Artificial lighting has become standard practice in modern horticulture, allowing growers to cultivate plants indoors year-round. These specialized fixtures, commonly known as grow lights, are designed to mimic or supplement the solar radiation plants use for growth. A frequent question for those employing this technology is whether these lights emit ultraviolet (UV) radiation, which is a component of natural sunlight. The presence of UV light in a grow environment is entirely dependent on the specific technology used, as manufacturers can either intentionally include or completely filter out these wavelengths to achieve different plant growth outcomes. Understanding the distinction between various UV bands and the light sources is necessary to determine the potential effects on both the plants and the people tending them.
Understanding the Light Spectrum
Light is a form of electromagnetic radiation, and the full spectrum ranges from extremely short gamma rays to very long radio waves. The visible light spectrum, which plants use for photosynthesis, spans wavelengths roughly between 400 and 700 nanometers (nm). Ultraviolet radiation sits just outside the visible spectrum, occupying the shorter, higher-energy wavelengths from approximately 100 nm to 400 nm.
UV light is further categorized into three bands based on wavelength and energy. Ultraviolet A (UVA, 315 nm to 400 nm) makes up the majority of the UV radiation that reaches the Earth’s surface. Ultraviolet B (UVB, 280 nm to 315 nm) is mostly filtered by the ozone layer but still reaches plants outdoors. The shortest and highest-energy band, Ultraviolet C (UVC, 100 nm to 280 nm), is completely absorbed by the atmosphere, so it is not found in natural sunlight.
UV Emission from Common Grow Lights
The UV output of any grow light fixture is determined by the underlying lamp technology and whether it has been designed for full-spectrum or specialized output. Most standard white Light Emitting Diode (LED) fixtures, which are the most popular choice in horticulture, typically generate little to no UV radiation. These LEDs often use a blue diode with a phosphor coating to create white light, a process that usually does not extend into the UV range. Specialized LED grow lights, however, may incorporate specific UV diodes to intentionally emit UVA (360–380 nm) or low-dose UVB (290–310 nm) wavelengths, allowing growers to utilize the biological effects of UV light on plants. Older lighting technologies, such as High-Pressure Sodium (HPS) and Metal Halide (MH) lamps, can emit UV radiation as a byproduct of their arc-tube operation, while fluorescent lights produce some UV that is typically filtered by the housing.
The Impact of UV on Plant Morphology and Chemistry
Growers who intentionally include UV in their lighting spectrum do so to elicit adaptive responses from the plants. While UV light is not directly used for photosynthesis, plants have developed photoreceptors that sense the presence of UV-B radiation. Sensing this radiation acts as an environmental signal, triggering defense mechanisms that can alter the plant’s physical structure and internal chemistry.
One morphological effect of UV exposure, particularly UVB, is the inhibition of stem elongation, which leads to plants that are shorter and more compact. This response is thought to be a defense mechanism to reduce the surface area exposed to potentially damaging radiation. Additionally, UV-B radiation can lead to the accumulation of secondary metabolites, which are compounds produced for defense.
These secondary metabolites include compounds like flavonoids, anthocyanins, and terpenes. Flavonoids and anthocyanins absorb UV radiation, acting as an internal sunscreen for the plant cells, and their increased production often results in more intense color development in leaves and flowers. For many crops, the accumulation of these compounds is desirable because it can enhance flavor, aroma, and nutritional content, and also boost the plant’s natural immune function.
Safety Guidelines for UV-Emitting Fixtures
When using grow lights that intentionally emit UV radiation, human safety precautions are necessary due to the higher energy carried by these wavelengths. Prolonged and unprotected exposure to UV light can cause damage to the skin and eyes. The shorter wavelengths, specifically UVB and UVC, pose the greatest risk, potentially causing issues like photokeratitis or increasing the risk of skin damage. Individuals working near UV-emitting fixtures should wear protective equipment to minimize exposure. This includes:
- Wearing UV-blocking eyewear or goggles designed for use with grow lights to shield the eyes.
- Covering exposed skin with long sleeves, pants, and a wide-brimmed hat.
- Limiting the amount of time spent directly under the lights.
- Applying broad-spectrum sunscreen with a high Sun Protection Factor (SPF) to any exposed skin.