If the # 1 houseplant problem during the winter months is giving them enough light, the # 2 problem is making sure they get enough atmospheric moisture (humidity). The air in our homes becomes incredibly dry during the winter because heating systems remove moisture from the air. The colder it is, the more you heat … and drier the air.
How to Tell If Your Plants Suffer from Dry Air
How can you tell if your plants are suffering from a lack of air humidity?
First, dry air especially affects the plants with thin leaves. Plants with thick, leathery or waxy foliage are relatively resistant to dry air, as are plants with very hairy leaves. They do suffer, but not as obviously as others. Succulents—crassulas, sedum, cacti, etc.—are in this category, as are several peperomias and also the rubber plant (Ficus elastica).
Plants with thin foliage—brugmansia, abutilons, palms, ferns, etc.—suffer most from dry air. Often their leaves curl slightly down, their edges or tip dry out or blacken, they may hang limply even after you water or, most obvious of all, they simply turn yellow and drop off, often in great numbers. Often, they become infested with spider mites which proliferate in on plantes stressed by dry air.
Logically, houseplants should need less watering in the winter, since they grow very slowly if at all. If you need to water them as much as during summer or even more, that’s because they are losing a lot of water to transpiration and are not very happy.
A secondary symptom is poor blooming. When the air is dry, flowers dry up, wilt or abort or just don’t last as long as they should, even if the plant itself is relatively resistant to air dry. The thick, hairy leaves of the African violet, for example, are quite resistant to dry air, but the flowers suffer.
Not Good for Humans Either!
People also suffer from dry air: dry skin, itchy eyes, irritated nasal passages, static electricity shocks, increased frequency of respiratory ailments, etc. It would be in the interest of both the mammalian and green inhabitants of your home to do something to correct it. In general, aim for a relative humidity of 50%. That’s just right for humans and pets and it’s “acceptable” for most plants (although many would prefer 70% and more).
A hygrometer can be very useful in determining atmospheric humidity. There are inexpensive digital models sold in almost every hardware store. Often they tell the temperature too: two birds with one stone! You can buy one and move it here and there to test local conditions. In most homes, you will indeed find both drier and more humid zones. It’s in the latter that plants will do the best.
How to Increase Atmospheric Humidity
For the reasons mentioned above, it is always wise to increase humidity in the rooms where you grow houseplants. The most obvious way is to run a humidifier. A central humidifier will increase the humidity throughout the entire house. A room humidifier will, as the name suggests, increase the humidity in a single room.
For those plants that need more than 50% humidity (again, mostly thin-leaved plants), you can create an extra-humid micro-climate by growing them over a humidity tray. Or simply group several plants together: since each plant gives off moisture through transpiration, the more plants you have and the closer they are together, the greater the atmospheric humidity.
Lowering the thermostat at night temporarily increases the ambient humidity and is also very beneficial. Or grow your plants in a room that is naturally more humid than others, such as a bathroom or laundry room (assuming that there is some sunlight!).
To maintain high humidity at all times for the most sensitive plants, place them in a “greenhouse” for the winter. This can be any structure with a transparent roof and walls, such as a terrarium … or a clear plastic bag. Inside such a structure, it’s easy to maintain humidity levels of 80% and more. Needy plants will thus remain in superb condition throughout the “dry season.” Humidity in a terrarium or a closed bag is as high as in a jungle and most plants simply adore it!
Spraying Is a Waste of Time
One thing that doesn’t work is spraying plants manually with water, traditionally done with a recycled Windex bottle. Yes, you often see this technique recommended, but people who do so haven’t done their homework. Studies going back as far as the 1970s have shown it is a total waste of time. The effect only lasts a few minutes, not enough for plant leaves to react. And it stains the leaves. Don’t waste your time doing it.
So there you go: a few tips on how to keep your houseplants happy during the heating season: put them into practice and you will soon have a real jungle of thriving foliage in your home!