UK compost
What compost for large-flowered petrocosmea in the UK?
Petrocosmea grandiflora
More about large-flowered petrocosmea in the UK
Which compost large-flowered petrocosmea needs
For large-flowered petrocosmea the mix to buy is peat-free multipurpose compost cut roughly half-and-half with horticultural grit or perlite. The RHS recommends a shallow pot of very well-drained, peat-free, loam-based compost with added grit and leafmould. A 1:1:0.5 ratio of loam compost, coarse grit, and leafmould suits the plant well. Use wide, shallow alpine pans for best results.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 large-flowered petrocosmea 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?
Large-Flowered Petrocosmea 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 large-flowered petrocosmea soil & potting-mix guide.
Compost for Large-Flowered Petrocosmea in the UK — frequently asked questions
What compost should I use for large-flowered petrocosmea in the UK?
Use peat-free multipurpose compost cut roughly half-and-half with horticultural grit or perlite. The RHS recommends a shallow pot of very well-drained, peat-free, loam-based compost with added grit and leafmould. A 1:1:0.5 ratio of loam compost, coarse grit, and leafmould suits the plant well. Use wide, shallow alpine pans for best results. 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 large-flowered petrocosmea?
Not on its own — multipurpose compost holds too much water for large-flowered petrocosmea 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 large-flowered petrocosmea perfectly well — they dry a little faster at the surface, so check moisture by feel rather than by the look of the top.
Does large-flowered petrocosmea need grit or perlite added?
Yes — large-flowered petrocosmea 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 large-flowered petrocosmea need?
Always a pot with drainage holes. Very well-drained, peat-free loam with grit and leafmould. 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 large-flowered petrocosmea care
See the full large-flowered petrocosmea care guide, its UK watering and UK hardiness.