Watering houseplants, like any other plants, depends on the type (species) of plant: some plants need more than others. As a Master Gardener, I’ve run into a lot of people who are disappointed when they ask me how to water houseplants; they want a quick, easy answer that requires as little thought as possible. Too bad.
Actually, it depends on more than the species of plant; it also depends on the individual plant: size matters, as well as overall bushiness and health. The type and size of pot it’s in, the type of growing medium it’s in, the location in the room, the humidity, the temperature, and the time of year (for plants that stay inside year-round) all have a part to play as well.
Plants in pots/containers need to be watered more than plants growing in the ground outside. Plants in the ground have the reservoir of groundwater to tap into even when the weather is dry, at least for a while. Conversely, a potted plant (inside or out) is like a housepet: it’s completely dependent on you for its welfare. Not only do such plants not have access to groundwater, but pots don’t hold a great deal of water when they’re already filled with soil and plant roots. Altogether, this means that potted plants must be watered regularly, even those that prefer drier conditions. How regularly – and how much – those are the factors that change from plant to plant.
In general, plants that grow in dry areas (the Mediterranean, deserts) need less water. Plants from moister areas (rainforests, valley floors, riversides) need more water. Common sense.
When it is warm, plants transpire (lose water through their leaves) more. Similarly, transpiration increases when the air is dry. These situations mean the plant needs more water. Plants with large leaves transpire more than plants with small leaves. Larger plants, or bushier plants, have more leaves and also transpire more. Many plants adapted to hot or dry climates (or both) have a waxy coating on their leaves and/or stems that drastically reduces transpiration, and thus the need for water.
Plastic, glass, glazed ceramic, metal or other nonporous containers hold water well. Terracotta pots, however, or other unglazed earthenware ceramics, are porous. They wick water out of the soil, so they are best for plants that like dry conditions. Something that prefers moist conditions will need to be watered more frequently if planted in a porous container.
I know I said growing medium has a part to play, and it does. However, ideally you will have chosen the medium according to the needs of the plant, so it’s less about the medium and more about the plant. For example, cactus mix is free draining while most standard potting mixes are pretty water retentive.
Most indoor plants slow down their growth during winter, a result of less light and lower temperatures. They need less water than plants in active growth. A plant in semi-dormancy needs even less water, and a fully dormant plant needs no water at all (although you probably won’t be thinking of a fully dormant plant as a houseplant).
In balance, this all means you water houseplants less in winter, especially if you keep the air moist (I will talk about humidity as a separate topic). Most plants will need only two-thirds to one-half the water they would need outside in summer, and sometimes less. For some plants, especially those that aren’t growing much and are kept in constant conditions, you can work out a more-or-less consistent routine of when to water and how much. In general, though, the best way to know when to water is to use your eyes and your brain (and possibly a finger).
To be continued.