The Climate Challenge in Poland's Growing Seasons
Poland's temperate continental climate produces wide seasonal temperature swings. Mean July temperatures in the central lowlands reach 18–20 °C, with daytime peaks under glass or foil regularly exceeding 35 °C without active cooling. Relative humidity rises sharply on calm, overcast days, creating conditions favourable to fungal diseases such as Botrytis cinerea and powdery mildew on susceptible crops.
In contrast, November through February brings short days, weak sunlight and external temperatures that can fall below −15 °C in the northeastern regions. During these months, any ventilation opening represents a direct heat loss, and growers must balance the need for CO₂ replenishment and humidity reduction against the cost of heating.
Natural Ventilation: Ridge Vents and Side Openings
The simplest and most widely used cooling mechanism in Polish greenhouse production is natural ventilation driven by the stack effect: warm air rises and exits through ridge openings, drawing cooler outside air through lower side openings. The efficiency of this system depends on the height of the structure, the area ratio of ridge to side openings, and the temperature difference between inside and outside air.
On multi-span glass or foil structures, ridge vents typically cover 15–20% of the floor area in well-designed installations. Continuous ridge vents running the full length of each bay provide more uniform air distribution than discrete window units. In older single-span structures with hinged glass panels, the vent area is often below 10%, which limits cooling capacity on still summer days.
Roll-Up Side Walls on Foil Tunnels
Many medium-scale foil tunnel structures in Poland use roll-up side walls — sections of the foil skirt that can be wound up mechanically or manually to expose a screened opening along the full side of the structure. When both ridge and side walls are open simultaneously on a multi-span tunnel, air exchange rates sufficient for summer cooling are achievable even without mechanical fans, provided there is some external wind. Insect screens fitted over the openings reduce pest ingress but reduce ventilation capacity by roughly 30–40%, depending on mesh density.
Mechanical Ventilation
On structures without sufficient natural ventilation — typically those with very low ridge-to-floor ratios, or those oriented along the prevailing wind direction with no effective cross-ventilation — exhaust fans mounted at one gable end draw air through the length of the tunnel. Inlet openings on the opposite gable control the direction of airflow. This approach is common on longer tunnel structures exceeding 80–100 m in length where natural stack ventilation alone proves insufficient in peak summer conditions.
Evaporative cooling pads combined with exhaust fans (pad-and-fan systems) are used on a smaller number of intensive operations, particularly those growing high-value crops such as sweet pepper. These systems can reduce air temperature by 5–8 °C in low-humidity conditions but are less effective during Poland's humid summer weather fronts.
Shading Systems
Direct solar radiation entering through glass or clear foil is the primary cause of heat build-up above the crop canopy. Shading reduces the incident radiation reaching the crop, lowering leaf and air temperature but also reducing photosynthesis. The management challenge is to shade enough to prevent heat stress without reducing yield-limiting light levels.
External Shade Nets
External shade nets installed above the roof of the structure intercept solar radiation before it enters the covering and converts to heat inside. They are significantly more effective than internal shading at reducing the greenhouse air temperature because the intercepted radiation is dissipated outside. Aluminium-thread nets with a shading factor of 40–60% are common on pepper and cucumber structures during June and July in Poland. The nets are typically deployed on a rail system and can be retracted on overcast days or during the September period when light becomes limiting again.
Internal Thermal and Energy Screens
Internal horizontal screens serve a dual function: they shade the crop from above during high-radiation periods in summer, and they form a thermal barrier between the plant zone and the cold glass or foil roof surface during winter nights. Aluminised fabrics reflect a portion of long-wave radiation back towards the crop and can reduce nighttime heating costs.
On commercial foil Venlo-type structures in Poland, energy screens are considered standard equipment for year-round operations. They are driven by motorised systems on overhead rails and can be opened and closed in a few minutes. The typical practice during clear winter nights is to close the screen above the crop at sunset and open it again after sunrise to restore CO₂ exchange with the upper greenhouse volume.
The ventilation and shading equipment on a commercial greenhouse in Poland can represent 15–25% of the total structure investment cost. Automated climate control systems — which adjust vent position, screen position and heating output in response to sensors measuring temperature, humidity and radiation — are standard on glass Venlo operations and increasingly common on large foil structures.
Humidity Management
High relative humidity inside the greenhouse — typically above 85–90% — promotes the germination and spread of fungal pathogens and can cause condensation on fruit surfaces. Humidity is managed through a combination of ventilation (to exchange moist air with drier outside air), temperature management (warmer air holds more moisture), and irrigation scheduling (avoiding large water volumes in the evening or on overcast, windless days).
In winter, when ventilation is restricted to conserve heat, growers sometimes use brief periods of "dehumidification ventilation" — opening vents slightly for 10–15 minutes before dawn when outside air, despite being cold, is relatively dry in absolute humidity terms.
CO₂ Enrichment and Ventilation Timing
On glass and high-specification foil structures with CO₂ enrichment systems, ventilation timing becomes a cost management issue as well as a climate one. Opening vents releases the enriched CO₂ atmosphere built up during the early morning hours. Automated systems balance the need to maintain temperature setpoints against the economic loss of venting enriched air. In Polish commercial tomato production, CO₂ enrichment is typically maintained up to the point at which outdoor temperature justifies ventilation, after which no additional CO₂ is supplied until vents close again in the afternoon.
