Why Is Your Greenhouse Killing Your Plants?

Purchasing and planting in a greenhouse can seem like a sure way to produce happy, healthy plants. But unfortunately, that is not always the case. Gardening is a science, and growing in greenhouses is no different. 

Your greenhouse is killing your plants most because it’s too warm, cold, or the humidity is off. Poor ventilation kills plants, which is why it’s important to have temperature and humidity gauges in your greenhouse. In addition, you should open your vents to allow fresh air to circulate as needed.

In the rest of this article, I will discuss several reasons why your greenhouse plants might be dying and how best to keep them alive.

1. Poor Ventilation Causes a Lack of Gas Exchange

The most common cause for plants dying in greenhouses is improper ventilation. To have good ventilation in your greenhouse, you will want to make sure that your structure has the ability to open paneling or vents that run along the roof and or sides. 

By opening up the panels or vents, your plants will get a good flow of fresh air, while the hot air that has been trapped can escape and bring the greenhouse back to a comfortable temperature. 

Not only is ventilation important for things like temperature control, but good ventilation also allows your greenhouse to replenish its oxygen and carbon dioxide supply and for built-up humidity to escape as needed. 

Without enough carbon dioxide, your plant will have stunted growth or even die. If the levels become too low due to poor ventilation (especially in winter), you may need to supplement carbon dioxide using compressed CO2 tanks, which can be costly.

2. Your Plants Are Too Warm

Another reason your plants aren’t thriving in your greenhouse is likely due to excessive heat build-up in the space. Most plants prefer temperatures between 70 to 80 °F (21 to 26 °C)

Unfortunately, these temperatures can easily be exceeded during the summer months and even the winter if you use a heater to warm the space but keep the vents closed to prevent cold drafts. 

Signs your plants are too warm:

  • The temperature reads significantly over 80 °F (26 °C). 
  • They are wilting.
  • Your plants’ leaves are curling.
  • Your plants’ leaves are dying and becoming brittle. 
  • Blossoms and fruit are dying or falling off.

If you recognize any of these signs in your plants, you must act quickly to remedy the situation. A great way to ensure that your greenhouse never gets too warm is to purchase a thermometer. I recommend using a wireless thermometer that pairs with your phone and notifies you in case of significant temperature changes.

A few other ways to keep your plants cooler are:

Move Your Greenhouse Into a Partial Shade

If you live somewhere with excessive heat during the summer months, placing your greenhouse in a bit of shade can benefit your overheated plants. 

Shade the Outside of the Greenhouse

You can also plant vines or other taller plants to partially shade the outside of your greenhouse. Shrubs or vines can be a fantastic way to add additional shade if your plants are often scorched from the hot sun. 

Circulate the Warm Air in Your Greenhouse

You can circulate the warm air in your greenhouse by opening your vents and placing a fan inside the greenhouse to help move the warm air out and cool down the space. 

3. Your Plants Are Too Cold

One of the most common causes of greenhouse plants dying is them getting too cold. This is more common in the wintertime, but it will sometimes happen that your greenhouse will be colder inside than the outside temperature.

How you can tell your plants are too cold:

  • It’s cold when you walk into your greenhouse. 
  • Your plants have droopy or curled leaves.
  • Your plant’s stems have started rotting. 
  • Their leaves have become discolored. 

If you are cold inside your greenhouse, there is a pretty good chance your plants are, too. Temperatures of -25 to 32 °F (-32 to 0 °C) will generally kill most plants even when safely planted in a greenhouse. 

This is why it’s so important to monitor your greenhouse’s temperature. In addition, it’s imperative to watch temperatures at night since they are more likely to plummet during that time. 

Ways you can prevent your greenhouse plants from freezing:

Add a Heater During the Cold Front or the Winter Months

There are several different heating options for a greenhouse during the winter months, the most popular being propane heaters. However, you can always take a more cost-effective approach and use compost to heat your greenhouse.

Add Insulation to Your Greenhouse

A popular greenhouse insulator is bubble wrap. Bubble wrap is cost-effective and can easily be placed along your greenhouse paneling to trap heat. 

Make Sure Your Greenhouse Is in Full Sun During the Day

The more sun your greenhouse gets during the day, the warmer it will remain. If your greenhouse is in partial shade, consider moving it to utilize the free solar heat. 

4. Inadequate Lighting During the Day

Another possible reason your greenhouse plants are struggling is the lack of sufficient light. Plants need plenty of light each day, and if they aren’t getting enough sunlight, they will start to die.

If you have a greenhouse in full or partial shade throughout the day, this could be the culprit behind your plants’ health decline. 

Plants typically need around 6 hours of full or filtered sunlight to grow. However, many sun-loving plants need 8-12 hours of sun to thrive. 

Signs your plants aren’t getting enough light:

  • Your plants look sparse and gangly. 
  • Your plants are leaning toward the limited light source.
  • The coloration of your plant’s leaves has changed.
  • Your plants aren’t producing flowers or fruit. 

If you happen to have a greenhouse that is placed somewhere shady, it might be time to consider making a change. Plants convert light into energy, and without this energy, they cannot grow to produce flowers, fruits, or vegetables. 

How you can improve your greenhouse lighting:

Move Your Greenhouse Somewhere Sunny

If your yard is large enough and there is a good sunny spot to move your greenhouse into, you should consider doing so. Usually, more sun is better unless you live in a place that frequently experiences extreme heat. In this case, you would want your greenhouse in partial shade. 

Trim Back Any Outside Plants That Are Casting Shade

If you have a tree or shrubs blocking some of your greenhouse’s light, consider trimming them back to allow more sun to penetrate your greenhouse. 

Add Some Artificial Lights

Artificial lights will never be as good as the sun, but they can be an excellent option if you can’t move your greenhouse and still need your plants to get enough light each day. 

Grow lights rich in blue light are good for foliage plants or leafy vegetables. On the other hand, a good balance of red and blue lights is necessary for fruiting or flowering plants. Check your plant’s light requirements to determine the intensity, duration, and distance of grow light exposure.

5. The Humidity Is Off

Last but not least, the humidity in your greenhouse might be off. Typically you should aim to keep your greenhouse humidity at about 80% during the day and 70% at night

However, it’s essential to do your research about all your plants to know what humidity they prefer. This will help you yield larger, healthier plants and produce. 

Signs your humidity is off in your greenhouse:

  • Your plants have a disease. 
  • Algae is growing on the dirt or other surfaces.
  • Your plants appear stressed.
  • Your plants are prematurely bolting.
  • Your plants are dying.

Humidity plays a huge role in the health of your plants. Too much humidity increases the chances of spreading diseases while having too little can stress your plants and increase the chances of bolting.

Use a hygrometer to monitor the humidity levels in your greenhouse. It will instantly inform you if your humidity is too high or low. 

To lower the humidity levels, you can choose from the methods below:

  • Improve air circulation. Allowing your greenhouse to get proper ventilation throughout the day can help you to lower the humidity. 
  • Don’t overwater your plants. Humidity is a measure of the amount of water particles the air can hold. Adding too much water to your plant’s soil can increase the greenhouse’s overall humidity due to the evaporation of the excess water.
  • Use a dehumidifier. If you are frequently experiencing high humidity levels, you can plug in a dehumidifier to help lower the amount of water in the air. 

On the other hand, you can use the following steps to raise the humidity in the greenhouse:

  • Use a humidifier. Although costly, this device can raise and maintain the humidity in the greenhouse to your desired level.
  • Place water trays or buckets below the plants. This works like pebble trays, which raise the local humidity for nearby plants as the water evaporates.
  • Wet the floor. For smaller greenhouses, wetting the floor with a water hose can raise the humidity, although briefly.
  • Close the vents. Keep the vents closed at night when the plants don’t photosynthesize or utilize carbon dioxide. This will help raise the humidity inside. However, ensure to open the vents during the day to allow fresh carbon dioxide in.

Keeping Plants Alive: Monitoring and Adjusting

You can keep your plants alive in a greenhouse by carefully monitoring the temperature and humidity and providing proper ventilation. If you notice the temperature is too warm or cold, you should adjust your greenhouses to suit the immediate need, such as providing more shade or a heater. 

The best way to keep your plants healthy is to observe them and act immediately if you notice changes in their health. 

Final Thoughts

Ultimately, there are several reasons your greenhouse might kill your plants.

However, the most common causes of greenhouse plants dying are:

  • Improper ventilation
  • Excessive heat
  • Extremely low temperatures
  • Inadequate lighting
  • Humidity levels are off

Addressing these issues as soon as you notice them will help you save your plants or learn how to improve the greenhouse conditions for your next batch of plants.

Dr. Moritz Picot

Dr. Moritz Picot is a horticulture enthusiast and the founder of TheGrowingLeaf.com, where he serves as the lead content writer. He established the website in 2022 as a valuable resource for both gardening aficionados and beginners, compiling all the gardening tips he has accumulated over the past 25 years. Alex has a passion for nurturing plants, transforming backyards into inviting spaces, and sharing his knowledge with the world.

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