Humidity By Paula Szilard As we all know, Colorado is a challenging environment for the tropical plant enthusiast. Many people who grow tropical rainforest plants in Colorado’s dry climate have learned the lessons of atmospheric humidity the hard way—a failure to thrive, wilting, decline, or sudden death by collapse. The term relative humidity is very descriptive. It’s the amount of water that the air can hold at specific temperatures. It’s just what it says---relative! It is the amount of water air holds relative to the amount it can hold at that specific temperature. It’s expressed as a percentage. In Colorado the relative humidity outdoors generally runs about 20-30% unless we have rain or snow. Just before and during rainfall, atmospheric humidity is 100%. Our indoor environments can be terribly dry in the winter because dry heat from our central heating systems is blown all over our living spaces. In the winter months, plantless, unhumidified interiors can have a relative humidity of as little as 10% and most plants from the humid tropics require anywhere from 70-90% humidity. So lovers of tropical plants have to work hard at creating an optimal environment for their plants. You can measure atmospheric humidity with a hygrometer. These instruments are not expensive and it might be useful for all serious growers of tropicals to own one. They are readily available via mail order. I purchased mine from a scientific instruments company in Denver. The warmer the air, the more moisture it can hold. Take my unheated front porch. On cold mornings when we first open the door, water has beaded up on the windows and the glass door. This is moisture that has precipitated out of the air. When the sun hits the room later in the morning, this water quickly evaporates and it’s being held by the air. If you have a greenhouse, then you are used to the experience of being hit by wall of moist warm air on a sunny day when you open the door. Or, at the extreme end, imagine the steam room at your gym. It’s very hot in there and there is so much water vapor in the air, it actually interferes with visibility. For health and adequate growth, plants try to maintain an equilibrium between the amount of water they lose through transpiration through their leaves and other tissues and the amount they take up. The hotter and sunnier the environment, the more transpiration occurs. This is because there is more photosynthesis, a process that produces carbohydrates for the plant and oxygen for us out of carbon dioxide and water in the presence of sunlight. That is why, when the weather is overcast, we need to do less watering and when it’s warm and sunny, we need to do more. To some extent, plants can control this process by opening and closing the stomates or pores on the underside of the leaf blade. When the stomates are open, carbon dioxide can enter and water is evaporated. Wilting causes the stomates to close, reducing water loss, but also reduces carbon dioxide uptake, thus reducing photosynthesis and resulting in less vigorous growth. This may be a partial explanation for the less luxuriant growth of plants in environments with reduced humidity. In areas where rainfall is high, atmospheric humidity is also high. When it’s raining and the humidity is 100%, plants are in a state of equilibrium. They absorb as much water from the air as they transpire. As temperatures increase, the air can hold more water and if more water is actually present, this equilibrium is maintained. If not, as the temperature rises, the humidity drops and the plant is no longer in equilibrium. If it can absorb the required water from the soil through its root system quickly enough, the equilibrium is likewise maintained. If not, the plant wilts, even though there might be sufficient moisture in the soil. The plant roots just can’t absorb it quickly enough to replace the lost moisture. If the temperature keeps rising and there is no increase in atmospheric moisture, then the plant could collapse and die. Many of us have had this experience. In its native habitat, the plant is usually in a state of equilibrium with its environment. The root system has grown to collect sufficient moisture to replace what is lost through transpiration. It is in balance with the amount of foliage it has to support. If the root system is too small to support the leaves, the plant wilts. Adding moisture to the soil won’t help because the roots cannot take it up fast enough. This is why, when propagating plants under lights, you need to remove the plastic covering very gradually and you don’t want to remove the covering at all before the plant has developed a strong root system. It’s why, when propagating, you should cut off most of the leaves. It’s also why, when you keep watering a wilted plant with a poor root system or a plant that needs less water than you give it, eventually the roots rot away and the plant dies. Since adding water to the soil won’t help, you have to increase the atmospheric humidity. As a quick emergency measure spraying the leaves with water can help. A longer term solution is a humidifier or just getting more plants. All the houseplant books suggest grouping plants together and putting pebble trays under plants to increase humidity levels around plants. Unfortunately, when you have a sizeable collection, this becomes unworkable. Nothing humidifies your interior environment like a lot of plants. Long-time members of the Tropical Plant Society have heard this story before, but I repeat it here because it is such a valuable lesson for newcomers. When my husband and I moved here from Hawaii 15 years ago, we were uncomfortable. We were used to a high humidity environment and we were having nosebleeds and problems with scaly, dry skin. My husband checked the humidifier attached to the furnace and determined that we needed a new one. We called the company that had installed the existing humidifier and had them install a new one. At the same time, I started to fill the house with tropical plants, harvesting every ray of sunshine that entered our windows. I even had plants in our basement windows! Our humidity levels began to rise and we felt better. We were quite pleased with our new humidifier! When the time came to clean it, my husband discovered that the installer had never actually finished hooking it up. All that increased humidity resulted from transpiration of water from the plants and evaporation directly from soil. Later, when I was in master gardener training I found confirmation of this phenomenon. I learned that approximately 90% of the water we pour on our plants is transpired or evaporated into the air. The moral of this story, of course, is that if you have enough plants you don’t need a humidifier. They are better than a humidifier! Plus, humidifiers don’t clean your air, and plants do! Plants can gradually adapt to reduced atmospheric moisture by growing a stronger root system and by growing fewer new leaves to reduce transpiration. This process is called ‘Hardening’ or ‘Hardening off.’ This is an absolute must for vegetable starts. It gradually gets the plant used to the outdoor environment. The important thing to remember is that your tropical plants do not like sudden changes. When you take them outside for the summer, you don’t just plunk them down in full sun. They will not only get sunburned, but they could be so badly damaged, they will die. Put them into a shady spot on the north side of your house for a while, then transfer them to part sun on the east side, and finally to their final destination, all over a period of a couple of weeks. So as you can see, there’s more to this humidity thing when you dig a little deeper. Here in the high desert, it sometimes involves a little bit of extra effort to provide conditions that allow tropical plants to thrive. Humidity is only one of the factors that must be addressed, but it is absolutely critical! And remember, the more plants you have, the less of a problem it is. We’ve taken these plants out of their natural habitat and placed them into an artificial environment, thereby creating problems, but we solve these problems a lot better if look to the plants themselves for solutions.

Humidity

Paula Szilard

As we all know, Colorado is a challenging environment for the tropical plant enthusiast. Many people who grow tropical rainforest plants in Colorado’s dry climate have learned the lessons of atmospheric humidity the hard way—a failure to thrive, wilting, decline, or sudden death by collapse.

The term relative humidity is very descriptive. It’s the amount of water that the air can hold at specific temperatures. It’s just what it says---relative! It is the amount of water air holds relative to the amount it can hold at that specific temperature. It’s expressed as a percentage. In Colorado the relative humidity outdoors generally runs about 20-30% unless we have rain or snow. Just before and during rainfall, atmospheric humidity is 100%. Our indoor environments can be terribly dry in the winter because dry heat from our central heating systems is blown all over our living spaces. In the winter months, plantless, unhumidified interiors can have a relative humidity of as little as 10% and most plants from the humid tropics require anywhere from 70-90% humidity. So lovers of tropical plants have to work hard at creating an optimal environment for their plants.

You can measure atmospheric humidity with a hygrometer. These instruments are not expensive and it might be useful for all serious growers of tropicals to own one. They are readily available via mail order. I purchased mine from a scientific instruments company in Denver.

The warmer the air, the more moisture it can hold. Take my unheated front porch. On cold mornings when we first open the door, water has beaded up on the windows and the glass door. This is moisture that has precipitated out of the air. When the sun hits the room later in the morning, this water quickly evaporates and it’s being held by the air. If you have a greenhouse, then you are used to the experience of being hit by wall of moist warm air on a sunny day when you open the door. Or, at the extreme end, imagine the steam room at your gym. It’s very hot in there and there is so much water vapor in the air, it actually interferes with visibility.

For health and adequate growth, plants try to maintain an equilibrium between the amount of water they lose through transpiration through their leaves and other tissues and the amount they take up. The hotter and sunnier the environment, the more transpiration occurs. This is because there is more photosynthesis, a process that produces carbohydrates for the plant and oxygen for us out of carbon dioxide and water in the presence of sunlight. That is why, when the weather is overcast, we need to do less watering and when it’s warm and sunny, we need to do more. To some extent, plants can control this process by opening and closing the stomates or pores on the underside of the leaf blade. When the stomates are open, carbon dioxide can enter and water is evaporated. Wilting causes the stomates to close, reducing water loss, but also reduces carbon dioxide uptake, thus reducing photosynthesis and resulting in less vigorous growth. This may be a partial explanation for the less luxuriant growth of plants in environments with reduced humidity.

In areas where rainfall is high, atmospheric humidity is also high. When it’s raining and the humidity is 100%, plants are in a state of equilibrium. They absorb as much water from the air as they transpire. As temperatures increase, the air can hold more water and if more water is actually present, this equilibrium is maintained. If not, as the temperature rises, the humidity drops and the plant is no longer in equilibrium. If it can absorb the required water from the soil through its root system quickly enough, the equilibrium is likewise maintained. If not, the plant wilts, even though there might be sufficient moisture in the soil. The plant roots just can’t absorb it quickly enough to replace the lost moisture.

If the temperature keeps rising and there is no increase in atmospheric moisture, then the plant could collapse and die. Many of us have had this experience. In its native habitat, the plant is usually in a state of equilibrium with its environment. The root system has grown to collect sufficient moisture to replace what is lost through transpiration. It is in balance with the amount of foliage it has to support. If the root system is too small to support the leaves, the plant wilts. Adding moisture to the soil won’t help because the roots cannot take it up fast enough. This is why, when propagating plants under lights, you need to remove the plastic covering very gradually and you don’t want to remove the covering at all before the plant has developed a strong root system. It’s why, when propagating, you should cut off most of the leaves. It’s also why, when you keep watering a wilted plant with a poor root system or a plant that needs less water than you give it, eventually the roots rot away and the plant dies. Since adding water to the soil won’t help, you have to increase the atmospheric humidity. As a quick emergency measure spraying the leaves with water can help. A longer term solution is a humidifier or just getting more plants. All the houseplant books suggest grouping plants together and putting pebble trays under plants to increase humidity levels around plants. Unfortunately, when you have a sizeable collection, this becomes unworkable.

Nothing humidifies your interior environment like a lot of plants. Long-time members of the Tropical Plant Society have heard this story before, but I repeat it here because it is such a valuable lesson for newcomers. When my husband and I moved here from Hawaii 15 years ago, we were uncomfortable. We were used to a high humidity environment and we were having nosebleeds and problems with scaly, dry skin. My husband checked the humidifier attached to the furnace and determined that we needed a new one. We called the company that had installed the existing humidifier and had them install a new one.

At the same time, I started to fill the house with tropical plants, harvesting every ray of sunshine that entered our windows. I even had plants in our basement windows! Our humidity levels began to rise and we felt better. We were quite pleased with our new humidifier! When the time came to clean it, my husband discovered that the installer had never actually finished hooking it up. All that increased humidity resulted from transpiration of water from the plants and evaporation directly from soil. Later, when I was in master gardener training I found confirmation of this phenomenon. I learned that approximately 90% of the water we pour on our plants is transpired or evaporated into the air. The moral of this story, of course, is that if you have enough plants you don’t need a humidifier. They are better than a humidifier! Plus, humidifiers don’t clean your air, and plants do!

Plants can gradually adapt to reduced atmospheric moisture by growing a stronger root system and by growing fewer new leaves to reduce transpiration. This process is called ‘Hardening’ or ‘Hardening off.’ This is an absolute must for vegetable starts. It gradually gets the plant used to the outdoor environment. The important thing to remember is that your tropical plants do not like sudden changes. When you take them outside for the summer, you don’t just plunk them down in full sun. They will not only get sunburned, but they could be so badly damaged, they will die. Put them into a shady spot on the north side of your house for a while, then transfer them to part sun on the east side, and finally to their final destination, all over a period of a couple of weeks.

So as you can see, there’s more to this humidity thing when you dig a little deeper. Here in the high desert, it sometimes involves a little bit of extra effort to provide conditions that allow tropical plants to thrive. Humidity is only one of the factors that must be addressed, but it is absolutely critical! And remember, the more plants you have, the less of a problem it is. We’ve taken these plants out of their natural habitat and placed them into an artificial environment, thereby creating problems, but we solve these problems a lot better if look to the plants themselves for solutions.