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Controlling Greenhouse Humidity During Summer’s Hottest Months

Summertime is primetime for greenhouse growers: vegetables are getting ripe, plants are growing quickly, and everyone's mind is on the harvest just a few months away. Now, more than almost any other time of year, it’s important to maintain peak conditions in your greenhouse - and that means keeping a close watch on heat and humidity.

But with large fluctuations in temperature between the high heat of the afternoon and the cool, dark nights, keeping your humidity levels in check can be a bit of a challenge. Here’s a look at some of the major factors contributing to high humidity in the summertime, and how you can make sure your greenhouse maintains the ideal levels throughout the warm season.

Why Humidity Levels Change With The Summer Season

If you’ve ever felt uncomfortable on those hot, humid, sticky summer days, you know just how much humidity can spike during the summer’s hottest months. Chances are, your plants feel that pressure too - and in some cases, that can really hinder their productivity.

Rising heat levels will cause some plants to perspire and water to evaporate - much in the same way we humans sweat in response to raised body heat. This perspiration will then evaporate into the air, raising humidity levels. In addition, hot air actually holds onto more humidity than cold air. This means that summer is naturally more humid than winter.

Let’s be clear - humidity itself isn’t necessarily a bad thing. In fact, keeping humidity high can actually help prevent overheating among your plants. This process, called “dampening down,” regulates high temperatures by introducing water into the greenhouse, which absorbs heat before evaporating and exiting the greenhouse. This can be good for heat-sensitive plants like tomatoes and cucumbers, which can quickly suffer as a result of overheating.

But importantly, too much humidity can be an open invitation for mold, fungi and botrytis - among other diseases and bacteria - to take hold on your plants, since these generally require relatively high moisture levels to survive. Let humidity build up to too high a level in your greenhouse and you may just end up cultivating a big batch of mold.

Tips For Raising Humidity Levels In Summer

As evidenced by the “dampening down” process, higher humidity can actually protect some plants from heat stress and other effects of high heat levels. For this reason, it may be beneficial for growers - especially those located in hot, dry areas of the country - to actively try and keep humidity at a slightly higher level than the outside air might naturally allow.

One simple method for introducing more moisture into your greenhouse is to spray water onto your structure’s flat surfaces early in the morning, allowing this water to evaporate throughout the day. This will safely and slowly raise humidity level in your greenhouse using the sun’s natural forces.

For those needing a little more water power, consider spray or drip irrigation system to deliver moisture efficiently when needed - a little pick-me-up for when the air gets a little too dry, which can be hooked up to your automated environmental controls for simple hands-free operation.

Similarly, growers can stop heat in its tracks by installing a greenhouse shade covering, or by utilizing a solar diffusing polycarbonate covering on your greenhouse walls and ceilings to redirect harsh sunlight and slow evaporation.

Tips For Reducing Humidity Levels In Summer

The major challenge for growers working to keep their greenhouse conditions ideal in the summertime generally isn’t increasing humidity, but reducing it - especially on those hot, humid days where there is so much moisture in the air already that the threat of mold is just too high.

In these cases, there are plenty of smart ways to mitigate rising humidity and keep your greenhouse at the right moisture level for your specific plants. For instance: work on watering plants early enough in day to allow their leaves and surfaces to dry completely before the cool evening comes. That way, you will minimize the amount of standing water available in your greenhouse during the night.

It’s also important to allow for adequate air movement throughout your greenhouse, which will help push warm, moist air away from your plants and out into the open air. Also ensure that your greenhouse floor drains well and does not allow water to pool, which can easily evaporate and add to your humidity problem. For this reason, you may also want to consider utilizing a steep enough roof pitch (or anti-drip poly) to prevent condensation from gathering and dripping onto plants from above.

Finally - and although it’s already a good practice for growers - it’s extra important to keep the ground inside of your greenhouses well weeded, since weeds can hold on to water and even perspire and release that moisture back into the air if left undisturbed in the hot sun.

Not Too Humid, Not Too Dry - Keeping Your Humidity Levels Just Right

Growing in a greenhouse offers major benefits over growing in the open air - namely, more efficient environmental, temperature, humidity, and light controls to keep your grow going strong not just through summer, but all year long. But for those unfamiliar with the difference in conditions inside a greenhouse versus outdoors under the sun, moisture levels can be challenging to control.

We recommend keeping a close eye on your greenhouse’s moisture and humidity levels using tools like a sling psychrometer, which compares wet and dry temperatures within your greenhouse to give a fairly accurate measure of relative humidity within your greenhouse walls. From there, you can take the right steps to adjust for the best possible outcome for your plants.

By keeping humidity under control and at the right level, you can seriously protect your plants, improve your output, and keep your greenhouse healthy and productive straight through until harvest.