First published in November 2019
When it comes to houseplant care, the spotlight is often on watering, light levels and temperature. It’s easy to overlook humidity, but for many plants, particularly foliage houseplants, this is an essential part of their care.
What is humidity?
Humidity is the concentration of water vapour in the air. In Britain, mist and fog are common forms of very cool, humid weather.
To measure humidity, we use a term called relative humidity (RH). The higher the percentage of water in the air, the higher the RH.
Humidity in the home is often below 30%, which is dramaticly different from the 80-100% in the equatorial jungle homes of many houseplant species.
Why is humidity important to plants?
In tropical rainforests, humidity levels can be very high a result of high temperatures, regular rainfall and high levels of transpiration.
Equatorial cloud forests, such as those in Central America, have the highest levels of humidity. Because of this, plants don't have to worry about losing water through their leaves, so they are often very big. They often grow in low light conditions, so larger leaves help them capture more light.
Large foliage makes for a beautiful houseplant, but they need a fair bit of care. Maintain high humidity around them to prevent issues.
The drier the air around a plant, the more it will need watering. So higher humidity levels are a bonus, even for tougher plants like this Devil's Ivy.
Problems caused by low humidity
Shrivelled foliage: This happens to very fragile plants, some of which are listed below (e.g. Maidenhair Fern). The room needs to be very dry and warm for foliage to shrivel, and plants near sources of heat are more at risk. Central heating and drafts are the bane of most foliage houseplants.
Brown leaf tips: This is very common in houseplants that are in centrally heated rooms or situated near a draught. Although it isn’t a terminal issue, the foliage looks unsightly and scruffy. You can remove damaged leaves, but sometimes removing the affected leaves results in the plant appearing bare.
Red spider mite: See link for symptoms and control:
This gardenia has a severe red spider mite infestation. This pest thrives in dry situations.
Tips for humidity-loving plants
Plants that demand high humidity (such as ferns) thrive in bathrooms, where they’re exposed to moisture at least once a day. In a regular room, follow the tips below:
Misting: Mist both the top and bottom of the leaves using lukewarm water in the morning. Humidity-loving flowering plants (e.g. Gardenia) don’t appreciate their flowers getting wet. If you live in an area with hard water, you may get chalky marks on the foliage over time.
Gravel tray: This method is often used for plants that cannot be misted (e.g. African Violets), but works well on most plants, particularly those that prefer consistently higher humidity. Fill a tray with gravel, add water to just below the height of the gravel and sit the plant on top. The water will evaporate according to the heat in the room, but the plant’s roots are kept dry.
Terrarium: Hands down the best way to enable a stable high humidity environment. Some plants (e.g. Spike Moss) are so demanding with their humidity levels that they can only really be grown with any success in a bottle garden or terrarium. For more info on creating your own, click here:
Grouping plants is an easy way to justify a houseplant addiction while raising humidity levels. Just be sure to check any newcomers are free of pests or diseases.
Houseplants that require high humidity levels
Indoor Azaleas are unusual in that they need high humidity alongside cool conditions. This is because they grow in cloudy forests at high altitudes in Asia, rather than tropical jungles.