Humidity in Your Greenhouse

Groundhogs day has once again came and went, with our favorite wily critter seeing their shadow. No, I’m not referring to Bill Murray. For those initiated with the tradition, each year in early February, the mayor of a small town in Pennsylvania drags a ground hog out of its cage in front of a large crowd to see how it reacts to the sun. The thought process being that the groundhog can sense weather patterns, based only around their viewing and reaction to, their own shadow. Now, I have a few hesitations about this holiday. Like, why does the sun being out mean we will have a longer winter, wouldn’t the opposite be the case? Also, why are we ignoring the fact that this groundhog is expected to perform in front of the largest crowd it will ever see in its short life? Surely it’s the shadow on the ground that scared the North American ground squirrel and not the thousands of people clamoring in the cold photographing the event… Despite the fact that this holiday makes no logical sense to me, it’s a fun little way to break up the monotony that can be the winter season. As the groundhog saw its shadow, we are not expected to have a late spring. While I’ll believe that when I see it, it does give me the opportunity to explore managing humidity in your greenhouse during the winter months. Humidity can be both a blessing and a curse on greenhouse owners. Too little humidity and your plants could suffer as the temperatures might drop below optimal levels. When kept at proper levels, humidity can even provide some gentle watering for your plants. However, too much humidity can be quite detrimental to your plants if they don’t require it. Too much moisture in your greenhouse can lead to growth of molds and fungi which can negatively impact your plants growth. Too much moisture can also waterlog your plants which can lead to root rot and other detrimental blights on your plants. So, allow me if you will, to regale you with some research on how to manage humidity in your greenhouse.

First things first, there is no one way to ensure “perfect” humidity levels. There isn’t even really a perfect level because, as with most things in life, humidity needs are relative. Different plants will need different levels of humidity. One of the ideas behind a greenhouse is to create a controllable micro-climate that can be set to mimic the environment which the plants you desire grow naturally. That said, you will need to understand the plants you want to grow before you can start growing them and worrying about humidity levels. So, as with most parts of gardening, some thinking and planning is required. Thankfully this isn’t a case of studying sun light patterns and doing substrate tests for your soil, but none the less, understanding the growing requirements for your chosen plants is a vital step in successful gardening. One you’ve decided what you want to plant, you may now start to think about humidity requirements.

When starting seeds, 95% humidity is recommended. However, this can usually be accomplished by using a humidity dome. This ensures that your seedlings are getting the necessary humidity without you having to regulate the climate of your entire greenhouse, that way you save time, money and effort. Once your plants are sizeable and actively growing, their humidity requirements will change. However, humidity levels for growing plants have a wide range, anywhere from 55-97%. This range is why it is so important to understand the specific needs of the plants you are growing, too much humidity for a plant that doesn’t need it can be deadly and vice versa. Why is humidity so deadly you ask? Well, higher humidity levels are generally accompanied by increased moisture levels which can be problematic. As moisture levels increase in your greenhouse, condensation will begin to build on the ceiling if the greenhouse is not properly vented or the air isn’t properly circulating. Built up condensation can lead to wet foliage which is a great way to get fungal infections or mildew. Things like powdery mildew and common fungal diseases like the Botrytis pathogen are often found in greenhouses when the proper precautions aren’t taken. When the built up condensation beings to drip onto your plants, the chances of infection spreading increase as droplets could splash onto healthy plants. During these winter months, moisture build up can become a big issue if you’re not careful.

Winter is funny in that some daytime temperatures can reach upwards of 50-60°F only to drop down to 20°F come nightfall. This sudden and drastic change in temperature can be troublesome for gardeners, especially on those sunny winter days. Temperature changes coupled with changes in sunlight levels can result in serious moisture build up. Proper ventilation and air circulation can really be your best friend when trying to regulate humidity levels in your greenhouse. Venting your greenhouse during the day will help combat the build up of moisture that results from the sun warming everything back up after the cold night. Moisture, originating in the leaves of your plants, goes into the soil to then evaporate when the sun comes out. This evaporation results in moisture suspended in your greenhouse, which without circulating air or proper ventilation, will lead to condensation later on. By venting your greenhouse during the day only, cold night temperatures can kill your crop, you can safeguard humidity levels don’t build too much ensuring suitable moisture levels. Once the sun has begun to go down, close your vents to trap in the heat of the day and keep out the cold night temperatures. Fans are also a great way to make sure the air in your greenhouse is circulating, regulating moisture levels.

Speaking of moisture, watering is something you can control that will aid you in your quest to conquer humidity. Water your plants sparingly and in the morning. Overwatering is especially easy during the winter as lower temperatures and less sunlight means plants aren’t as thirsty. By watering in the morning you ensure what sunlight and warmth the day does provide can help excess water evaporate from the soil. When watering your plants you should stick to room temperature/warmer water. During the winter, like yourself, your plants want to stay warm. You’re more than likely spending some money/energy on heating your greenhouse, keeping your soil and plants warm. Cold water will shock your plants which can stifle their growth.

If you’re looking for a low cost yet physically intensive method to removing excess moisture from your greenhouse, look no further than whatever reflective surface is nearest you. Grab a squeegee, or maybe just an old towel, and get after it! Like I said, this method isn’t for those who don’t want to be physically involved. If done correctly, you can even capture and store the condensed moisture for the next time you need to water your plants. If you do try this method, let me know how it goes. You might save yourself some money, but you’ll lose a few hours of you day for sure.

Heaters and dehumidifiers are more energy intensive solutions, but at least it doesn’t require you stand on a step stool to get the drops of water off your greenhouse ceiling… Heaters alone likely will not solve your problem as trapped heat can create moisture when interacting with a room full of plants. Proper venting or air circulating fans in addition to heaters would likely do the trick. Dehumidifiers are also a great way to take the moisture out of the air in your greenhouse. You won’t want to run the dehumidifier 24/7 like my dad did in the basement during the New England summers of my youth, but it could be a helpful tool. Running it too often will take away essentially all of the humidity in the greenhouse, which again the general range required for growing plants is anywhere from 55-97%. Running it on a strict schedule can help you manage humidity and moisture levels in your greenhouse without negatively impacting your plants. The mornings and sunset when light levels and temperature changes are more prevalent would be optimal times to mechanically dehumidify your greenhouse. An extra bonus with a dehumidifier is that most models will capture the moisture in a removable water tank, which, you can use to water your plants!

Now that we’ve discussed in length what to do to keep your humidity level from getting too high, I’m sure some of you are wondering, what if I need to increase humidity? Well, luckily for you, this is a pretty easy task. A simple and cost effective way to deal with this dilemma is to bring in large buckets of water into your greenhouse. This solution will actually help you in two ways. The first being that the buckets of water will evaporate over the course of the day as things warm up, increasing moisture and humidity levels in your greenhouse. The second benefit is that water acts as a great temperature buffer, in that, water has a tendency to more or less absorb cold temperatures at a faster rate than substances around it. This to mean that water will get colder quicker than the soil in your greenhouse and while doing so, absorbing the cold thus slowing the rate at which the overall ambient temperature in your greenhouse decreases. In summation, water buckets will raise humidity, moisture and slow your greenhouse from freezing. Just make sure you clean the buckets out every now and again to keep away insects. Humidifiers and misters are also adequate ways to increase humidity. Humidifiers are generally somewhat energy intensive, especially if you’re running heaters in your greenhouse. Misters can be more physically taxing if you have a large greenhouse. Stationary misters need to be moved around periodically and handheld misters require you walk around your greenhouse. You also have to mind not to get the foliage too wet using a mister, that or have a fan running during and after your misting sessions. I’d warn you to keep an eye on how much moisture and humidity you’re adding but, we covered that earlier.

Once again, I’d like to thank you all for reading. Have yourselves another great week and best of luck in your greenhouse!

-Brian Bill