While humidity is somewhat less important than temperature and light, it is still an essential thing for plant people to understand and monitor to keep their plants thriving.
Many of the most popular houseplants require a lot of humidity, much more than is typically found in the average home. Many houseplants evolved to live in dense, humid jungles with an incredible amount of humidity, e.g. moisture contained in the surrounding air.
What is humidity?
Humidity refers to relative humidity, a ratio expressed in a percent. This ratio is the amount of atmospheric moisture relative to the amount of moisture that would be present if the air were saturated. Thus, for the same absolute amount of moisture in the air, relative humidity will change.
As seen in the table below, relative humidity and temperature go hand in hand. The higher the temperature the more humidity it can hold. Read all about temperature and houseplants here.
| Temperature | 0% | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
| 40 °C | 0 | 5.1 | 10.2 | 15.3 | 20.5 | 25.6 | 30.7 | 35.8 | 40.9 | 46.0 | 51.1 |
| 35 °C | 0 | 4.0 | 7.9 | 11.9 | 15.8 | 19.8 | 23.8 | 27.7 | 31.7 | 35.6 | 39.6 |
| 30 °C | 0 | 3.0 | 6.1 | 9.1 | 12.1 | 15.2 | 18.2 | 21.3 | 24.3 | 27.3 | 30.4 |
| 25 °C | 0 | 2.3 | 4.6 | 6.9 | 9.2 | 11.5 | 13.8 | 16.1 | 18.4 | 20.7 | 23.0 |
| 20 °C | 0 | 1.7 | 3.5 | 5.2 | 6.9 | 8.7 | 10.4 | 12.1 | 13.8 | 15.6 | 17.3 |
| 15 °C | 0 | 1.3 | 2.6 | 3.9 | 5.1 | 6.4 | 7.7 | 9.0 | 10.3 | 11.5 | 12.8 |
| 10 °C | 0 | 0.9 | 1.9 | 2.8 | 3.8 | 4.7 | 5.6 | 6.6 | 7.5 | 8.5 | 9.4 |
In a greenhouse, relative humidity is 50 percent or higher, plant stores are often quite humid too. The average home in the U.S. is closer to 15 or 20 percent, vastly lower than a typical houseplant’s growing conditions or garden center.
When bringing that lush new plant home, it may suffer. When the environment around a houseplant is dry or not as humid as a plant needs, it will quickly brown at the tips of leaves or wilt. This is moisture leaving the plant as the surrounding air absorbs that moisture.
Seasonality affects humidity dramatically. Anyone in a northern climate knows that the air gets dry in the winter, but as the temperature changes, homes across the country adapt. Heaters and radiators kick on, air conditioning cools the air. Both of these tend to remove humidity from the air. When they do, plants that were thriving in the summer tend to wilt or brown.
Boosting humidity
There are some ways for plant people to foster a more humid microclimate, but it’s essential to get the right plant for the environment. Short of a high-output humidifier, some plants will just never do well in a very dry home.
To bolster humidity, there are some tactics to get a few percent more humidity and help plants do better:
- Use a shallow dish of rocks, marbles or oddly shaped rocks. The fill breaks up the surface of the water and allows for faster evaporation and increases surrounding humidity.
- Cluster plants together to create a humid microclimate. As soil dries, that moisture will enter the surrounding air.
- Use a humidifier, it can be bothersome but essential for some plants, especially in the winter.
- Create a greenhouse, there are dozens of Ikea hacks or small-format solutions to do this. It’s a lot of work and the greenhouse will require maintenance, but for serious plant collectors, it’s a nice solution.
What not to do:
- Mist plants. This does almost nothing at all unless someone is misting nonstop all day. And misting many houseplants can lead to disease, rot or damage leaves.
- Water more frequently. This is a common issue, people see their plant looking sad and add more water. Oftentimes times of low humidity are also when plants require less water because there is less light (e.g. winter). More water will just harm the plant.
How to measure humidity
Measuring humidity requires a hygrometer, there isn’t a good way to measure humidity without this special piece of equipment. They’re not expensive, and for serious plant owners they’re nice to have next to humidity-loving plants. They are, however, kind of ugly.
A good practice is to routinely check areas where plants live in a home. Humidity doesn’t change nearly as often as light, so checking every couple months or when a plant doesn’t seem to be doing well is fine for most houseplant owners.
The Sprout Houseplant Tender makes this a lot easier. It monitors temperature every minute of every day, and sends alerts if a plant needs more light or might fare better in another window.