Managing light is the primary factor determining plant health and productivity in a controlled greenhouse environment. When considering how much “sun” a greenhouse needs, the focus must shift from simple brightness to measurable energy that fuels photosynthesis. This usable energy is quantified by specific horticultural metrics, allowing growers to treat light as an actively managed resource. The goal is to optimize the total quantity of light received over a day, ensuring plants get the energy they require for optimal growth.
Measuring Light Requirements
The standard measurement for usable light is Photosynthetically Active Radiation (PAR). PAR defines the spectral range (400 to 700 nanometers) that green foliage absorbs to convert carbon dioxide and water into energy. Light intensity is measured in Photosynthetic Photon Flux Density (PPFD), but this momentary reading is insufficient for long-term planning.
The most meaningful metric for greenhouse management is the Daily Light Integral (DLI), which integrates the PPFD over a full 24-hour period. DLI is expressed in moles of light per square meter per day (mol·m⁻²·d⁻¹), representing the total usable light energy delivered to the plant canopy. Different plant groups have distinct DLI needs that guide management decisions throughout the year.
Low-light foliage plants and young cuttings, for example, thrive with a relatively modest DLI between 4 and 8 mol·m⁻²·d⁻¹. Medium-light crops, such as lettuce, herbs, and flowering annuals, generally require a DLI in the range of 12 to 18 mol·m⁻²·d⁻¹. High-light fruiting vegetables, including tomatoes, peppers, and cucumbers, need a significantly higher DLI, often requiring 20 to 30 mol·m⁻²·d⁻¹ for maximum yield and quality.
Factors Influencing Natural Light Availability
The amount of natural light that reaches the plants is heavily influenced by the greenhouse’s location and structure. Geographical latitude and the season profoundly affect light intensity and day length, causing the outdoor DLI to fluctuate drastically from summer to winter. Cloud cover also plays a major role, creating unpredictable dips in available light that must be accounted for.
The orientation of the greenhouse structure impacts light transmission. In northern latitudes, a North-South alignment maximizes winter light exposure and provides uniform distribution. Conversely, an East-West orientation is beneficial in hot climates, mitigating intense midday summer sun, though it can result in uneven light distribution.
The glazing material covering the greenhouse also dictates how much light successfully penetrates the structure. Single-pane glass offers high light transmission (90-95%) but provides minimal insulation. Multi-wall polycarbonate transmits slightly less light (70-88%) but diffuses it, distributing light more evenly across the canopy. Polyethylene film is a cost-effective option (80-90% transmission), though its rate tends to degrade faster over time due to UV exposure.
Managing Light Deficiencies
When natural light is insufficient to meet the crop’s DLI requirements, supplemental lighting becomes necessary, particularly during winter or periods of heavy cloud cover. The goal is to supply the precise number of photons needed to achieve the target DLI. This is calculated by determining the difference between the target DLI and the natural DLI, then calculating the required intensity and duration of the supplemental light.
Light Emitting Diode (LED) fixtures are the preferred option due to superior energy efficiency, producing more PPFD per watt than traditional High-Pressure Sodium (HPS) lamps. LEDs also offer precise spectral control, allowing growers to tune the light spectrum with specific red and blue light ratios to influence plant morphology and growth phases. Although the initial cost is higher than HPS, operational savings from lower energy consumption and a significantly longer lifespan (50,000+ hours) justify the investment.
In contrast, HPS lights are a lower-cost entry point but generate a significant amount of heat in addition to light. This waste heat can be advantageous in cold climates as a supplemental heat source, but it increases the burden on cooling systems in moderate or warm weather. HPS lights also have a fixed, non-customizable spectrum and a shorter lifespan, requiring more frequent bulb replacements.
Managing Light Excess (Shading and Cooling)
Just as insufficient light can hinder growth, excessive light intensity can cause plant stress and damage. High light levels frequently coincide with high temperatures, leading to a risk of overheating and a condition known as sunscald, where foliage is bleached or burned. Managing light excess is therefore intrinsically linked to managing the greenhouse’s internal temperature.
External shade cloths are a common and effective tool, measured by the percentage of light they block, such as 30%, 50%, or 70% shade. A 30% shade cloth, for example, allows 70% of the light to pass through, providing a minimal reduction suitable for light-loving plants during peak summer sun. Choosing the appropriate percentage is based on the crop’s tolerance and the local climate’s intensity, with higher percentages reserved for heat-sensitive or shade-loving plants.
Another method involves applying liquid shading compounds, which are specialized coatings sprayed onto the exterior glazing. These compounds, often chalk-based, reflect excess solar radiation and heat while still transmitting photosynthetically active light. Some advanced coatings are photo-selective, blocking infrared radiation (heat) more effectively than PAR light, and they can be easily washed off at the end of the season. Reducing the solar load lessens the demand on mechanical ventilation and fan systems, helping to maintain a manageable temperature and prevent crop damage.