How to Control Temperature in a Greenhouse? | Keep Plants in the Safe Zone

Getting the temperature right in a greenhouse is a balancing act between keeping plants warm enough to thrive and cool enough to avoid heat stress. The most consistent guidance from extension services is straightforward: measure the temperature accurately first, then ventilate and shade to cool things down, and insulate and heat to warm them up. Warming the air to a comfortable level for you, rather than the plants, is one of the easiest ways to throw the whole system off.

What Temperature Should a Greenhouse Be?

The ideal temperature depends entirely on what you are growing, but general guidelines provide a useful starting point.

During the winter, the goal shifts from cooling to frost protection. A common recommendation for frost-sensitive crops is to set heaters to a modest 37–41°F (3–5°C)—just enough to keep plants alive, not to make the space comfortable.

Because no single setpoint suits all plants, it helps to know your crop’s specific tolerance. A good rule of thumb is to research the ideal range for your specific plants and use that as your target rather than a universal number.

How to Cool a Greenhouse in Summer

Cooling a greenhouse relies on moving hot air out and keeping solar radiation from turning the space into an oven. Here are the most effective methods, starting with the most essential:

• Ventilation: Roof vents, louvre vents, and open doors are your first line of defense. Cross-ventilation, using low-level inlets and high-level outlets, dramatically improves airflow.
• Automatic vent openers: These mechanical devices open roof vents when temperatures rise, which is a lifesaver when you are away. Most models require no wiring and can be fitted in under an hour.
• Air circulation: Horizontal airflow (HAF) fans or exhaust fans with an intake pull heated air out and replace it with cooler outside air, eliminating hot spots.
• Shading: Shade cloths or shade paint reduce incoming solar radiation. Shade cloths are typically designed for a 30% to 50% light reduction—enough to cool things down while still allowing for photosynthesis. Avoid excessive shading, which can reduce the light your plants need to grow.
• Evaporative cooling: Misting or evaporative cooling pads lower temperatures by evaporation, but they increase humidity. This is most effective when outside air is dry; in humid climates, it can worsen disease pressure.

If you are dealing with intense summer heat and need a more permanent solution, a well-designed cooling setup is worth the investment. For a look at tested products, see our roundup of the best small greenhouse temperature control options for summer.

How to Keep a Greenhouse Warm in Winter

Keeping the cold out is a matter of insulation, sealing drafts, and supplementing heat only when necessary. The following methods will help you protect tender plants through the colder months:

• Seal drafts: Check for gaps around doors, vents, and the structure itself. Sealing these is the cheapest and most effective way to retain heat.
• Insulate: Use bubble wrap or thermal screens on the interior glazing to reduce heat loss. Thermal screens or curtains also help by trapping a layer of air between the screen and the roof.
• Add thermal mass: Water barrels, bricks, or stone absorb heat during the day and release it slowly at night, moderating temperature swings. This is a passive, low-cost method of evening out the extremes.
• Heating: For frost protection, use a thermostat-controlled heater set to your crop’s minimum threshold. Heating a greenhouse to human comfort levels is a wasteful mistake that can also stress plants.

Regularly monitoring with a maximum-minimum thermometer placed at plant height, in the center of the greenhouse and away from glass and heaters, will tell you how well your system is actually working.

FAQs

Why is my greenhouse getting too hot even with a fan?

An undersized fan or a lack of proper intake vents limits airflow. Ventilation works by replacing hot internal air with cooler outside air; without a low-level inlet, the fan simply recirculates the hot air. Adding roof or louvre vents on the opposite side will improve cross-ventilation and make your existing fan far more effective.

Can I use a regular household heater in a greenhouse?

No. Greenhouse environments are wet and humid, and electrical equipment must be rated for these conditions to prevent shocks or fires. You should also be aware that some heaters produce combustion gases that are harmful to plants. Always use a heater designed specifically for greenhouse use and follow the manufacturer’s safety specifications.

How does shade cloth affect plant growth?

Shade cloth reduces light intensity, which can slow photosynthesis if used too aggressively. While it is an effective cooling tool, you should balance cooling with light transmission. Using cloth with a light reduction of 30% to 50% is a common compromise, but you should observe your plants for signs of stretching or reduced vigor, which indicate they are not getting enough light.