Houseplants & Humidity: Tips That Help (& Those That Do Not)

Popular tips like using drip trays or misting plants to increase humidity levels do not work but there are few things you can do to keep your houseplant plants happier during the dry, winter months.

For more helpful tips also see How Often Should I Water My Houseplants? and How to Keep Houseplants Watered While You’re On Vacation.

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Houseplants & Humidity

Forget the pebble trays! And sorry, plant mister, you’re not doing much good.

Humidity, which is the amount of water vapor or moisture in the air, is vital for plant growth. I’ve described why it is important for plants here.

Most of the houseplants we grow—whether tropical or subtropical—are native to parts of the world with moderate to high humidity.

For those of us in cold climates, indoor humidity levels during the summer months tend to be fine (around 50% or higher) for our indoor plants.

It’s during winter, with heat from forced air furnaces, space heaters, fireplaces, or woodstoves, that indoor humidity levels drop—ours commonly hovers around 30%. You may feel it with dry, cracked skin and see some of your houseplants getting dry, brown leaf edges or showing other signs of dehydration.

What can we do to increase humidity levels in our homes?

While there are some things you can do to increase indoor humidity levels, the most commonly recommended tips do not actually help.

If you want to know your humidity levels, a simple device like this hygrometer at Amazon measures both indoor room temperatures and humidity levels.

Contents

Methods That Do Not Work

Pebble Trays

• Do not increase indoor humidity in any significant way.

Known as “pebble trays,” “drip trays,” or “humidity trays,” these trays are popular for growing plants like orchids and African violets.

There are different variations but basically the bottom of the tray acts as a water reservoir. Some trays come with a fitted plastic shelf or you can add a few inches of pebbles to the tray to create a place to sit plant pots just above the surface of the water.

The idea is that the water will evaporate into the air surrounding the plant to provide additional humidity.

It sounds feasible but makes little impact.

The problem is, it’s very little water and we can’t control where that water goes.

Yes, it goes into the air but it doesn’t linger around the plants. Instead it dissipates within the air, circulates throughout the house, and likely exchanges with outdoor air as well.

There are few studies on this but one published years ago in the American Orchid Society Bulletin [¹] illustrates the point. The experimenter measured humidity levels around a drip tray indoors in his home in Minnesota.

• Levels were highest just above the water surface where a 3% increase was measured.
• This dropped to 2% about four inches above the water and to zero around a foot up.

While this might bump up the humidity a few points around the base of your plants, it’s too small a difference to make any meaningful difference to the plants.

On a side note, I like drip trays for exactly that—to catch water that overflows when deeply watering my houseplants. But you can’t expect them to resolve low humidity issues.

Spraying Water

• Does not increase indoor humidity in any significant way.

Just like pebble trays, misting plants with water with a hand mister has little or no impact on humidity levels. It adds a tiny amount of water to the surface of the plants which soon evaporates. Plus, plants take in almost all their water from the potting mix, not their leaves.

Water misting can help retain humidity levels in closed environments like greenhouses or terrariums equipped with continuous spray units on timers.

This, of course, would be totally impractical to do at home and there are always concerns with mold and mildew when humidity levels stay really high. Sticking around 50% indoors in winter would be great.

Spray misters are, however, useful for removing dust from leaves. A gentle spray and wiping with a soft cloth can remove dust build-up which in turn may improve photosynthesis (how plants use sunlight to synthesize food from carbon dioxide and water). But for humidity? Not so much.

Growing in Bathroom

• No significant long-term benefits.

What about keeping plants in a bathroom?

While steamy showers will provide some temporary humidity, it’s not going to raise humidity levels up significantly or consistently enough to make an impact.

That said, if the bathroom happens to provide the desired amounts of light and warmth, it’s certainly an option to grow houseplants there.

Helpful Methods

Increased Winter Watering

When humidity levels are moderate or high, certain plants like succulents including aloe vera and cacti like their potting mix to dry out between waterings.

During months when the air is dry and humidity is low, it may help to avoid letting the soil dry out so much as there is little other moisture available.

You never want to over-water but just keep an eye on things so the potting mix never dries out completely. This could help reduce some stress on the plants.

Home Humidifiers

The most reliable way to increase indoor humidity levels is to use a humidifier designed to maintain a set humidity level ongoing. There are a range of options from ones that humidify your whole house to room-sized units.

Why Humidity Matters

Why is humidity important for houseplants?

Plants need humidity—water vapor or moisture in the air—to function.

Let’s have a look at how it works.

Temperature & Relative Humidity

How much humidity air can hold depends on the air temperature.

This is known as relative humidity—the amount of water that the air could hold at its current temperature.

As air gets warmer, it’s able to hold more water.

Cooler air holds less water.

This means the same amount of water in the air at a higher temperature will be a lower relative humidity that at a cooler temperature.

Now let’s see how lower humidity in winter causes plants to release more water and risk drying out.

How Humidity Affects Plants

There are lots of ways that humidity affects plants but the main one is its effect on transpiration (the loss of water vapor).

Plants take up water from their roots and distribute it from there to every part of the plant—all of their stems or branches and then to the leaves.

This is pretty impressive since plants don’t have anything like a heart to pump the water to all these place. Instead it happens passively, like rolling down a hill, except here the water is moving against gravity, sometimes up hundreds of feet in tall trees.

Ninety-five percent or more of the water a plant takes up through its roots is released back into the air as water vapor through transpiration.

The process is controlled by guard cells around the stomata (microscopic plant pores) that open and close as needed to release or retain the moisture.

This is all driven by differences in pressure and concentration. Humidity, specifically humidity around the leaves, plays a major role in how it all works.

In the plant, once the water gets to the leaves, it is considered to be at very high concentration—maybe even full saturation (100%).

The difference in water concentration between the leaf and the air—the humidity level of the air—drives the transpiration, which plants need to live.

Soil moisture is important for transpiration as well, but humidity plays a big part.

• When humidity is high, that lowers the difference in concentration between the leaf and the air, which slows down the movement of water from the plant to the air.
• When humidity is low, there’s a greater difference in concentration between the leaf and air, which means the water can move quickly from the plant to the air. If too quickly, the guard cells may need to step in, close the stomata, and reduce the outward flow of water so the plant doesn’t get dehydrated.

This can create a conflict because the plant needs its stomata to be open to take in carbon dioxide from the air which it needs for photosynthesis but it doesn’t want to give up too much of its water either.

The plant can either leave the stomata open for more carbon dioxide to get in but more water to get out or close the stomata to save water but reduce photosynthesis.

Plus, the reduced water movement also means a reduction in the flow of nutrients since nutrients travel with the water.

It’s quite a juggling act!

This is why, when we take plants outside their humidity comfort zone—either the levels their species have adapted to over thousands and thousands of years or just what the particular plant has become accustomed to—essential functions can be impacted, sometimes resulting in stress on the plant.

While we can’t replicate their native climates in our homes, keeping humidity levels stable around 50% makes indoor living better both for plants and their humans.

Realistically, some plants will adapt, others will not.

To summarize, don’t expect drip trays or water misters to combat dry air indoors in winter. A household humidifier can help and keep up with your watering so the plants always have some water available.

Resources

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Plant Tagging System

This is very simple but helpful: use this houseplant tagging system to keep track of what your plants like both for water and light.

Houseplant Tips

References

~Melissa the Empress of Dirt ♛