How Spectral Light Influences Plant Growth

Light can be expressed as a wave. Imagine the ripples that form on a pond when you throw a pebble into the water. This is very similar to what light waves look like.

The size of the waves, or wavelengths, vary depending on where the color of the light falls on the electromagnetic spectrum.

Red light such as radio, microwave, and infrared wavelengths are wide, long, and contain less energy. Violet light such as UV, X-rays, and gamma rays are short, narrow, and high energy.

The spectrum’s wavelengths are measured in nanometers, and it ranges from zero to 5,000,000,000 nanometers (nm). Visible light, which falls between red and violet light, and is a small slice of the entire electromagnetic spectrum.

All the colors of the rainbow fit in the very narrow range of 400-700 nm. ll of these colors together create white light, and they are separated into individual colors when they pass through a prism (this is why rainbows appear when sunlight passes through water droplets).

This section of the spectrum has the most impact on plant growth, although more recent studies are showing that infrared and UV rays play a role as well.

Understanding the Spectrum

It is important to understand how color temperature is measured when shopping for an indoor grow lamp. Kelvin measures a light’s degree of warmth, ranging from zero to 10,000 K. Cooler blue lights will have a higher temperature, and warmer red lights will have a lower temperature. The temperature of daylight is about 5,600 K.

Light plays an essential role during photosynthesis. In the photosynthesis process, plants convert light, carbon dioxide, water, and minerals into glucose and oxygen. Plants primarily use the red and blue spectrums of light in varying amounts during this process, depending on their stage of growth.

Although different plants prefer different light spectrums, in general, certain spectrums perform specific tasks. Blue light in the 400-500 nm range promotes root growth and intense photosynthesis. Red light in the 640-720 nm range stimulates stem growth, flowering, and chlorophyll production. Together, this entire range is known as photosynthetically active radiation.

Sunlight contains the full spectrum of colors, though the wavelength will vary according to the time of day or year. Spring sunlight is more violet and high energy, encouraging the vegetative growth of plants, whereas fall sunlight has a warmer color temperature and lower energy, producing the fruit and flowers for a fall harvest.

You can also see the color spectrum change throughout the day as the sun changes position in the sky, which is what creates vibrant red and orange sunsets as the day winds down.

Indoor growers traditionally use metal halide (MH) lamps in the vegetative stage and high pressure sodium lamps (HPS), or a mix of both, in the flowering stage. Metal halide lamps have more blue light, while HPS lamps have more red light.

Many light emitting diode (LED) lamps allow you to change the color temperature as needed throughout the vegetative and flowering cycles. There are also MH and fluorescent “daylight” lamps that produce a white light closer to that of the sun.

Some growers swear by using HPS lamps alone throughout the entire cycle, which works but will often grow taller plants with greater internodal spacing and produce a smaller harvest.

Although a plant can grow under red lights alone for its entire life cycle, indoor growers usually prefer shorter, bushier plants due to limited vertical space, and have found that using a fuller spectrum of light will produce a higher yield come harvest.

Studies on Light and Plant Growth

In a 1997 study published in the Oxford Journal of Experimental Botany, wheat was grown under red LEDs with and without supplemental blue lighting, and was then compared to wheat grown under white daylight fluorescent lamps.

The study found that wheat grown under red lights alone demonstrated less development during vegetative growth, grew longer stems and leaves, and yielded fewer seeds than wheat grown under white light.

Wheat grown under red LEDs with supplemental blue fluorescent lighting produced similar growth and yield to wheat grown under white light. The study concluded that wheat can, in fact, complete its life cycle under red LEDs alone; however, larger plants and greater seed yields are produced with the fuller spectrum of light.

A 1987 study published in Scientia Horticulturae studied the effects of blue, natural, green, yellow, and red light in chrysanthemums, tomatoes, and lettuce. It found that blue light reduced the dry weight, height, and leaf area in the plants compared to other light spectrums.

Green and yellow light enhanced the leaf area of tomatoes compared to natural light. They also found that blue light produced darker green leaves than natural light, whereas green and yellow light produced light green leaves.

A 2012 study published in Scientia Horticulturae investigated the influences of three different qualities of light on lettuce. They used a red and blue LED; a red, blue, and white LED; and a fluorescent lamp. It was found that the dry weights, crispness, sweetness, and shape of plants treated with the red-blue-white LED and fluorescent lamp were higher than in plants treated with only red-blue LEDs. Once again, a fuller spectrum of light throughout the plants’ life cycle yielded better results than blue or red light alone.

As a gardener, it is critical to recognize the importance of lighting in your garden. Outdoors, you as the gardener must consider which plants grow best at your latitude. You also need to consider your garden’s microclimate and how much sun is received at certain times of day.

You might find it helpful to shade certain plants at certain times depending on your location and which plants you’re growing. Indoors, you must choose the correct grow lamp for each part of a plant’s life cycle, keeping in mind that blue light will enhance vegetative growth and a fuller spectrum lamp will enhance the overall yield and quality of your harvest.