The Best Ways to Increase Humidity for Your Houseplants

Now that you’ve understood the importance of atmospheric humidity and what’s at stake for plants, humans and our homes, by reading Humidity: Plants, Health and Our Homes, how can we increase humidity in our homes in winter? Several methods are suggested for doing so, but I’m warning you now, not all of them are good! Let’s take a look at the most frequently suggested approaches to see what’s what.

Humidifiers

A humidifier is simply a household appliance that produces steam or fine water droplets to increase humidity in a small space. Most can cover the space of a single room, but there are models, often combined with heating, ventilation and air-conditioning systems, that can regulate the relative humidity in an entire building (I dream of not having to constantly refill my humidifiers!).

The most common devices are either hot-steam, cold-steam or ultrasonic. Hot-steam humidifiers work by evaporating water, heating it and then cooling it, so as not to cause burns. Cold-steam humidifiers pass air through a water-soaked filter to charge it with moisture. Finally, an ultrasonic humidifier uses a surface vibrating at an ultrasonic frequency to create micro-droplets of water.

In all cases, it’s important to clean and disinfect humidifiers regularly to prevent mold and mildew. Manufacturers often suggest weekly cleaning, but the U.S. Environmental Protection Agency (EPA) recommends every three days. To do this, you can use a mixture of water and white vinegar, to which you can optionally add bleach or hydrogen peroxide. It’s also a good idea to let your appliance dry out before refilling with water.

Humidifiers are probably the most effective way of increasing humidity in the air, but need to be filled and cleaned regularly.

Pebble Trays

This method is very simple: place gravel, clay balls or other small stones in a tray. This can be a grow tray, but any flat container will do. We fill the bottom of the tray with water and place your plants on top.

The principle is simple. Water rises to the surface of stones or marbles by capillary action and evaporates. In fact, any water in contact with the air will evaporate if the air is dry. So you end up with a localized humid zone around your plants.

The problem is that this zone is more localized than you might think. Of course, the first few centimetres above the surface of the tray will be wetter, but thanks to the phenomenon of diffusion, this moisture will soon spread throughout the entire room, rather than creating a small damp bubble around our plants, as we might imagine. And to humidify an entire room, your tray would probably have to dry out completely every day, even several times a day depending on conditions, which is not the case.

Worse still, since the humidity is localized just above the tray, it may prevent the potting soil from drying out. This might be a good thing for plants that need a constantly moist substrate, but for the majority, this situation can lead to root rot, in the worst case scenario.

Grouping Plants

When they breathe, plants, like humans, emit water vapour. So, by grouping plants together, we can create a more humid zone around them, with each plant benefiting from the moisture emitted by its neighbor.

Have you ever visited a greenhouse? When you step inside, you immediately feel that the air is heavy and full of water. It’s much the same when plants are grouped together at home.

Unfortunately, as with the humidifying tray, the effect is usually very limited. And since the diffusion principle is involved, that lovely moisture will soon spread throughout the entire room. However, those with a large collection of houseplants kept in a small space, such as a dedicated room, will see an effect. Maybe even too much!

Misting

Misting not only has a very limited effect on ambient humidity, it can also cause problems for your houseplants. This technique should be avoided at all costs!

For a start, the water particles produced by a sprayer are too large to penetrate the pores on the leaf surface. So your plants can’t absorb the water. However, these droplets will settle on the leaf surface and evaporate, increasing atmospheric humidity… for a while. When these water droplets have dried out, after about fifteen minutes, the already limited humidifying effect will be over, because, as we’ve seen with other methods, the humidity will spread into the ambient space by diffusion.

But misting is not only useless, it can also be harmful. Firstly, tap water contains minerals. When it dries out on the leaves, the minerals remain, leaving white spots that reduce the plant’s capacity for photosynthesis. Secondly, keeping plant leaves wet can lead to the development of leaf diseases.

Stop it right now!

Greenhouse cabinet, terrarium or other

Glass cabinets make excellent greenhouses for indoor plants. Not only do they prevent moisture from escaping, they can also be fitted with artificial lighting. This blend of built furniture and vegetation is a thing of beauty! They are particularly useful for tropical plants with high humidity requirements, keeping the relative humidity easily between 60 and 80% without the need for a humidifier. What’s more, by keeping humidity high, you probably won’t have to water your plants as often.

The same is true of terrariums. Since humidity can’t spread outside the closed enclosure, the same benefits apply. Even an uncovered terrarium can help. Since water vapor cannot diffuse horizontally, but only vertically, humidity will remain high for longer. You can even change the terrarium’s degree of openness by moving the lid, thus controlling the humidity level.

If you don’t have a glass cabinet or terrarium, a simple transparent plastic bag will do the trick.

Be careful, however, not to place these cabinets, terrariums or bags in direct sunlight, as the greenhouse effect can raise the temperature enough to kill your plants in a matter of hours.

Keep Your Plants Cool

Another idea, which doesn’t apply to every situation, is to keep your plants cool. The colder the air, the less water it can hold, and the higher the relative humidity. This makes it easier for you to keep atmospheric humidity at an acceptable level for your plants. For example, at 20 C (68 F) with a relative humidity of 30%, the air contains around 5 g/m3. At a temperature of 15 C (59 F), with 5 g/m3, the relative humidity is 40%, an acceptable level for the vast majority of plants. I understand that this is not for everyone, but if you have a special room for your plants, it becomes an option to consider.

With the reduced light and dryness that winter brings, it’s perfectly normal for our houseplants to suffer a little. Increasing humidity and light levels can put a balm on this period which is difficult for some of us and our plants.