It’s the middle of winter, and with daytime temperatures getting into the teens or single digits, the heat is running constantly. This makes the air in my house dry, and some of my plants seem to be suffering from the low humidity. I decided to do some research to find out what humidity level is best for plants.
So, what is the best humidity level for plants? An ideal humidity range for most mature plants is 50% to 60%. Some tropical plants, such as pineapple, are accustomed to humidity levels up to 90%. Many succulents, such as cacti, will be fine with humidity levels as low as 10%. As a general rule, plants with thicker leaves can tolerate lower humidity levels.
As you can see from the ranges mentioned above, your target humidity level will depend on the type of plants you are trying to grow. The ideal humidity level also depends on the stage of a plant’s development. In addition, temperature has an effect on humidity, and sometimes you will optimize one at the expense of the other.
Let’s start with the basics of what humidity is, and how to measure it. Then, we’ll take a look at how a plant’s humidity needs change as it grows. Finally, we’ll get into some of the symptoms displayed by plants when the humidity level is too low or too high, along with methods to adjust the humidity to a more appropriate level.
What is Humidity?
Humidity tells you how much water vapor is in the air. When temperatures rise, water evaporates faster, turning into water vapor and increasing humidity. Higher humidity increases the chances of rain, fog, or dew (the water droplets you sometimes see on grass first thing in the morning).
How Do You Measure Humidity?
A hygrometer is one tool that can measure humidity. If you have a dehumidifier in your home, you can use it to provide an estimate of the humidity level.
First, raise the humidity setting until the dehumidifier stops running. Let’s say this happens at a setting of 50%. Then, you know that the humidity level is 50% or below.
Then, decrease the humidity setting until the dehumidifier starts running again. If you decrease the setting to 45% and it starts running, then you know the range is 45% to 50% in your house.
Of course, remember that the humidity can vary by room, due to air currents. Basements tend to be cool and damp, with higher humidity levels than the rest of the house.
The Ideal Humidity Level Changes with Plant Growth Cycle
As your plant goes through its growth cycle, its humidity needs will change.
For seeds to germinate, the humidity level should be high, in the 90% to 100% range. If you are starting plants from seeds, keep them under a humidity dome to keep the humidity high.
A humidity dome is a clear cover made of plastic or glass that allows light through and keeps moisture under the dome, to raise the humidity level for plants inside the dome.
A humidity dome will also have openings that you can seal off to increase the humidity, or open to decrease humidity. For more information, check out my article on humidity domes.
Seedlings are seeds that have sprouted and grown leaves, but are still small. Once your seedlings emerge from the soil and are strong enough, remove the humidity dome. Otherwise, you run the risk that your plants will succumb to the growth of fungus or disease.
If you are interested in growing seedlings on a large scale, check out this article on greenhouses from the University of Massachusetts at Amherst.
The vegetative stage of a plant is an intermediate stage of growth between germination and flowering. During this phase, the plant extends its root system to absorb more nutrients and water from the soil.
The plant uses the nutrients and water, along with light and carbon dioxide, to create energy through photosynthesis. The plant uses this energy to grow taller and stronger, and creates leaves to absorb more light.
A plant will want far lower humidity during the vegetative stage than during germination. An ideal humidity range for the vegetative stage is 50% to 60%.
The flowering stage of a plant is the stage where flowers are produced, followed by fruit. In this stage, the plant devotes the energy it has produced (and is still producing) to flowers and eventually seeds, nuts, fruit, or vegetables.
A plant will want even lower humidity during the flowering stage than during the vegetative stage. An ideal humidity range for the flowering stage is 40% to 50%.
Symptoms of Plants When Humidity is Too Low
In a process known as transpiration, water moves from the plant’s roots to the leaves, where it evaporates into the air. When humidity levels are low, the plant loses more water due to transpiration. Combined with dry soil, this can lead to a lack of water for the plant.
Some symptoms of plants when the humidity is too low include:
- Brown edges on leaves
- A wilting or drooping look (a “lazy” plant that looks like it wants to lie down and rest)
- The flowers fall off, or they do not form at all
Often, low humidity will occur in your home or office, due to HVAC (heating, ventilation, and air conditioning) systems, which remove humidity from the air. In fact, humidity levels can get as low as 10%! This is especially common in the winter months, when cold temperatures cause heating systems to run almost constantly.
Ways to Raise Humidity
There are several ways to raise humidity at home. One way is to use a humidifier, which adds water vapor to the air. Just make sure to replenish the water supply at regular intervals.
Another way to increase humidity is to boil water or run a hot shower. This will use more energy and water than a humidifier, so it is not cost effective. However, you can use this method together with a humidifier if your house is extremely dry in the winter.
You can also leave open containers of water throughout your house. The water will eventually evaporate, adding to the humidity of the air. Again, make sure to replenish the water regularly, and put the containers in places where they won’t get spilled by your kids or pets (I am clumsy, and so I would probably be the one to spill them).
Finally, you can hang clothes to dry in your house. This takes up more space than any of the other methods, but if your plants like higher humidity, then hanging clothes in a room with plants may help them to grow.
To raise the humidity level in a small area, try placing plants close together. You can also try spraying their leaves, but do this sparingly to avoid fungus or mold. Another trick is to put plants that like high humidity in the bathroom. They will enjoy the steam from showers and baths!
Maybe you are growing tropical plants or germinating seeds in a greenhouse. If the humidity levels are too low, the tropical plants won’t do well, and you will get a poor germination rate.
To increase the humidity level, make sure the greenhouse is sealed, and leave the door closed. Also, make sure to leave an open container of water inside, so that it can evaporate into the air and increase the humidity level.
Symptoms of Plants When Humidity is Too High
When humidity levels are high, the plant cannot evaporate water from its leaves by transpiration. This prevents the plant from cooling. When temperatures are high, this can lead to the plant overheating.
If the soil is wet, it will stay wet longer, and the plant may suffer from root damage due to rot. You may also see signs of decay on your plants.
Some symptoms of plants when the humidity is too high include:
- Mold on leaves or flowers
- Rotten leaves
- Root damage
Another red flag: if the humidity in your home is too high, you may also see mold growth, especially in basements.
Often, high humidity will occur in the basement of your house. Your greenhouse can also become too humid if it is sealed tightly and the outside humidity is also high.
Ways to Lower Humidity
Luckily, there are several ways to lower the humidity level so that your plants can thrive.
First, you can use a dehumidifier to remove water vapor from the air. The dehumidifier will turn the vapor into liquid, and you will need to empty the water tray regularly.
Sometimes, you don’t feel like doing this, or the tray fills up too often for you to change it on time, as in a basement. With some dehumidifiers, you can connect one end of a hose to your dehumidifier and put the other end directly into a sink to drain the water out continuously.
If you have a room with lots of plants and it becomes too humid for some of them, then space them further apart.
You can also use a fan, which will bring dry air from other rooms into a humid room with lots of plants.
Finally, remember that warmer temperatures mean that the air can hold more water vapor. For example, air at 60 degrees Fahrenheit will hold twice as much water vapor as air at 40 degrees Fahrenheit. Every time the air temperature increases by 20 degrees Fahrenheit, the air will be able to hold twice as much water vapor.
Don’t run to crank up your thermostat just yet, though. As mentioned before, there are ideal levels for both temperature and humidity. It is a balancing act, and optimizing one comes at the expense of another.
If you want more information on humidity levels for a greenhouse, check out this resource on lowering humidity from the University of Massachusetts at Amherst.
Hopefully, you have a better idea of how humid your house should be in order for your plants to thrive. You also have some ways to increase or lower the humidity, if necessary.
I hope you found this information helpful. If you have any questions, please leave them in the comments.