can be said that overall plants need light in all colors of the spectrum, not just narrow peaks in the blue and red ends as is commonly assumed. For a more in depth look at why this is the case, check out this study by Dr. Keith McCree or read Inda-Gro’s summary of his findings.

What about Ultraviolet (UV) and Infrared (IR) light outside the PAR range?

While plants don’t use UV and IR light to conduct photosynthesis, these segments of the spectrum can still provide several benefits to plant growth. Benefits can include disease resistance, reduction of bacteria and fungus, and more robust growth.

Relative Photosynthetic Response Curve (Quantum Weighted)

Quantum weighted Relative Action Curve adapted from McCree, 1972.

 

So what should I look for when researching grow lights?

A Spectral Distribution Curve (SDC) is important to look for when researching grow lights. It provides a visual weighting of the PAR wavelengths produced by a particular bulb. The SDC of a bulb is measured using a spectroradiometer.

There are two things to keep in mind when reading an SDC:

  • The value of the Y axis may be labeled a number of things or not labeled at all depending on the manufacturer. Common labels include µMol/S, Watts, mW/nm, µW/nm, Intensity, Spectrum, and so on. While this changes the meaning of the data somewhat, overall the idea is the same in that it provides a breakdown of the light colors produced in the PAR region relative to each other.

  • In a grow space where the fixture will be providing the main source of light, it’s more important to look at the overall PAR curve rather than spiked peaks at specific nm values. While a large spike say at 635 nm may work ok for supplemental lighting (Fig. B), it would not work well as a primary light source. Ideally, the SDC for a primary light source will display a broad range of colors (Fig. A).

Protopia CMH315W4K CMH SDC

Fig. A – Broad spectrum of the Protopia 315W 4K CMH bulb

Growstar 45W LED SDC

Fig. B – Narrow spectrum of the Growstar 45W LED

What about Kelvin and CRI?

A good grow light doesn’t (or shouldn’t) provide light quality measurements in Kelvin or CRI. Why? Humans process green and yellow light better than blue and red light. Kelvin and CRI scales take this human perception into consideration by assigning higher value to green and yellow light. This may result in light that looks good to us, but since plants need all colors of light, CRI ratings don’t tell us much about whether it will “look good” to the plants. Kelvin and CRI do have their place in helping us figure out color though and are often included in light specifications so we’ll cover them briefly.

Plant Response vs. Human Perception

Kelvin (K)  – With respect to light, Kelvin refers to the overall color of light a bulb produces based on human perception. Colors typically range between 2700K (warm orange) to 6500K (cool blue/white). While Kelvin tells us little about the color wavelengths a light emits, it can be used to ballpark what end of spectrum is favored. Warm lights will lean towards the red end of the spectrum and cool lights will lean towards the blue end.

Kelvin Temperature Scale

Color Rendering Index (CRI) – A CRI rating tells us how accurately a light can display colors compared to sunlight. The sun has a CRI of 100. A CRI of at least 85 is good if the aim is to have plants appear naturally colored. A high CRI can indicate a wide range of colors are emitted across the PAR region, but not necessarily since it is weighted for human perception. The only way to know for sure would be to find the SDC of the light.

King Plus 1200w LED SDC

SDC for the KingLED 1200W (original model) – Giving plants a purplish hue, this light would measure poorly on the CRI scale and yet provides a decent spectrum of light for plants.

Sylvania Octron 950 Full Spectrum T8 SDC

SDC for the Sylvania Octron 950 Full Spectrum T8 – With a CRI of 90, this light looks good to the human eye and would also make a decent grow light though it may need a little help in the blue and red ends of the spectrum.

If you’d like more information about Kelvin and CRI, see this video:

 

A note on LEDs labeled as PAR 20, PAR 30, etc.

You may notice some LEDs will have labels like “PAR 20” or “PAR 30”. This has nothing to do with the PAR region for plants. Instead, it stands for Parabolic Aluminized Reflector and is referring to the type of reflector the light uses along with the diameter of the bulb. Just a little note as this can cause some confusion.

 

Next steps

In this part we’ve covered what type of light carnivorous plants need including the colors of light necessary for healthy growth, the importance of SDCs when researching grow lights, and a little about Kelvin and CRI. Next, we’ll take a look at how much light carnivorous plants need in Part 2.