A greenhouse is a specialized structure designed to create a controlled environment for growing plants by regulating air temperature and humidity. These structures rely on solar energy to provide light while maintaining a warmer internal climate than the outside air. The common query about the color of these structures stems from a misunderstanding of how light interacts with the building materials and the plants housed inside.
Understanding the Visible Light Spectrum
The light we perceive as white sunlight is actually composed of a spectrum of different colors, each corresponding to a different wavelength of electromagnetic radiation. This visible spectrum ranges from red light, which has the longest wavelength, through to violet light, which has the shortest wavelength. The colors we see are determined by which wavelengths an object absorbs and which wavelengths it reflects or transmits back to the eye.
An object appears a certain color because its surface molecules absorb all other wavelengths of the visible spectrum. For example, a red apple absorbs all colors except red, which is reflected. If a material absorbs light, that energy is converted into heat, whereas if it transmits light, the light passes through the material unchanged. This interaction between light and matter is fundamental to the operation of both the greenhouse structure and the plants within it.
How Greenhouse Materials Trap Heat
The function of a traditional greenhouse is to trap thermal energy, known as the greenhouse effect. The glass or transparent plastic materials used in construction allow high-energy, short-wave radiation, including visible light, to pass through unimpeded. This incoming radiation is absorbed by the soil, plants, and other surfaces inside the structure.
Once absorbed, this solar energy is re-radiated back into the enclosed space as lower-energy, long-wave infrared (IR) radiation, which we perceive as heat. The glass or polyethylene material is largely opaque to this longer-wavelength IR radiation. This opacity prevents the thermal energy from escaping, causing the temperature inside the greenhouse to rise significantly. This selective transparency—allowing short waves in and blocking long waves out—maintains the elevated temperature necessary for plant growth.
Why Plants Need Specific Colors of Light
The life-sustaining process within a greenhouse is photosynthesis, which requires specific wavelengths of light to convert carbon dioxide and water into energy. Chlorophyll, the primary pigment responsible for photosynthesis, dictates which parts of the visible spectrum are utilized by the plant. This pigment exhibits two major peaks of high absorption: one in the blue-violet range, between 400 and 500 nanometers, and another in the red range, around 650 to 680 nanometers.
These highly absorbed blue and red wavelengths provide the necessary energy for the plant to drive its metabolic processes. Conversely, chlorophyll absorbs green light, which falls roughly between 500 and 600 nanometers, much less effectively. Because the green light is not efficiently absorbed, it is instead reflected and scattered back toward our eyes, which is why plant leaves appear green.
Addressing the Green Appearance
Effective, traditional greenhouses are not green; they are clear, utilizing transparent glass or plastic to maximize the transmission of the full light spectrum. The association of green with greenhouses often results from confusing the structure’s material with the color of the plants housed within. The collective reflection of green light from thousands of leaves makes the entire structure appear green from a distance.
Some smaller, less expensive greenhouses use tinted green plastic films. This green tint is not intended to aid growth, but rather to reduce the intensity of sunlight, acting as shade. By reflecting some of the green light that plants use least, the film reduces the total heat load and prevents overheating in smaller, poorly ventilated structures. A green-tinted covering filters out the light plants reflect, making it less optimal for maximum growth than a clear covering.