Optimizing Climate in Your Greenhouse

Greenhouses are an excellent way to extend your growing season. They allow you to enjoy a warm oasis and eat fresh vegetables throughout the winter. However, if your greenhouse’s climate is not dialed in, you’ll have nothing more than a hothouse full of shriveled plants with moisture dripping from the walls. When growing in a greenhouse, you take on Mother Nature’s job of creating the correct temperature, humidity, and air flow. Optimizing the climate in your greenhouse is essential, and there are many options and techniques to help you create this ideal environment.

Grow Room Temperature

Greenhouses capture heat efficiently, even in the cold winter months. Your goal is to keep the temperature in the proper range for growing. In the summer, you’ll want to maintain your daytime temperature at about 75-85˚F and nighttime temperature at about 60-75˚F. In the winter, keep temperatures between 60-70˚F during the day and as low as 45-50˚F at night. The exact temperatures you maintain depends on the crops you grow and their preferences.

Many factors influence the temperature in your greenhouse, including light, outside temperatures, and air flow. Four methods commonly used for cooling your greenhouse are shading, evaporative cooling, fogging systems, and thermal mass reservoirs.

While shading is effective at reducing temperature, it also reduces the amount of light reaching your plants by 50-60 percent, which can inhibit growth rates. Sometimes, however, this trade-off is necessary to ensure your plants’ survival. To create shading, you can purchase shade cloth, paint-on materials, or adhesive compounds that can be applied directly to the glass for a more permanent solution. There are also roll-up screens made of wood, aluminum, vinyl, or plastic that come with pulley and rope systems to allow for adjustment based on light and outside temperature. You can grow trees and plants next to the greenhouse as a source of shade too. You can even grow trees or trellis grapes inside your greenhouse to supply shade to other plants. The large size of the grape leaves have the additional benefit of acting as a natural evaporative cooling system.

Evaporative cooling systems, also known as swamp coolers, turn hot air into a cool breeze by passing air through a soaked screen, pad, or spray of water. These systems are most effective in environments with low humidity as they add moisture to the air.

Fogging and misting systems are also often used to cool greenhouses. These two systems are similar in that they both spray water to cool the air. The main difference being the size of the water droplets. Foggers create a very fine mist, while misters spray larger water droplets. These systems can be put on a controller to spray water when temperatures get too high. While effective, the main drawbacks to these types of systems are excess humidity causing fungal issues within the greenhouse and clogged emitters from water with high mineral content.

You can also control temperature by keeping a black reservoir filled with water in your greenhouse. During the day, the water will absorb heat. When it cools down at night, heat is released and warms up the greenhouse. This is the principle of thermal mass at work. You can even put your tank to good use and grow fish while you’re at it. As a rule of thumb, fill your reservoir with two to three gallons of water per square foot of greenhouse space. Finally, if you need to increase temperatures in the winter, you can use heaters or even wood stoves.

(More on indoor grow room temperatures: The Best Temperatures for an Indoor Grow Room.)

Greenhouse Ventilation and Air Flow

Air flow is essential for temperature control and fresh air exchange. Without fresh air, there will not be enough carbon dioxide for photosynthesis unless you install a CO2 generator. Even with a generator, you’ll need fans to move air within the greenhouse to help prevent fungal disease.

Vents, exhaust fans, and oscillating fans can be used for fresh air exchange and circulation. You can also install roof vents that need to be opened and closed throughout the day depending on the temperature. Roof vents are often operated by hand, but you can automate them with a thermostat controller to eliminate the manual work.

Exhaust fans can also be set on a timer so air is exchanged as often as once per minute. If it is cold outside, you will want to exchange the air less often, about a few times per hour. You can also hook your fans up to a thermostat, so air is exchanged when temperatures start to rise. Mount these fans high on the end walls and make sure they have a protective hood so that rain is not blown into the greenhouse.

Ceiling fans or oscillating fans keep air circulating within the greenhouse. This will control humidity levels and strengthen your plants’ stems, just as the wind strengthens plants outdoors.

Finally, if you are going low-tech, you can install screened doors on the ends of your greenhouse to create cross-ventilation and prevent stagnant air.

(More on greenhouse ventilation with Maintaining Consistency With a Greenhouse Ventilation System.)

Grow Room Humidity

Humidity levels should be maintained between 70 and 85 percent. When levels begin reaching 90 percent and higher, growth slows, fungal diseases start to occur, and early bolting becomes a problem. Keep in mind it can be a challenge to measure humidity levels when air is constantly circulating. Placing multiple hygrometers around your greenhouse can help you get a more accurate reading.

If the humidity gets too high, you can add a dehumidifier to dry it out. You might also try pruning your plants. Leaves perform a process called transpiration in which they release moisture into the air from pores on their surface, so reducing the foliage should help reduce humidity levels. Proper ventilation and air circulation are also important keys to managing humidity levels in a greenhouse. So, open doors or vents to reduce humidity. Controlling your temperature will also help to manage moisture levels as heat dries up moisture.

If you live in an arid environment and need to increase humidity, you can do this by spraying the floor with water or installing a misting system.

Greenhouses can make a wonderful addition to your garden as long as they are managed properly. When you control your temperature, air circulation, and humidity, you can take advantage of extended growing seasons and eat fresh year-round.