Light is the primary environmental factor controlling plant growth and development within a greenhouse. Plants use photosynthesis, converting light, water, and carbon dioxide into the energy required for growth. The quantity of light a crop receives directly influences overall yield and physical quality, including stem thickness and flower production. Managing the light environment is a fundamental task for growers seeking to optimize plant health and maximize productivity. Meeting a crop’s daily light needs ensures a consistent, high-quality harvest.
Quantifying Greenhouse Light
Growers use specific scientific metrics to quantify the light energy available for plants. Instantaneous light intensity is measured using Photosynthetic Photon Flux Density (PPFD). PPFD counts the number of photons within the photosynthetically active radiation (PAR) range (400–700 nm) striking a square meter per second (\(\mu\text{mol}/\text{m}^2/\text{s}\)). It provides a snapshot of intensity but not the duration of light exposure.
The standard for horticultural light measurement is the Daily Light Integral (DLI). DLI represents the total accumulated amount of PAR photons received by a plant over a 24-hour period. Measured in moles per square meter per day (\(\text{mol}/\text{m}^2/\text{d}\)), DLI combines intensity and duration, correlating strongly with plant growth and yield.
A plant’s growth response is tied to the total energy it receives daily. Establishing a target DLI helps growers determine if natural sunlight is sufficient or if supplemental lighting is required. Monitoring DLI allows for precise control of the light environment, minimizing the risk of poor growth or stress.
Determining Light Requirements by Crop Type
Different plant species thrive under varying light conditions, meaning their ideal DLI targets differ significantly. Horticultural crops are grouped into categories based on their light needs to establish appropriate management strategies. High-light demanding crops require a greater DLI to achieve maximum productivity.
Fruiting vegetables like tomatoes, cucumbers, and peppers are high-light crops, often requiring a DLI between 20 and 30 \(\text{mol}/\text{m}^2/\text{d}\) for optimal yield. Insufficient light leads to reduced fruit size, quality, and poor flower set.
Medium-light crops, including most leafy greens like lettuce, spinach, and herbs, generally require a DLI of 12 to \(17\text{mol}/\text{m}^2/\text{d}\). Low-light crops, such as ornamental plants and foliage, perform best with a DLI between 5 and \(10\text{mol}/\text{m}^2/\text{d}\). Shade-loving plants can suffer from leaf burn or stress if exposed to excessively high light levels.
Maximizing Natural Sunlight
Optimizing natural sunlight begins with the structural design of the greenhouse. In the Northern Hemisphere, orienting the longest side East-West generally maximizes light capture, especially during winter. A North-South orientation provides more even light distribution across crop rows, benefiting tall crops like tomatoes. The best choice depends on the latitude, crop, and primary season of operation.
The choice of glazing material significantly impacts the amount and quality of light entering the structure. Traditional glass offers excellent light transmission, often over 90%, providing maximum direct light for high-light crops. However, direct light can create “hot spots” that may scorch sensitive foliage, and glass offers minimal insulation.
Polycarbonate panels transmit slightly less light (80-85%) but offer the benefit of light diffusion. This scattering helps light penetrate deeper into the canopy and distributes it more evenly, promoting uniform growth. Polyethylene film is the least expensive option, offering comparable light transmission and good light diffusion properties.
When natural light is excessive, measures must be taken to prevent plant stress. Shading compounds applied to the exterior glazing reflect excess sunlight and can be washed off seasonally. Internal shade cloths or screens can also be deployed mechanically to reduce light intensity, preventing sunburn and managing temperature.
Strategies for Supplemental Lighting
Supplemental lighting is necessary when the natural DLI inside the greenhouse falls short of the crop’s target DLI. The required supplemental DLI is calculated by finding the difference between the crop’s optimal DLI and the actual sunlight DLI received. This “light deficit” determines the energy supplemental fixtures must provide over the photoperiod.
Growers must select the appropriate lighting technology and determine the necessary PPFD intensity and duration. High-Pressure Sodium (HPS) lights have historically been standard due to their high output and lower initial cost. HPS fixtures emit a broad spectrum weighted toward the yellow-red end, effective for flowering and fruiting crops.
Light-Emitting Diodes (LEDs) are favored because they are more energy-efficient, using less energy than HPS fixtures for the same light output. LEDs produce less radiant heat, reducing cooling costs and allowing fixtures to be placed closer to the canopy. LED systems also allow customization of the light spectrum to manipulate plant development.
The photoperiod, or duration the lights are on, is determined by dividing the supplemental DLI requirement by the fixtures’ PPFD intensity. For example, a high-light crop might receive a supplemental PPFD of 200 \(\mu\text{mol}/\text{m}^2/\text{s}\) for 16 hours to meet its DLI goal. Careful fixture placement ensures uniform light distribution across the canopy, preventing uneven growth.