Summary of Controlled Environment Agriculture: Glasshouses and Polytunnels

## Introduction Protected crop production means growing plants in structures that give more control over the growing environment than open fields. These structures help increase yields, extend growing seasons and protect crops from weather and pests. Examples include glasshouses and polytunnels, but this material will focus on general principles of protected cropping rather than specific details about polytunnels or plant water loss. ## Why protect crops? - Extend the growing season so warm-season vegetables can be grown earlier and later in the year - Improve yield and quality by creating optimal temperature and light conditions - Reduce damage from frost, wind, hail and heavy rain - Help manage pests and diseases more effectively > Definition: Protected crop production — Growing plants inside a structure or under a cover to control environmental conditions such as temperature, light and humidity. ## Main environmental factors managed in protected crops ### Light - Light intensity and quality affect photosynthesis and plant growth. (See Section 2.1 for more on how light intensity affects photosynthesis.) - Growers can control light by using glazing materials, shade cloth, or artificial lighting. ### Temperature - Temperature controls metabolic rates and enzyme activity in plants. (See Section 1.4 for enzyme–temperature relationships.) - Protected structures trap heat and allow temperature control using heaters, vents, or thermal screens. > Definition: Microclimate — The local climate inside a protected structure that can differ substantially from outside conditions. ### Humidity and ventilation - Humidity affects transpiration and disease risk. Good ventilation balances humidity and CO2 supply without drying plants excessively. - Fans, vents and controlled opening systems are used to manage air movement. ### CO2 concentration - Higher CO2 can increase photosynthesis for many crops. Some glasshouses enrich CO2 to boost growth. - CO2 enrichment must be controlled to avoid waste and ensure safety. ### Pest and disease management - Physical barriers and controlled entry reduce pest introduction. - Integrated pest management (IPM) combines biological controls, monitoring and targeted treatments. ## Structure types and features (comparison) | Feature | Glasshouse | Polytunnel | Shade house | |---|---:|---:|---:| | Light transmission | High | High to medium | Low to medium | | Cost | High | Medium | Low | | Temperature control | Precise | Moderate | Limited | | Typical use | High-value, year-round crops | Seasonal production | Shade-loving crops | ### Practical example: Growing lettuce in a glasshouse - Maintain daytime temperature range suited to lettuce (cool-season crop): keep temperatures lower than for tomatoes - Provide uniform light: use diffused glazing or supplemental LED light on low-light days - Control humidity and ventilation to avoid fungal diseases - Use biological controls (e.g., predatory mites) to reduce chemical pesticide use Fun fact: Protected cropping dates back to Roman times when growers used frames and covers to protect plants from cold weather and extend the growing season. ## Managing energy and costs - Heating is the largest energy cost in cool climates; growers balance heat with crop value - Thermal screens and heat storage (water tanks, phase-change materials) reduce heating needs - LED lighting is energy-efficient for supplementing light but must be costed against expected yield gains ## Cultural practices inside protected environments - Spacing and pruning: plants are sited to optimise light use and airflow - Training systems: trellises and support wires save space and increase yield per area - Fertiliser management: often delivered via fertigation (nutrient solution through irrigation) for precise control > Definition: Fertigation — Applying soluble fertilisers through the irrigation system to deliver nutrients directly to plant roots. ## Environmental and sustainability considerations