A greenhouse is a structure designed to capture and maximize natural sunlight, converting it into the energy plants need for growth. By enclosing a growing space with transparent materials, the environment can be controlled to extend the growing season and improve productivity. However, reliance on the sun means external factors, such as weather patterns and the time of year, introduce significant variability into the daily light supply. Understanding plant light needs is the first step in determining if supplemental grow lights are necessary to ensure a healthy and consistent harvest.
Understanding Light Requirements in a Greenhouse
The energy source for plant growth is Photosynthetically Active Radiation (PAR), the segment of the light spectrum between 400 and 700 nanometers. This wavelength range is used by plants for photosynthesis, the process that converts light energy into chemical energy. Instantaneous light intensity is measured as Photosynthetic Photon Flux Density (PPFD), which quantifies the number of photons hitting a square meter per second. However, PPFD only provides a momentary snapshot of the light available to the plant.
A more comprehensive measure is the Daily Light Integral (DLI), which accumulates the total amount of PAR received over a 24-hour period, expressed in moles per square meter per day (mol/m²/d). DLI is a strong indicator of a plant’s potential for growth and biomass accumulation. Even in a structure designed to maximize light, the greenhouse itself reduces the light available to plants.
Glazing materials, such as glass or polycarbonate, typically transmit between 70% and 90% of incoming sunlight, losing some light to reflection and absorption. Structural components, including the frame and internal equipment, cast shadows that can collectively reduce light transmission by another 5% to 15%. These factors mean the light reaching the plant canopy is always significantly less than the outdoor ambient light, making DLI management a central part of greenhouse operation.
When Supplemental Lighting Becomes Essential
Supplemental lighting is necessary when natural light conditions fail to meet the minimum DLI required for optimal crop development. This deficit is most pronounced during winter months, especially at higher geographic latitudes where day length is short. For example, high-latitude greenhouses can see DLI plummet to below 10 mol/m²/d on cloudy winter days. Without supplemental light during these low-light periods, growth slows dramatically, leading to extended crop cycles and reduced quality.
The specific crop type is the largest determinant for the need for extra light. High-light demanding plants, such as tomatoes, cucumbers, and peppers, require a consistently high DLI, often needing 15 to 20 mol/m²/d for optimal yield. Low-light crops like leafy greens and herbs can thrive with a DLI as low as 10 to 12 mol/m²/d. Supplemental lighting is often necessary for high-value fruiting crops to maintain commercial viability outside of peak summer light conditions.
Light supplementation is standard practice for accelerated production schedules, allowing growers to force growth or harvest out of the natural season. Providing a consistent, high DLI allows for predictable crop timing and faster maturity, which is crucial for meeting market demands. Precise light management is also important for young plants, such as seedlings and cuttings in the propagation stage.
A consistent DLI encourages strong, compact growth and prevents seedlings from becoming “leggy,” a sign of light starvation. Even in a well-lit greenhouse, localized shading can create light deficits. Dense foliage, internal equipment, or nearby buildings can cast shadows, necessitating targeted supplemental light to ensure uniformity across the entire crop canopy.
Selecting the Right Grow Light Spectrum and Intensity
Once the need for supplemental light is established, choosing the correct light involves balancing the plant’s spectral needs with technological efficiency. The light spectrum, or light quality, dictates how the plant grows and develops, as different colors trigger specific biological responses. Blue light (400 to 500 nm) is essential for vegetative growth, promoting strong stems, compact structure, and efficient chlorophyll production.
Red light (600 to 700 nm) is highly effective for photosynthesis and is the primary driver for flowering and fruiting. For most crops, the best results are achieved with a balanced spectrum combining red and blue wavelengths, often supplemented with green light to penetrate the canopy. Selecting an adjustable or full spectrum fixture allows growers to fine-tune light quality to match the specific growth stage, such as increasing the red ratio during flowering.
The two dominant grow light technologies are Light-Emitting Diodes (LEDs) and High-Pressure Sodium (HPS) lamps. LEDs have become the modern standard due to their energy efficiency, consuming 50 to 80% less electricity than HPS fixtures for equivalent light output. LEDs also emit significantly less heat, which reduces the load on cooling and ventilation systems, leading to operational cost savings.
HPS lights remain a choice for some due to their lower initial purchase price and high intensity, particularly in the red-orange spectrum beneficial for flowering crops. HPS lamps have a shorter lifespan (around 10,000 hours), and their substantial heat output requires them to be placed farther away from the plant canopy. LEDs, with lifespans often exceeding 50,000 hours, can be placed closer to the plants due to passive cooling, which increases the amount of usable light reaching the foliage.
The overall intensity and placement of the chosen lights must be calculated to achieve the target DLI without causing plant stress. Too little light leads to weak growth, while excessive light intensity can cause leaf burn, known as photoinhibition. Growers must use light meters to measure the PPFD at the canopy level. This ensures fixtures are positioned to provide uniform light distribution, preventing hot spots and ensuring every plant receives the necessary daily photons.