Environmental Controls: Temperature, Humidity, Ventilation, and CO2

Why the Greenhouse Environment Matters

Plants do not grow in isolation. Every process — photosynthesis, transpiration, respiration, cell elongation — responds to the air around the plant. As a greenhouse technician you control four variables that together determine crop quality: temperature (and its day-to-night pattern), relative humidity, air circulation, and carbon dioxide (CO₂) concentration. Learning to read and adjust an environmental controller is the hands-on skill that ties all four together.

Temperature and DIF

Greenhouse growers distinguish between the day temperature (DT) and the night temperature (NT). The difference between them is called the DIF (differential). When DT is higher than NT the DIF is positive; when NT is higher than DT the DIF is negative.

DIF directly affects stem elongation. Under a positive DIF (+10 °F, for example), the plant hormone system promotes internode stretching — internodes are the stem segments between leaves. Under a negative DIF the opposite occurs: internodes remain compact. Commercial growers sometimes use a brief period of cooler temperatures just before and just after sunrise (a "cold pulse") to shorten internodes without chemical growth regulators. The DIF concept applies broadly across bedding plants, potted flowering crops, and vegetable transplants.

For your measurements today: measure internode length with a ruler from the base of one leaf to the base of the next. Average at least five internodes per flat and compare the two DIF treatments.

Relative Humidity and Vapor Pressure Deficit

Relative humidity (RH) expresses how much water vapor the air holds compared to the maximum it could hold at that temperature, reported as a percentage. Air at 80 °F can hold more water vapor than air at 60 °F, so the same absolute amount of moisture produces a lower RH reading in warmer air.

A more precise measure for plant science is vapor pressure deficit (VPD) — the difference between the amount of water vapor in the air and the maximum the air could hold at that temperature. A high VPD means dry air that pulls water out of leaves aggressively, increasing plant stress and wilting risk. A very low VPD means the air is nearly saturated; water does not evaporate from leaf surfaces efficiently, leaves stay wet, and fungal pathogens thrive.

Botrytis cinerea (gray mold) is the pathogen most commonly associated with high-humidity conditions in greenhouses. It sporulates and infects when free moisture sits on plant tissue. Keeping VPD in a moderate range — generally achieved by maintaining RH between roughly 50 and 80 percent depending on crop and temperature — reduces infection risk. Timed ventilation at dusk and dawn, when outside temperatures drop and RH spikes, is a standard cultural practice to exhaust humid air before condensation forms.

Ventilation and Air Circulation

Greenhouse ventilation has two purposes: temperature management and humidity control. Natural ventilation uses roof vents and side walls; mechanical ventilation uses exhaust fans paired with evaporative cooling pads on the opposite wall — the pad-and-fan system. In cold weather, heating and minimal ventilation preserve warmth; in warm weather, fans pull air across the wet pads to cool and dehumidify.

Air circulation fans (horizontal airflow or HAF fans) are different from exhaust fans. They recirculate air inside the greenhouse without exchanging it with outside air. This breaks up stagnant boundary layers around leaves, which strengthens stems, improves gas exchange at the leaf surface, and reduces localized humidity pockets where disease can establish.

CO₂ and Photosynthesis

Outdoor air contains approximately 420 parts per million (ppm) CO₂. Inside a sealed greenhouse during peak daylight hours, plants can deplete CO₂ to levels that limit photosynthesis. Supplemental CO₂ injection — typically raising concentration to 1,000–1,500 ppm — can increase growth rates and yield in tightly managed crops. CO₂ enrichment is most effective when light levels are high; injecting CO₂ on cloudy days or at night has little benefit. When vents are open for cooling, CO₂ enrichment is generally paused because the enriched air escapes immediately.

Reading and Adjusting the Environmental Controller

Modern greenhouse controllers integrate sensors for temperature, humidity, and sometimes CO₂ and light into a single panel or digital interface. Typical adjustable parameters include:

• Heat set point — the temperature at which the heating system turns on.
• Ventilation (vent) set point — the temperature at which roof vents or exhaust fans open.
• Pad-and-fan set point — the temperature at which the evaporative cooling system engages.
• Humidity set point — the RH level that triggers ventilation or heating to reduce moisture.

To reach an instructor-specified set point, locate each parameter on the controller, enter the target value, and confirm the change is saved. Always allow 10–15 minutes for the greenhouse environment to stabilize before recording your readings, because temperature and humidity sensors respond to real air conditions, not to the number you entered.

Record your readings every 3 minutes on your data sheet. Your goal is to stay within ±2 °F and ±5% RH of the target range across all monitoring periods.

Safety in the Greenhouse

• CO₂ enrichment equipment uses compressed gas or combustion generators; never operate or service these without training. High CO₂ concentrations (above roughly 5,000 ppm) are hazardous to humans. Ensure sensors and alarms are functional before entering a CO₂-enriched zone.
• Electrical panels and controllers must never be serviced while energized. Follow lockout/tagout procedures if any wiring or sensor replacement is required.
• Wet floors near pad-and-fan systems and irrigation lines create slip hazards; wear slip-resistant footwear.
• Chemical growing media and fertilizer solutions require gloves and, when mixing concentrates, eye protection.
• Do not climb on benches or heating infrastructure to reach high vents; use an approved step stool or ladder.