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How Different Lights Make Plants Grow

Phytochromes and Photosynthesis

Every plant contains phytochromes--a family of proteins that allows plants to sense each of the different light colors within the spectrum. Sunlight contains the full spectrum of light needed to make plants grow, but not all plants have access to the sun’s rays. Other types of lighting can supplement the light quality for house plants or greenhouse plants. Light quality refers to the colors of light, or the light's wavelength.

Plants use all colors of the spectrum to grow and thrive, but blue and red are the primary colors necessary to complete photosynthesis. Photosynthesis is the conversion process by which plants transform light energy into necessary food and oxygen. The amount of red and blue light within a light source impacts plant growth in different ways, but a combination of these two colors is most important. Green is the only color in the light spectrum that is not absorbed by the plants. Plants reflect the color green, which is why they appear green to the human eye.

Red Light

Red and orange lights are crucial to plant growth, as these colors trigger hormones that increase flowering and budding. Red light also induces germination and works together with blue light to promote seed growth. Far-red light (between red and infra-red) inhibits germination.

Even though red light stimulates flowering and foliage growth, if a plant receives too much red light it will become spindly. High pressure sodium lights, or HPS, are commercially available grow lights that emit a red-orange glow. HPS lights are good companion lights for growing conditions with limited natural sunlight, or in combination with other light sources that have strong levels of blue light.

Blue Light

Blue light controls and regulates the rate of a plant's growth, along with many other plant responses. These include phototropism and stomata opening. Phototropism defines how a plant responds to light--stems grow up to reach the light source, and roots grow down, away from the light source. Stomata are the openings beneath or on the surface of a plant’s leaves. Moisture loss is primarily due to the size of the stomata openings. Because the amount of blue light a plant receives controls the degree of stomata opening, this light controls the amount of water a plant keeps or loses, which will ultimately impact the plant’s growth.

Metal halide lights are commercially available grow lights that emit more light in the blue spectrum. Metal halide lights or full-spectrum LED grow lights are good choices for growing plants indoors if there is no sunlight available.

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