Farm Operations Management
Temperature Management in Vertical Farms: Physiological Fundamentals and Practical Control
Articles for Farm Operations Managers
Temperature quietly shapes the yield and quality of a vertical farm. Even with the same cultivar, the same LEDs, and the same nutrient solution, a difference of just a few degrees changes growth rate and disease risk.
What makes it difficult is that temperature inside a vertical farm is not uniform. Heat from LEDs, lighting ON/OFF cycles, top-to-bottom differences in multi-tier cultivation, air-conditioning outlets, and condensation all overlap, and the averaged value from a sensor alone cannot capture what is really happening on the floor.
In this article, I organize the meaning of temperature from a plant-physiology standpoint, the temperature unevenness that arises in a closed space, the thinking behind air conditioning, airflow, and insulation, and the signs from the crop that you should be watching in day-to-day management.
The Deep Relationship Between Plants and Temperature
Inside the plant body, thousands of chemical reactions are constantly in progress: photosynthesis, respiration, protein synthesis, hormone production, and many more. Most of these reactions are regulated by enzymes (biological catalysts), and enzyme activity depends heavily on temperature. The enzymes that work inside a plant function most efficiently within a specific temperature range, and a 10 °C rise roughly doubles the rate of an enzymatic reaction. Within the optimum range, reactions run faster at the higher end, but once the upper limit is exceeded, function is lost.
When the temperature is too low, enzyme activity drops sharply and metabolic reactions slow down. A decline in the fluidity of cell membranes hinders the transport of substances, the absorption efficiency of water and nutrients falls, and the translocation of photosynthetic products is suppressed, all in a chain reaction. Conversely, when it is too high, proteins denature and enzymes stop working, the respiration rate exceeds the photosynthesis rate, and energy consumption becomes excessive. The permeability of cell membranes rises excessively, the ion balance breaks down, and damage from an increase in reactive oxygen species accumulates in the cells.
How Temperature Affects Yield and Quality
Because temperature changes the metabolic rate of a plant, its impact on growth rate is direct. A deviation of just 2 °C from the optimum can change the days to harvest by more than 10%. Take lettuce as an example: at the optimum temperature (around 20 °C), it reaches harvest size in about 35 days, but at 17 °C this stretches to about 40 days (roughly 14% longer), and at 23 °C it drops to about 32 days (roughly 9% shorter). Changes in days to harvest translate directly into the number of production cycles per year, which has a large effect on the productivity of the farm as a whole. A deviation from the optimum temperature range can reduce harvest by up to 30% from the same cultivation area.
Impact on Quality
Plants that suffer temperature stress become worse in both appearance and taste. Typical symptoms include morphological abnormalities such as leaf shrinkage, curling, and deformation; discoloration such as yellowing, reddening, and browning; an increase in bitterness caused by shifts in the balance of secondary metabolites; and a decrease in vitamins and functional components. Changes in appearance in particular are directly tied to commercial value, so in a vertical farm that competes on high added value they cannot be taken lightly.
Increased Disease Risk
Poor temperature management is also directly linked to disease risk. Temperature stress lowers disease resistance, condensation at low temperatures becomes an entry point for pathogens, and a hot, humid environment accelerates pathogen growth. Secondary infections also become more common when a physiological disorder makes it even easier for pathogens to enter. Because poor growth, quality deterioration, and disease risk chain together, a mistake in temperature management spreads further than you might expect.
Understanding the Characteristics of a Vertical Farm from a Temperature Standpoint
Because a Plant Factory with Artificial Lighting (PFAL) is a closed space isolated from the outside environment, it develops its own unique temperature environment. Understanding this is the starting point for effective management.
Lighting Equipment Generates a Large Amount of Heat
LED lighting is called energy efficient, but even the latest LEDs release about 40% of the power they consume as heat. Under high light intensity, a temperature rise of 2 to 5 °C has been observed directly under the lights, and this local heat generation is one of the main causes of temperature unevenness.
Temperature Fluctuates Easily in a Closed Space
Temperature changes sharply when the lights go on and off. The fluctuation is especially pronounced at lights-on in the morning and lights-off in the evening. Transpiration from the plants also changes humidity and temperature, and the effect grows as leaf area increases.
Top-to-Bottom Temperature Differences in Multi-Tier Cultivation
In a multi-tier grow rack layout, heat naturally accumulates upward. A difference of up to 3 °C can arise between the top and bottom tiers, and the absence of air movement is part of what sustains this gap. The temperature difference between tiers appears directly as a difference in growth rate.
Temperature Management Tailored to Vertical Farm Characteristics
Insulation and Heat-Shielding to Curb Temperature Fluctuation
Keeping fluctuation inside a closed space to a minimum starts with properly controlling the flow of heat in and out. The main measures are strengthening the exterior walls with high-performance insulation aimed at a thermal transmittance (U-value) of 0.25 W/(m²·K) or lower, suppressing inflow and outflow of air at openings with air curtains or double-door structures, and thorough insulation of piping and ducts. Heat losses from places that are easy to overlook add up and cannot be ignored.
Designing and Placing the HVAC System Strategically
Dealing with the temperature distribution unique to a vertical farm requires careful HVAC design. Capacity should be decided based on an accurate calculation of heat load that includes lighting, other heat sources, and plant transpiration, and air conditioners and outlet vents should be placed so that cool air reaches directly under the lights. Airflow simulation is useful here. Dividing the room into zones by growth stage and designing to minimize thermal interference between zones also contributes to stable yield.
Eliminating Temperature Unevenness with Airflow Design
Circulating air at a wind speed of 0.3 to 0.7 m/s is a good target for eliminating temperature unevenness while keeping plant stress low. Supplementary fans in corners and at the back of shelves, duct design that encourages vertical air circulation, and ensuring airflow at canopy level are all ways to remove blind spots.
Practical Points of Temperature Management on the Floor
It is important to treat recommended temperatures as nothing more than a guide. The optimum temperature differs even within the same crop depending on the cultivar, and it also shifts in combination with light intensity, CO2 concentration, and humidity. For example, in a cultivation zone with high light intensity, heat generation from photosynthesis also increases, so you may need to adjust by setting the target temperature 1 to 2 °C below the recommended value.
Responding to the characteristics of each cultivation location is also essential. The upper tiers of a multi-tier rack tend to run 1 to 3 °C hotter, so either increase the volume of cool air reaching the top tier or lower the temperature setpoint on that side. Near air-conditioning outlets, watch out for localized chilling injury where cool air hits the plants directly. On days with extreme outdoor temperatures, it may also be necessary to adjust the setpoint and operating hours to ease the load on the HVAC equipment.
Management Based on Observing the Crop, Not Just the Data
The most important thing in temperature management is reading the signs the plants themselves show, not just the sensor readings.
For leaf color and shape: a deep blue-green suggests possible cold stress (especially in lettuce), yellowing or a reddish-purple tint is a sign of chilling injury (clearest in young leaves), browning along the leaf margin is an early symptom of heat injury, and leaf curling can be a sign of water stress caused by excessive transpiration under high temperatures.
For stem condition, the check points are legginess (the classic symptom of high temperature combined with weak light), widened internode distance (a possible sign that temperature is too high), and insufficient hardening of the stem (which appears when nighttime temperatures are too high).
For root condition, browned roots point to excessive root respiration or oxygen shortage under high temperatures, and stalled root elongation indicates reduced metabolic activity under low temperatures.
Preventing and Dealing with Condensation
Condensation tends to be overlooked, but if you leave it alone the impact grows. The risk of Botrytis (gray mold) and similar diseases rises, and photosynthesis is blocked until the condensation evaporates. There are also cases where water droplets act as a lens and cause leaf burn.
As concrete preventive measures, it is effective to ventilate appropriately (especially just before lights-off in the morning and just after lights-on) to keep humidity at 70% or below, to prevent excessive transpiration by keeping plant density appropriate, and to use dehumidifiers as needed. Ramping the lights on and off gradually over 30 minutes to avoid abrupt temperature changes is also effective, and the transitions between seasons require particular care. Running fans 24 hours a day keeps air from stagnating, and placing them so that airflow reaches the leaf surfaces directly helps prevent condensation. A constant low wind speed of 0.3 to 0.5 m/s is a good guide.
Stable temperature affects growth rate, quality, and disease risk all at once. Management that combines both sensor data and crop observation is what leads to stable yield and stable quality.