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Well, the winter soil requirements for houseplants aren’t any different from any other season. As usual, choose a growing medium that suits the needs of the plant.

I say “soil” but soil as such is usually not considered a great choice for potted plants. Certainly the potting mix manufacturers want you to believe that. There is an element of truth to it, however.

Soil from the garden tends to get too compacted in a pot after a while. Plant roots need oxygen, and water needs spaces between soil particles in order to penetrate. When the soil in a pot settles too much, the plant can suffer. Also, if you use soil from the garden, you can bring all sorts of unwanted guests into the house with it. If your soil is naturally sandy, you could get away with it; clay soil is just the opposite and will start to compact very quickly.

Outside, frost action in winter and the activity of soil fauna (worms, insects) naturally keep the soil aerated. Earthworms don’t do well in enclosed spaces (composting worms are different) and most people don’t want insects skittering about their houses. As for frost action, well, if you keep your potted plants in a place where frost hits them, they’re not really houseplants, are they?

The addition of lots and lots of organic matter more-or-less solves the problem – compost, well-rotted manure, and leafmould are all good choices for most houseplants. Orchid mixes contain lots of large pieces of bark, and cactus/succulent mixes contain various components to keep the mix lean and free-draining. Perlite, vermiculite, and crushed pumice are inorganic minerals that can also keep the mix light.

I personally don’t believe that peat (or peat moss) is harvested in a responsibly sustainable manner, so I avoid using it, or mixes based on it, when possible.

Having said all that, there are ways you can get away with using topsoil or even garden soil in containers. As I said above, adding various materials slows down or even prevents compaction; in effect you make your own potting mix. If you intend to repot the plant on a regular basis (at least once a year) with fresh soil, that essentially makes the compaction problem irrelevant. There are also a few plants that don’t seem to be bothered by compaction at all. People as gardening-besotted as I am often make their own potting mixes based on what they have at hand (or can get), but many people will find it easier to just buy the manufactured stuff.

A potting mix doesn’t need to contain soil at all – a lot of the prepared ones are soilless. Pure garden compost, or compost mixed with leafmould, perlite, or vermiculite, is an excellent potting medium, IF the plant likes a lot of fertility and moisture. Other plants that prefer their growing media to be drier or poorer (or both) include succulents, carnivorous plants, and the various Mediterranean herbs (thyme, sage, oregano, lavender, rosemary). Don’t put those in a rich, fertile soil – they will not do well.

Remember that just as containers don’t hold a great deal of water, the soil in them has a limited amount of fertility. Most containerised plants need to be fertilised regularly during the growing season, some more than others, and a few not at all. However, during winter most houseplants slow down and fertilising should be reduced or stopped entirely.

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