I think humidity is a reasonable topic to follow watering. For some reason, it doesn’t get as much attention as other factors such as light and water, but it can have a huge impact on houseplants.

There are two measures of humidity: absolute and relative. Absolute humidity is a simple measure of the amount of water vapour present in the air. Relative humidity is a comparative measure of how much water vapour is present vs. the amount of water vapour the air is currently capable of holding. The capacity for water vapour changes according to such factors as temperature and atmospheric pressure. Both measures are expressed in percentages. The same sample of air could have an absolute humidity of, say, 3% and a relative humidity of 90%. Weather reports generally give relative humidity.

Most newer urban and suburban buildings in developed temperate-climate countries these days use central heating in winter. Well, they do around here. I think. And central heating is very good at stripping the moisture from the air. Plants like a certain level of humidity in their environment, although most will eventually adapt to lower humidity (up to a point) if they survive long enough. Even many plants from naturally dry regions often dislike the arid air of a centrally-heated house. On the other hand, most succulents barely notice, being naturally adapted to the extremely dry air of a desert.

When I was much younger, I learned a hard lesson about acclimatising plants when bringing them in for winter. I wanted them to stay outside and growing for as long as possible and only thought about bringing them in when temperatures got too cold. By then the heating would already be on and I couldn’t understand why many of the plants would suddenly drop most of their leaves while they were still green. I eventually figured out it was the (lack of) humidity, but not before I lost a citrus bush and a couple of miniature roses and store-bought bonsai. Now I bring them inside well before the heat goes on, even if the weather is fine. Experience teaches which plants are bothered more.

A random note about bonsai: despite how they are depicted on television and in movies, they are NOT houseplants. However miniaturised, they are trees and need to be outside, and deciduous trees need a dormant stage during winter. Any given bonsai should be kept indoors for only a couple days out of any week and only about five days in any month (or better yet, not at all). The only time you keep bonsai in the house for extended periods is to overwinter tender species, and those usually need special measures to keep them healthy.

It can be difficult to regulate humidity in the house, but it’s well worth it for both humans and plants. For most of us, it’s more pleasant and healthier to breathe somewhat moist air and it reduces chapped lips and dry skin. Moist air feels warmer than dry air, even when it isn’t. In fact, drying the air was one of the basic principles behind early air conditioning.

As far as plants go, the air can’t be too humid, but that probably won’t be a problem. Arid-climate plants will be just fine in moist air. Stagnant moist air can encourage fungal diseases, however, so if you manage to make your house, or at least your plant spaces, very humid, good air circulation is important. Anyone with a greenhouse can tell you that. And you wouldn’t want it humid enough to make the room damp, even if you could.