Indoor gardening has grown significantly in popularity. Grow lights serve as artificial light sources, replicating the essential light energy plants need for photosynthesis and helping them thrive by providing the necessary spectrum for their development.
Why Grow Lights Appear Purple
Many LED grow lights emit a distinctive purple hue because they combine specific red and blue light-emitting diodes (LEDs). Plants primarily rely on red and blue wavelengths for photosynthesis, converting light energy into chemical energy. Chlorophyll, the pigment responsible for a plant’s green color, efficiently absorbs light in these red and blue regions. Green light, being less absorbed, is largely reflected, which is why plant leaves appear green to the human eye.
When red and blue light are combined, human vision perceives this mix as purple or magenta. This visual effect results from the light spectrum, intentionally tailored for plant needs rather than human aesthetics. The design provides the most photosynthetically active wavelengths, minimizing energy output on less utilized parts of the spectrum. This ensures efficient energy use by the plant for growth.
How Purple Light Benefits Plants
The specific wavelengths within purple grow lights, blue and red light, each play a key role in plant physiology. Blue light (400-500 nm) promotes strong vegetative growth, leading to robust stems and healthy leaf development. It also influences stomata, tiny pores on leaves that regulate gas exchange and water loss, supporting overall plant metabolism.
Red light (600-700 nm) is important for plant development, including flowering and fruiting. It plays a role in seed germination and biomass accumulation, contributing to the plant’s final yield. Red light also interacts with plant hormones, such as auxins, involved in stem elongation and flower formation.
The combination of these two light colors in purple grow lights optimizes both vegetative growth and reproductive stages for indoor plants. This balance ensures plants receive the specific light signals needed to progress through their life cycle. Both blue and red light are highly photosynthetically efficient wavelengths, well-absorbed and utilized for energy production.
Practical Use of Purple Grow Lights
Effective use of purple grow lights involves understanding placement and duration for different plant stages. The distance between the light source and plants is important, as light intensity, measured as Photosynthetic Photon Flux Density (PPFD), decreases with increasing distance. While precise distances vary by light model and plant type, growers adjust light height to provide sufficient intensity without causing stress or burning.
The duration of light exposure, or photoperiod, varies depending on the plant’s growth stage. Seedlings benefit from 16 to 18 hours of light per day for early development. For plants in their vegetative stage, an extended light period of 14 to 18 hours daily is recommended to support foliage growth. When plants transition to the flowering stage, reducing light duration to about 12 hours of light followed by 12 hours of darkness can simulate natural conditions that trigger blooming and fruiting. Plants require a period of darkness for essential metabolic processes.
Light intensity is discussed in terms of Photosynthetically Active Radiation (PAR), which refers to the wavelengths (400 to 700 nanometers) plants use for photosynthesis. PPFD specifically quantifies the amount of PAR that reaches the plant surface per second. While growers do not need complex calculations, understanding that higher PPFD supports faster growth is useful.
Are Purple Lights Always Best
While purple grow lights efficiently deliver the red and blue wavelengths important for plant growth, they are not the only effective option for indoor gardeners. Grow lighting has seen advancements, with the emergence of “full-spectrum” or “white” LED grow lights. These full-spectrum lights emit a broader range of wavelengths, aiming to mimic natural sunlight more closely than purely red and blue combinations.
Full-spectrum lights include green and yellow wavelengths. While not as efficiently absorbed by chlorophyll as red and blue light, these can penetrate deeper into the plant canopy. This deeper penetration allows for more uniform light distribution throughout the plant, benefiting lower leaves shaded by upper foliage. For human observers, white lights create a more natural and comfortable viewing environment compared to the intense purple hue.
Modern LEDs offer greater energy efficiency and heat management. Ultimately, the choice of grow light, whether purple or full-spectrum, depends on the specific needs of the plants, the grower’s preferences, and aesthetic considerations for the growing environment.