Humidity and Houseplants
Grant Wood
Humidity is a measure of the amount of water that air will hold. The water is usually in the form of invisible droplets. At 100 percent humidity the air cannot hold any more water. The highest humidity often occurs on hot days, creating a "muggy" feeling. Fog occurs when the air is saturated and the invisible water now becomes visible.
Humidity is measured relative to temperature and is called relative humidity (RH). The measurement is taken this way because humidity and temperature are directly related: the warmer the air, the more water it can hold.
Humidity in the Home If warm air holds more moisture than cold, then why is it so dry inside the house in winter? Remember that the furnace is taking dry outside air and warming it. If no water is added to this outside air, then it will still be dry. You can increase the humidity inside to a certain extent by adding water to the air. Warm, moist air is always being lost from the house, and cold dry air is always being brought in, so high humidity in the entire house is not possible.
When warm, moist air comes in contact with a cold, dry surface, the water in the air condenses. This is very common on windows, and is an indication that the humidity inside the house is higher than outside. If the inside walls of the house are cooler than the air inside the house, water can condense on the walls, and can cause wallpaper to come unstuck, but don't rely on this as a means of stripping wallpaper!
Humidity and Plants
Humidity is important to plants because it partly controls the moisture loss from the plant. The leaves of plants have tiny pores in them called stomata. Carbon dioxide enters the plants through these pores; oxygen and water leave through them.
The humidity inside a plant is close to 100%. A plant growing in a dry room will lose moisture because water always moves from high to low humidity. When the difference in humidity is large, the loss of moisture from the plant is rapid and severe.
Most houseplants prefer a humidity of about 60%. Cacti, succulents and plants native to desert environments tolerate much lower humidity (30-35%), but prefer not to drop below 20%. House plants that are native to tropical rain forests require much higher humidity, 90% for example, and thus pose problems for most home owners. Plants that require a very high humidity are best grown in terrariums or closed containers where it is possible to regulate the humidity.
Damage from excess humidity - Under very humid environments, fungal diseases can spread. This seldom happens during winter, but can be a problem in fall when the temperature is cool and the humidity is high. Mildew on plant leaves is an indication of excess humidity and lack of ventilation.
Damage from dryness - Plants that prefer a more humid environment, but that are forced to grow in a dry environment will commonly suffer damage to younger leaves and to leaf tips. New leaves and leaf tips are the area of the plant with the most actively growing cells, and these cells are the most susceptible to dry air. Older leaves that are fully formed may be shed as a result of lack of humidity, but they will not be deformed or damaged by the dry air. Plants stressed in this way very frequently shed flower buds, or flowers die soon after opening.
Increasing Humidity
There are a number of ways that a home owner can increase the humidity in the room or around the plants.
Humidifiers - Using a humidifier is by far the most effective way to increase humity. Humidifiers that attach directly to the furnace will increase the humidity throughout the house. Portable humidifiers can be used to increase the humidity in one or more rooms.
Changing locations - Bathrooms and kitchens, if they are sunny, often have a higher humidity than other areas of the home, and may be more suitable for house plants requiring extra humidity.
Double potting - Take a small potted plant or a number of small plants, and put them in a larger pot. Fill the area underneath and around the small pots with peat moss. Keep the peat moss constantly moist. As water evaporates from the peat moss, it increases the humidity around the plants. Make sure the large pot has a tray underneath to catch excess moisture from the peat moss. A similar approach is to place a house plant in a basket lined with moist peat moss.
Pebble trays - Fill a large plant saucer with pebbles or stones. Place a number of small pots (or a large pot) on top of the stones. To assure that the pots do not contact the water, you may wish to place them on saucers which sit on the pebbles. Now fill the large plant saucer with water up to the level of the pebbles. Make sure the saucer with pebbles is large enough to be effective - the larger the surface area of pebbles, the more effective the method will be.
Totems for climbing plants - Take chicken wire and roll it into a totem (tube). Fill the tub with peat moss. Anchor the tube in the plant pot and then wind the climbing plant around the tube. Keep the peat moss inside the tub moist. Do not worry if the plant forms roots into the totem, but if this occurs make sure the totem is kept evenly moist.
Grouping - Moisture loss from one plant can benefit the plant next to it. Try and group plants with similar watering requirements together, and keep them close to each other. The closer together they are, the more effective the method.
Misting - This is the least effective but often the most used method. Misting plants with tepid water will result in a layer of water on the leaves, which will reduce the transpiration of water from the leaves. However, soon after misting, the water will evaporate, and once this occurs, the air is once again low in humidity. If plants are misted too often or too much, however, fungal growth and tissue rotting may result. Plants with hairy leaves cannot be misted, for leaf spotting will likely occur as a result.
