Plants require light for photosynthesis, the process by which they convert light energy into chemical energy to fuel their growth and development. The rate of photosynthesis is directly related to the color and intensity of light that plants receive. Different wavelengths or colors of light can have varying effects on plant growth and health. Understanding how light color affects plants can help gardeners and indoor plant owners optimize growth.
Effects of Light Color on Plant Growth
The color of light is determined by its wavelength within the electromagnetic spectrum. Light wavelengths range from low energy infrared to high energy ultraviolet. The visible light spectrum that humans can see includes red, orange, yellow, green, blue, indigo and violet light.
Plants are most sensitive to blue (430-475 nm wavelength) and red (625-740 nm wavelength) light. Chlorophyll, the primary photosynthetic pigment in plant leaves, absorbs red and blue light most efficiently. However, other plant pigments like carotenoids and anthocyanins absorb other colors.
Here are some general effects that different light colors have on plants:
- Blue light: Stimulates chlorophyll production and chloroplast formation, improving photosynthesis. Influences vegetative leaf growth and compact, sturdy plants.
- Red light: Most powerfully drives photosynthesis and biomass accumulation. Promotes flowering and fruiting.
- Green light: Less efficient for photosynthesis since green is reflected by plant leaves. Provides brightness that aids growth.
- Yellow/orange light: Enhances carotenoid pigments and influences chloroplast formation.
- UV light: At low levels, can increase production of phenolics, antioxidants and flavonoids. Too much can damage DNA.
- Far red light: Promotes stem elongation, shade avoidance and influences flowering.
While blue and red light are most critical, plants generally grow best under full spectrum white light that includes a balance of wavelengths. Indoor grow lights are designed to provide an ideal blend of colored light for plant growth and development.
Blue Light vs Red Light
Both red and blue light play important roles in plant growth. Here is a more in-depth comparison:
- Blue light – Has shorter wavelengths and higher frequency than red light. Blue photons possess more energy, which allows deeper penetration into plant tissues. Promotes chlorophyll biosynthesis, stomatal opening and chloroplast formation for improved photosynthesis. Influences phototropic responses to light and vegetative leaf growth. Too much can result in compact, stocky plants.
- Red light – Has longer wavelengths and lower frequency than blue light. Drives photosynthetic activity at peak levels and promotes flowering and fruit production. Most efficiently converted to chemical energy during photosynthesis. Promotes stem elongation, branching and flowering/fruiting in many species. Too much can result in overgrown, leggy plants.
Most horticulture experts recommend providing both blue and red light in an appropriate ratio tailored to each plant species and the grower’s objectives. For example, blue light favors leafy greens while red light optimizes fruiting in tomatoes. In nature, the blue light component is higher in spring/summer when vegetation is vigorous while the red component increases in late summer/autumn to stimulate flowering.
Recommended Light Color for Plant Types
The ideal light color balance differs among plant varieties depending on whether the desired outcome is vegetative leaf growth or flowering/fruiting. Here are some general recommendations:
| Plant Type | Recommended Light Color |
|---|---|
| Leafy greens/herbs | Higher blue light component |
| Flowering plants | Higher red light component |
| Fruiting plants | Higher red light component |
| Seedlings | Balanced red and blue light |
| Succulents/cacti | Balanced red and blue light |
Blue light is ideal for promoting healthy green growth in leafy plants like lettuce, spinach and basil as well as houseplants with colorful foliage. Red light drives flowering and fruit production in plants like orchids, roses, tomatoes, peppers and cannabis.
Seedlings and young plants require balanced light to support all stages of development. Succulents and cacti grow well under full spectrum white light. Adjusting the color balance based on the plant can optimize yields.
Effects of Green, Yellow and Orange Light
Though less critical than blue and red light, other light colors can still influence plant growth:
- Green light – Not efficiently used in photosynthesis since green wavelengths are reflected by plant leaves. However, contributes brightness that aids growth. May hasten flowering in some long-day plants.
- Yellow light – Absorbed by carotenoid pigments and enhances chloroplast development. Can improve flowering and fruit production.
- Orange light – Also absorbed by carotenoids. Supports chloroplast formation and influences stomatal opening.
Full spectrum grow lights include green, yellow and orange wavelengths to provide a diversity of light colors, even though plants do not use these colors as efficiently as blue and red. Nearly all plants grow best under broad spectrum white light supplemented by additional red/blue light based on the plant’s needs.
Ultraviolet Light Effects
Ultraviolet (UV) light has shorter wavelengths and higher frequencies than visible light. Low UV levels can benefit some plants by increasing production of phenolics, antioxidants and aromatic compounds like essential oils. However, too much UV radiation can damage plant DNA and tissues, causing stunted growth.
Outdoor sunlight contains some UV rays, while indoor grow lights filter out most UV light. Small doses from specialized UV LEDs can provide benefits without harming plants. However, UV light requires cautious use to avoid plant injury.
Far Red Light Effects
Far red light has longer wavelengths (700-800 nm) just beyond what humans can see. It has minimal impact on photosynthesis, but strongly regulates plant morphology and flowering.
Exposure to far red light at dusk triggers the shading avoidance response, making plants elongate stems and avoid neighboring plants competing for light. It also influences the photoperiodic flowering response in some long-day plants, stimulating flower bud formation.
Providing small amounts of far red light from LEDs can control plant shape and prompt flowering. However, too much far red can result in overgrown, leggy plants with sparse foliage.
Conclusion
While plants can use light across the color spectrum, blue and red light drive photosynthesis and growth most efficiently. Blue light favors leafy growth while red light stimulates flowering and fruiting. Full spectrum white light with supplemental red/blue LEDs tailored to the plant provides ideal balanced illumination.
Adjusting light color based on the plant variety and desired outcome offers a useful way to manipulate plant morphology and development. Proper use of colored lights allows growers to optimize yields.
