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How Humid Is Too Humid for a Greenhouse?

Bobbi PetersonA greenhouse is a great way to grow flowers, green plants, and even vegetables. It allows you to enjoy flowers, like orchids, that may not thrive in your home or yard. African violets, gloxinia, and ferns that might be temperamental in your living room will be colorful, verdant and lush in your greenhouse. You can continue to grow vegetables like tomatoes and peppers in a greenhouse when it’s too cold outdoors. Plus, you can grow herbs and greens all year.

You probably know greenhouses need humidity. Many plants, like ferns, thrive in a humid climate. Others, like orchids, absolutely need a humid climate to be healthy.

However, a greenhouse can become too humid for healthy plants. Read on to see why, and ways to fix too much humidity.

How Humidity Affects Plants

All plants need water to survive. Humidity is moisture in the air. Plants breathe it in through their leaves, so to some degree, humidity is good for plants needing moisture. That’s why greenhouses, as a rule, are humid.

Too much humidity, though, may cause disease in plants. Why? Because plant leaves will get wet, and fungi grow in wet plants. Mildew can get started in plants just as it can in damp areas in a house. In fact, fungi like the Botrytis pathogen and powdery mildew are all too common in greenhouses across the country.

Once these diseases start, they may spread rapidly. Humidity rises. If it builds up on the greenhouse’s ceiling, it will start to drip. When it falls on plant leaves, the drips can splash on healthy plants and infect them.

An excess of humidity can also simply cause plants to not be healthy, even if they don’t contract a specific disease. They fail to thrive. One of the reasons for this is the effect of relative humidity on plants. Relative humidity is the ratio of current moisture in the air and the moisture-holding capacity of a specific air volume at a certain temperature. Plants utilize the water in their soil through transpiration, which is the ability of the roots to take in water and send it up through the leaves, providing the plants with nourishment. The moisture is eventually extruded into the air, which cools plants down. If the relative humidity changes rapidly, the pace of transpiration can change as well. If relative humidity rises, transpiration will slow down and can damage plant tissues. So can a quick change in the opposite direction.

Causes of Excess Humidity

There are three primary causes of excess humidity in greenhouses:

The first stems from improved greenhouse technology. While technology has had many great effects on greenhouses, better seals and improved construction can create conditions that cause humidity. There is less chance of moisture evaporating.

The second is the temperature outside. Greenhouses are not a universe unto themselves! When days are sunny and warm but nights are cold, conditions for excess humidity are created. Why? Because moisture is rising from plant leaves and soil, and moisture hangs in the air. More moisture exists in warm air than in cool air. Warm temperatures cause moisture to be held as vapor, and when nights become cold, the vapor condenses. That can cause drops of water or excess moisture. It might drip down from the ceiling during the night, or it may show up as dew on the plants and soil.

The third is the temperature in the greenhouse. As a general rule, warm air can hold much more moisture than cooler air. Air that is 70 degrees Fahrenheit holds double the moisture of air that is 50 degrees Fahrenheit.

Because of this, seasons where there might be dramatic swings in temperature between day and night, such as spring and fall, are peak potential times of excess humidity.

How to Fix Excess Humidity in Your Greenhouse

The diseases that affect plants with excess humidity can spread throughout your greenhouse. It’s important, then, to fix too-high humidity quickly.

First, make sure you have adequate ventilation and circulation. Fans, doors, and greenhouse vents will all increase ventilation and cause the air to circulate. Venting the greenhouse during the daytime will lessen moisture from both plants and soil. As a result, the chances of it condensing and dripping down during the night become greatly reduced.

Second, using bottom heat to keep the greenhouse warm when the air outside is cooler will help eliminate condensation. If you’re experiencing difficulty with excess humidity, purchase heaters for the greenhouse.

The third method is to purchase equipment that can give you a reading of humidity, such as a psychrometer. Using information from the psychrometer, you’ll be able to heat more accurately.

While humidity is essential to healthy greenhouse growing, so is the prevention of too much of it. With time, you will learn to adjust excess humidity and plan for its prevention.

small greenhouse
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