Simply put, plants grow best in sunlight, which—although it looks white or golden—contains all colors in the spectrum. The visible spectrum of light is defined by wavelength measured in nanometers. And, while sunlight contains all colors, in terms of intensity, there is more blue light than red light.
For a time, the knowledge of this fact helped to foster the myth that plants only need blue and red light to grow. But science has since verified that, while more of some colors promote higher growth in specific stages, plants use all light for photosynthesis, including green and yellow.
Science of Light
Studies conducted by NASA determined the following roles which different light wavelengths within the PAR range play in plant growth and development.
- 400-520 nm (Blue) Light: Affects the chlorophyll content in the plant, which affects leaf thickness and plant height. Too much can actually stunt growth, so it needs to be used within a full spectrum.
- 500-600 nm (Green) Light: It was originally thought that plants didn’t use this at all, however the majority of green light is used and penetrates through thick canopies, actually supporting growth and sustaining the leaves beneath.
- 630-660 nm (Red) Light: This light is essential for germination and stem growth, as well as leaf expansion. It is also used to regulate periods of growth and flowering. Too much red light, or red light used alone, will produce tall plants that appear stretched with thin leaves.
- 720-740 nm (Far Red) Light: Exposure to IR (infrared) light can help plants develop larger leaves as well as reduce the amount of time required for flowering. It also penetrates the canopy to help support lower stems and leaves.
Many LED grow light manufacturers provide light spectrum charts for specific grow lights which indicate the type and intensity of the light produced.
Three Key Measurements
There are basically three measurements of light that indoor growers should be aware of and understand how they affect photosynthesis and plant development.
- PAR: PAR stands for Photosynthetically Active Radiation. This is the wavelength of light from 400nm to 700nm—roughly the light used by plants for photosynthesis
- PPF: Photosynthetic Photon Flux is the total amount of photons in the PAR range produced by the source of light every second. This is measured in micro moles per second (µmol/sec) and tells you how much light is being produced that is actually usable by your plants.
- PPFD: Photosynthetic Photon Flux Density is the measurement of the PPF in a defined area. It is measured in micro moles per square meter, per second (µmol/m2/s).
A good LED light will have the PPFD data available in the form of a chart similar to the one below. It tells you how much usable light is being produced at specific points in a defined area. Depending on your growing space, you can determine how many lights you will need to grow specific plants.
And while these do not measure the quality of light in the spectrum, they are valuable when used along with that data. For example, a smaller full-spectrum light will outperform a larger one providing only red and blue light.
Note: These measurements only cover light produced in the PAR range and do not measure light outside of that range. Light outside the PAR range, such as far red light, is also valuable for plant growth.
Indoor Growing with LED Lights
LED grow lights are more widely used than any other type, and a full-spectrum LED grow light closely mimics natural sunlight. These are available in a number of sizes and are easy to install in a grow room or tent.
In addition, there are full-spectrum LED grow lights that include additional blue or deep red diodes which can be used at specific stages of growth and development to produce the desired effect.
Which types of plants being grown will determine what color light you want to use at different stages of growth, to force blooms, or encourage plants to become dormant.
Specific combinations of blue and red light can encourage growth in dwarf varieties without allowing them to become too large. The same is true for growing plants to be more compact than they normally would develop in natural sunlight. The inclusion of green light and small amounts of far red light contribute to the growth of the lower stems and leaves of larger plants so that they don’t appear “leggy.”
There are many resources available for best indoor growing practices for all types of plants, including state extension services, college and university websites, and a multitude of indoor growing and gardening sites and blogs.
Summary
Full-spectrum light that mimics natural sunlight is the best for indoor growing. For controlling specific aspects of plant growth and flowering, combinations of higher intensity light in the red and blue wavelength ranges are useful. PPFD charts and light spectrum graphs are valuable tools to use when selecting indoor LED grow lights.
