The air in our homes is particularly dry during the winter. Indeed, relative humidity drops like a rock when heating systems start operating. You see, when you heat air, its relative humidity decreases, and the more you heat it, the drier it becomes. By midwinter, the air in our homes is often drier than the air in the Sahara Desert! And many plants suffer terribly from this atmospheric drought, with leaves that curl, turn yellow and drop off, flower buds that abort, greater susceptibility to insects, etc.
If you are looking for ideas on how to increase the atmospheric moisture for your plants during the winter, I suggest you read High Humidity Makes for Happy Houseplants. But maybe you don’t have to be concerned about dry air. It all depends on the type of plants you’re growing.
That’s because not all plants suffer from dry air. Those that do, like flowering maples, brugmansias, palms, ferns, etc., usually have thin leaves that numerous stomata (breathing pores) that transpire abundantly. Many quickly wilt when the air is dry; others sulk and fail to thrive; some die. But plants with thick, waxy leaves, like succulents (crassulas, aloes, cacti, etc.), rubber plants (Ficus elastica) and peperomias, are able to tolerate dry air without any difficulty. How come?
Evolution Provides a Helping Hand
Most thick-leaved plants come from climates where the air is naturally very dry and have had thousands if not millions of years to get used to it. The wax that covers their leaves helps reduce water loss and often too they have proportionally fewer stomata that plants with thin leaves.
Many also take advantage of what is called an crassulacean acid metabolism (CAM), a type of photosynthesis in which the plants essentially close their stomata during the day, when the air is drier, and only carry out respiration only at night, when it is more humid (as temperatures drop, as they usually do at night, relative humidity increases). CAM greatly reduces water loss.
Others have abandoned leaves completely and carry out photosynthesis through their green stems. This again reduces the number of stomata and therefore water loss. Cactus are the best known “stem succulents”, but there are others, notably stapelias and euphorbias.
Even the sap of succulent plants is designed to reduce water loss: it is thick and mucilaginous, reluctant to release the water molecules it contains. In fact, succulents got their name from their thick sap. The term derives from the Latin “suc”, which refers to a thick juice.
Although they may tolerate dry air, succulents and other plants with thick foliage are not bothered by humid air. So if you set up, for example, a humidifier to help your thin-leaved houseplants get through the winter, your succulents will be just fine. It’s simply that they don’t really get much of an advantage from more humid air.
So, do what you can to help your thin-leaved houseplants to survive the winter, but those with succulent leaves or thick stems readily put up with the air around them, be it dry or humid. One less thing for the indoor gardener to worry about!