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).