A plant light is generally not an Ultraviolet (UV) light, but the two are related in the science of growing plants indoors. A standard grow light is designed to support photosynthesis, the process plants use to convert light energy into chemical energy for growth. UV light is a form of non-visible electromagnetic radiation found just outside the visible spectrum. While most commercial grow lights focus on visible light, UV light is increasingly incorporated as a supplement to influence plant development.
The Photosynthetically Active Radiation Spectrum
Standard plant lights primarily emit light within the Photosynthetically Active Radiation (PAR) spectrum, which ranges from 400 to 700 nanometers (nm). This visible light range is what plants use most efficiently to drive photosynthesis. Grow lights often prioritize the blue light range (400–500 nm) and the red light range (600–700 nm).
Blue light is effective for promoting compact, bushy vegetative growth and healthy leaf formation. Red light strongly influences flowering, fruiting, and overall biomass production. Green light (500–600 nm) is less efficiently absorbed by chlorophyll but contributes to photosynthesis by penetrating deeper into the leaf canopy. UV light falls below the 400 nm threshold, placing it outside the optimized PAR range and making it not the primary focus of typical grow lights.
Ultraviolet Radiation Types and Plant Effects
Ultraviolet light is categorized into three types based on wavelength, each having a different effect on plant biology. UVA (315 to 400 nm) has the longest wavelength, is closest to visible light, and is the least energetic. UVA is not highly damaging and positively influences photomorphogenesis, the light-regulated change in plant form. It promotes pigment formation, such as chlorophyll and flavonoids, and can enhance the aroma and color of fruits and flowers.
UVB light (280 to 315 nm) is significantly more energetic and triggers a defensive reaction in plants. Plants possess a specific photoreceptor, UVR8, which detects UVB and initiates a stress response. This response increases the production of protective secondary metabolites, including antioxidants, phenolic compounds, and resins, which act like a plant’s natural sunscreen. In some crops, this stress leads to desired quality enhancements, such as higher concentrations of beneficial compounds.
UVC light (100 to 280 nm) is the most energetic and highly damaging to living cells, which is why the Earth’s ozone layer blocks almost all of it. Due to its high energy, UVC light is germicidal and destroys DNA, meaning it should not be used on actively growing plants. Its application in horticulture is reserved strictly for sterilizing surfaces, water, or air to kill molds, bacteria, and other pathogens. High-intensity UV exposure, particularly UVB, can cause reduced growth, DNA damage, and leaf discoloration, highlighting the need for careful dosage.
Incorporating Supplemental UV Lighting
Since standard plant lights focus on the PAR spectrum, growers use specialized fixtures to add UV light as a supplement to achieve specific goals. This practice focuses on photomorphogenesis and quality enhancement rather than simple photosynthesis. Growers might add UVA or low-dose UVB light to enhance a crop’s flavor, aroma, or nutritional value by stimulating the production of desirable compounds. UV exposure has been shown to boost the potency and pigment content in certain herbs and fruits.
The goal of supplemental UV is to introduce a controlled, mild stress that prompts the plant’s natural protective mechanisms without causing damage. This carefully managed stress can strengthen the plant’s cuticles and cell walls, increasing resistance to pests and diseases. However, it is essential to control the dosage, as overexposure, especially to UVB, can stunt growth or cause leaf burn. For safety, UVC should be avoided entirely for plant growth and reserved only for sterilization, requiring caution to protect both the user and the plant.