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HOUSEPLANT COMFORT ZONE..Black Thumb Series part 3

Sad plant

As I have stated before, I don’t believe that anybody has a “black thumb.”

However, many people may lack a basic understanding regarding the needs of houseplants. In my last article of this series I discussed watering and lighting, now let’s consider a couple of other factors in our quest for happy, healthy houseplants.


As I mentioned before, most houseplants hail from tropical regions.  Their preferred temperatures range from sixty to eighty degrees (all temperatures that I will mention are Fahrenheit, so if you use the Celsius scale, you get to do the math!), which is the average comfort range of humans as well.  Some tropical plants, such as the Madagascar Dragon Plant (Dracaena Marginata) or the Chinese evergreen (Aglaoenma) will begin to suffer cold damage at fifty-five degrees.

Abrupt temperature changes can stress your plants, so consider their placement carefully.  A plant placed near an outside door where it gets a draft of outdoor air each time the door is opened very well may not thrive, particularly if the outside temperature is much cooler or hotter than the interior.  The same principle applies to plants placed where they may catch a breeze from heating or air conditioning vents.  And even a light loving plant may not appreciate being placed on a sunny windowsill in winter months when the temperature can drop drastically at night and they can feel the chill through the windowpane.  Consider placing them on a table or plant stand in front of the window a couple of feet or so away from the glass.

I don’t know about you, but I like it cooler at night when I’m sleeping.  Houseplants share this preference; they like nighttime temperatures of about sixty to sixty-five degrees, but prefer daytime temperatures of seventy-five to eighty degrees.  However if you’re one of those people that like to keep your house at an even seventy-two degrees or so day round, most houseplants will adapt to this, it’s the extremes of temperature that will stress them out the most, temperatures fifty-five degrees and below, and temperatures above eighty or eighty-five degrees.


In my neck of the woods “humidity” is almost a dirty word!  Summers here are hot!  But it’s the high humidity (moisture in the air) that makes one feel as if they were going to melt; high humidity makes it feel hotter than it actually is.

In the winter, however, when the heating system comes on, the dry air it produces can be even more uncomfortable.  Your lips crack, the mucous membrane in your nose and throat dries up making it almost painful to breathe and your skin dries out and flakes.  Your eyes itch.  Dry air makes the human body more susceptible to asthma, sinusitis, bronchitis, nosebleeds and dehydration.  And dry air makes it feel like the temperature is colder than it actually is.

But wait a minute, we were talking about plants!

Excess humidity and overly dry air are hard on plants, too.  Plants have little openings or pores, mostly located on the underside of its leaves, called stomata.  Stomata are roughly equivalent to the human mouth; it is through these openings that plants “breathe.”  Each stoma is flanked by two cells known as “guard cells.”  Guard cells are roughly equivalent to our lips as it is by these cells retracting or pressing together that the stoma opens and closes.

Generally stomata open in the presence of light and close at dark.  When they are open the plant exchanges gases through them, taking in carbon dioxide and releasing oxygen.  Interestingly enough, this is the exact opposite of what humans do when they breathe in and out.  We utilize the oxygen in the air and breathe out carbon dioxide.  Sort of sounds like plants and people were meant to go together in the grand scheme of things, doesn’t it?

“But what’s all that got to do with Humidity?” you ask.

When stomata are open something else happens.  Through a process known as transpiration plants release evaporated water, or moisture into the air through these stomata.  As this moisture leaves the plant it creates a sort of vacuum effect on the entire plant system, causing more water to be drawn up through tissue called xylem from the roots of the plant, which absorb water through their cell walls from the surrounding soil.  Also through this process dissolved minerals present in the soil (such as those in fertilizer) are distributed throughout the plant.

In periods of high heat and humidity, if the humidity in the surrounding air molecules is too high to absorb any more moisture, evaporation cannot take place and the process of transpiration stops.  This means that the fertilizer the plant needs to remain healthy is no longer being distributed through the plant and, as the process of evaporation has a cooling effect on the leaves, the plant may retain too much heat and effectively ‘cook’ itself.  Fortunately, air conditioners remove a goodly amount of humidity from the air, making the indoor environment more comfortable for both plants and people during hot and humid summer months.

Conversely, if the air surrounding a plant is too dry, it will cause the process of transpiration to speed up.  The plant will keep putting water vapor into the air and keep pulling water up from the roots, which keep sucking moisture out of the soil.  If dissolved minerals (as in fertilizer) are present in any quantity in the soil, too much of it may be distributed through the plant too rapidly; causing such problems as leaf-tip burn.  The soil will dry out quickly and the plant will need to be watered more often.

So just what is the ideal relative humidity for houseplants?  Serendipitously enough, plants are happiest at the same humidity levels that humans are the most comfortable in; between forty and sixty per cent.  And a roomful of houseplants will help to maintain humidity at that level.

But what if you only have a few houseplants?  How do you keep them happy in the presence of overly dry air?  Well, there are a few things you can do.

#1.  Ideally, of course, for both you and the plants would be to use a humidifier to bring the humidity level to the comfort zone that both you and your plants enjoy.

#2.  Lacking that, plants placed in groupings will create their own little island of humidity provided they are placed in a draft-free place where a random breeze won’t blow away their moist environment.

#3.  The other thing that you could do would be to place a tray with pebbles in it under the plant(s) and pour water in the tray almost to the top of the pebbles.  But you don’t want to set the planter pots directly in water; this could result in soggy soil and root rot.


Houseplants and people really do go very well together.  In addition to filtering harmful toxins out of the air that you breathe (see previous post: Why Have Houseplants?), plants take in the carbon dioxide that you breathe out and exhale the oxygen that you need to breathe in.  And by maintaining household temperature (sixty to eighty degrees) and humidity (between 40 and sixty percent) levels that are ideal for plants, you will be maintaining the temperature and humidity levels that are the most healthy and ideal for you as well.

And one added bonus: if you raise the humidity in your house during the cold winter months when your heating system is drying out the air, you will be able to lower your thermostat and save on heating expenses while maintaining your (and your plants) comfort zone, remember; humid air feels warmer and dry air feels colder!

Happy Indoor Gardening, and y’all come back and see me, ya hear?




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