UK compost
What compost for square-stemmed pelargonium in the UK?
Pelargonium tetragonum
More about square-stemmed pelargonium in the UK
Which compost square-stemmed pelargonium needs
For square-stemmed pelargonium the mix to buy is peat-free multipurpose compost cut roughly half-and-half with horticultural grit or perlite. Use a cactus compost or a 1:1 mix of loam-based compost and coarse grit or perlite; this species must never sit in moisture-retentive growing medium as the semi-succulent stems rot quickly in wet conditions.In British garden centres the bagged growing medium is sold simply as “compost” (multipurpose, ericaceous, or loam-based John Innes), which is a different thing from the rotted garden “compost” you make in a heap — for a pot you want the bagged kind.
Peat-free compost
Buy peat-free. The sale of peat compost to home gardeners is being phased out across the UK, and the RHS recommends peat-free on environmental grounds. A good peat-free multipurpose grows square-stemmed pelargonium perfectly well; the one habit to change is watering — peat-free dries faster at the surface while still moist below, so check by feel a knuckle deep rather than trusting the look of the top.
Ericaceous or multipurpose?
Square-stemmed Pelargonium does not want a rich, water-holding compost — it wants sharp drainage. Cut peat-free multipurpose roughly half-and-half with horticultural grit, sharp sand or perlite, and always pot into a container with drainage holes. A "cactus and succulent" bagged mix is a ready-made shortcut.
For the full recipe, pH and drainage detail (US wording), see the square-stemmed pelargonium soil & potting-mix guide.
Compost for Square-stemmed Pelargonium in the UK — frequently asked questions
What compost should I use for square-stemmed pelargonium in the UK?
Use peat-free multipurpose compost cut roughly half-and-half with horticultural grit or perlite. Use a cactus compost or a 1:1 mix of loam-based compost and coarse grit or perlite; this species must never sit in moisture-retentive growing medium as the semi-succulent stems rot quickly in wet conditions. In UK garden centres this is sold simply as "compost" — the bagged growing medium, not garden-made leaf-mould — so match the description above rather than a brand.
Can I use ordinary multipurpose compost for square-stemmed pelargonium?
Not on its own — multipurpose compost holds too much water for square-stemmed pelargonium and will rot the roots. Cut it roughly 50:50 with horticultural grit, sharp sand or perlite so it drains fast.
Should the compost be peat-free?
Yes. Sales of peat compost to home gardeners are being phased out in the UK, and the RHS recommends peat-free for environmental reasons. Modern peat-free multipurpose composts grow square-stemmed pelargonium perfectly well — they dry a little faster at the surface, so check moisture by feel rather than by the look of the top.
Does square-stemmed pelargonium need grit or perlite added?
Yes — square-stemmed pelargonium must have sharp drainage. Add about one part horticultural grit or perlite to one part compost, and always use a pot with drainage holes.
What pot and drainage does square-stemmed pelargonium need?
Always a pot with drainage holes. Very gritty, fast-draining cactus or succulent compost, pH 6.0-7.0. Stand it on a saucer, empty any water that collects after watering, and never leave the pot sitting in a full outer cover — waterlogged compost in a cool UK room is the commonest cause of root rot.
More square-stemmed pelargonium care
See the full square-stemmed pelargonium care guide, its UK watering and UK hardiness.