How Does the Color of Light Affect Plant Growth?

The color of light, meaning its wavelength within the visible spectrum, affects plant growth because different wavelengths are absorbed differently by chlorophyll and other plant pigments. This matters primarily when choosing grow lights for supplemental lighting: natural sunlight contains the full spectrum of wavelengths and plants have no issues with it. Artificial light sources vary considerably in the wavelengths they produce, and understanding which wavelengths support plant growth helps you choose a suitable grow light.

Chlorophyll and Light Absorption

Plants use chlorophyll as the primary pigment for capturing light energy in photosynthesis. Chlorophyll absorbs light most efficiently at two wavelength ranges: blue light in the range of 430 to 450 nanometers and red light in the range of 640 to 680 nanometers. The green wavelengths in the middle of the spectrum are largely reflected rather than absorbed, which is why plants appear green to us.

This does not mean that only blue and red light support plant growth. Plants also have accessory pigments including carotenoids that absorb yellow and orange wavelengths. The total amount of photosynthetically active radiation, abbreviated as PAR, across the 400 to 700 nanometer range determines how much light energy is available for growth. But the most efficiently used wavelengths are blue and red.

Blue Light

Blue light supports vegetative growth: the growth of leaves, stems, and roots. Plants grown under predominantly blue light tend to be compact and stocky with thick leaves and short internodal spacing. Blue light also triggers phototropism, the bending of stems toward a light source, and supports stomatal opening for gas exchange.

Red Light

Red light supports flowering and fruit production. Plants grown under predominantly red light tend to grow taller and produce larger leaves, and red light is particularly important for triggering the flowering response in photoperiod-sensitive species. Red light combined with blue light provides the balance that supports both vegetative growth and flowering.

Full-Spectrum Grow Lights

For most houseplant growers, a full-spectrum LED grow light that covers the blue and red wavelengths and includes green for a natural appearance is the most practical choice. Look for lights labeled as full-spectrum with a color temperature of 5000 to 6500 Kelvin for vegetative growth, or 2700 to 3500 Kelvin for bloom-supporting spectrum. Lights specifically for indoor plant growing list their PAR output, measured in micromoles per square meter per second, which is the most useful measure of how much usable light the plant receives.

For a review of a specific widely available grow light model, the Viparspectra grow light review covers one LED panel option in detail. For the difference between heat lamps and grow lights, the heat lamps for plants guide covers that distinction.