Greenhouse Study Guide

📖 Core Concepts Greenhouse – enclosed structure that lets sunlight in, traps long‑wave infrared, and maintains a warm, humid microclimate for plants. Heat capture – short‑wave solar radiation passes through the covering, is absorbed by floor/plants, then re‑radiates as infrared that the covering (glass or plastic) blocks, raising interior temperature. R‑value – measure of thermal resistance; typical plastic coverings ≈ 2 (°C·m²·W⁻¹). Vapour‑pressure deficit (VPD) – difference between actual air moisture and saturation; controls transpiration and disease risk. CO₂ enrichment – raising atmospheric CO₂ to 1100 ppm can boost photosynthesis when CO₂ is the limiting factor. Supplemental lighting – LEDs provide specific wavelengths and photoperiod extension, especially at night. Photovoltaic‑greenhouse integration – semi‑transparent panels let photosynthetically active radiation (PAR) through while generating electricity on the same footprint. 📌 Must Remember Sunlight → short‑wave IR → absorbed → long‑wave IR trapped = greenhouse warming. Typical covering R‑value ≈ 2 → low insulation → heating is a major cost. Optimal CO₂ level for most crops ≈ 1100 ppm; higher levels only help if CO₂ is limiting. LED wavelengths: blue (≈ 450 nm) for vegetative growth, red (≈ 660 nm) for flowering/fruit set. Semi‑transparent PV panels transmit 60 % of PAR; spectrum‑separating systems send 70 % to plants and >30 % to PV cells. Ventilation (natural or fan‑assisted) is required to keep VPD in the range 0.8–1.2 kPa for most vegetables. 🔄 Key Processes Heat Capture & Retention Sunlight → transparent covering → floor/plant absorption → long‑wave IR emission → covering reflects IR → interior warms. Ventilation Control Sensors detect temperature/VPD → controller opens vents or runs fans → excess heat/moisture expelled → fresh CO₂ in. Heating Cycle (active) When interior < set point → furnace (natural gas/electric) fires → warm air circulated → maintain set temperature. Cooling Cycle When interior > set point → open windows/shade house deployed → evaporative cooling or exhaust fans reduce temperature. CO₂ Enrichment CO₂ source (compressed gas or on‑site generation) released → mixed by circulation fans → concentration monitored to stay 1100 ppm. LED Supplemental Lighting Timer/controller sets photoperiod → LEDs emit targeted spectra → photosynthesis continues during dark periods. 🔍 Key Comparisons Glass vs. Polycarbonate vs. Polyethylene film Glass: high light transmission, high cost, heavy, high R‑value (better insulation). Polycarbonate: good impact resistance, moderate light transmission, low R‑value (2). Polyethylene film: cheapest, flexible, lowest R‑value, shorter lifespan. Passive vs. Active Heating Passive: solar heat storage, waste heat, geothermal – lower operating cost, dependent on climate. Active: gas/electric furnaces – reliable temperature control, higher energy bills. Shade house vs. Open greenhouse Shade house: reduced solar intensity, cooler microclimate, suited for shade‑tolerant crops. Open greenhouse: maximizes light, higher temperature, best for high‑light crops. ⚠️ Common Misunderstandings “More CO₂ always equals higher yield.” – Yield only rises when CO₂ is the limiting factor; other factors (nutrients, light, water) must also be adequate. “Plastic covers provide good insulation.” – Their R‑value (2) is low; expect higher heating costs than glass structures. “LEDs replace sunlight completely.” – LEDs supplement; they cannot fully replace the full spectrum and intensity of natural sunlight for most crops. 🧠 Mental Models / Intuition “Thermal blanket” model – Think of the covering as a blanket that lets heat in but keeps it from escaping, just like a sleeping bag. “Ventilation as breathing” – Plants “inhale” CO₂ and “exhale” water vapor; opening vents is like taking a deep breath to bring fresh air in and let excess moisture out. 🚩 Exceptions & Edge Cases High‑latitude winter – Sun angle low; supplemental lighting becomes essential despite good insulation. Extreme heatwaves – Even shade houses may overheat; evaporative cooling or misting may be required. Water‑logged soils – Excess humidity can raise VPD, encouraging Botrytis; need precise irrigation control (e.g., drip vs. sprinkler). 📍 When to Use Which Covering material: choose glass for high‑value, long‑season crops needing stable temps; polycarbonate for medium‑value, impact‑prone sites; polyethylene film for seasonal, low‑cost production. Heating method: use passive solar storage when climate provides sufficient winter sun; switch to active gas/electric furnace for consistently cold periods. Cooling method: open vents + shade cloth for moderate heat; add evaporative misting or forced‑air fans for extreme temperatures. Lighting: LEDs for night extension or low‑light seasons; supplemental HPS only if budget permits and spectrum isn’t critical. 👀 Patterns to Recognize Rising temperature + stagnant humidity → likely pathogen risk (Botrytis). Low VPD + high leaf temperature → water stress despite ample moisture. Drop in photosynthetic rate despite adequate light → CO₂ limitation (consider enrichment). 🗂️ Exam Traps “Glass always has a higher R‑value than plastic.” – While glass transmits more light, its R‑value can be comparable or lower if single‑pane; multi‑wall polycarbonate may outperform single glass. “CO₂ enrichment eliminates the need for ventilation.” – Enrichment raises CO₂ but also increases VPD; ventilation is still required to control humidity and temperature. “LEDs automatically improve yields.” – Yield gains depend on matching spectrum and intensity to crop stage; wrong wavelength or over‑intensity can cause photoinhibition. “Passive heating alone can keep a greenhouse warm in all climates.” – In high‑latitude or very cold regions, passive storage may be insufficient; active heating is still needed.