Choosing the right compost for rare alpine plants

Why alpines refuse to behave like ordinary garden plants

Alpines come from places most garden plants would consider a hardship posting: mountain scree, limestone pavements, high meadows where the snow lingers until June and the summer sun bakes the ground dry by August. What they get there is sharp drainage, thin soil, plenty of air around the crown, and roots that run deep into cool, moist crevices rather than sitting in a soggy pan of water.

Bring that plant into a British garden and the two things that kill it fastest are rich compost and winter wet. Not cold — most alpines shrug off temperatures that would see off a pelargonium. It is the combination of a nutrient-heavy mix and months of drizzle that does the damage, rotting the crown from the inside out while the leaves above still look perfectly healthy. Get the compost right and you have solved perhaps eighty per cent of alpine growing.

The golden rule: drainage first, fertility a distant second

Most alpines are adapted to low-nutrient conditions and actually resent being fed. A rich, moisture-retentive compost encourages soft, sappy growth that is vulnerable to botrytis and crown rot, and it pushes the plant into flowering itself to exhaustion. What you want instead is a mix that holds just enough moisture to keep roots plump through a dry spell, then lets the surplus drain away within seconds.

A good working ratio for most hardy alpines is roughly one part loam, one part leaf mould or composted bark, and two parts horticultural grit by volume. That gives you a lean, open, gritty medium that behaves more like scree than like border soil. For cushion-forming species, dwarf bulbs in troughs, or anything with a tight rosette, push it further — one part loam to three parts grit is not excessive.

• Loam provides body and a little nutrition, but use a sterilised, low-fertility loam rather than scooped garden soil.
• Leaf mould or composted bark adds gentle humus and helps the mix hold together without becoming claggy.
• Horticultural grit is the backbone. Without it, the rest is just potting compost.

If you are growing in a trough or pan, add a layer of crocks or coarse grit at the base, then fill with your mix. Never use ordinary multi-purpose compost straight from the bag for alpines, and never, ever use garden soil in a container.

Choosing your grit — and getting the pH right

Grit is not simply grit. For lime-hating alpines such as most rhodohypoxis, gentians and dwarf rhododendrons, use granite, quartzite or a washed river grit, and keep the mix on the acid side with leaf mould. For lime-lovers — encrusted saxifrages, dianthus, most androsace and the silvery lewisias — a limestone or dolomitic grit is ideal, and a handful of limestone chippings worked through the mix will keep them happy.

Particle size matters too. Aim for grit in the four to six millimetre range. Dust and fine sand are the enemy: they fill the gaps between larger particles and turn an open mix into something resembling concrete. If you want to lighten a mix further without adding weight, a little perlite works, though it tends to float to the surface over time.

Planting, top dressing and the collar question

When you plant, set the crown slightly proud of the surrounding mix rather than burying it. For cushion plants and anything with a tight rosette, a collar of grit — clean, coarse, three to five millimetres — fanned out around the neck of the plant is one of the single best defences you can give it. Rain runs off, air circulates, and the base of the plant stays dry even after a fortnight of drizzle.

This top dressing also suppresses liverwort and moss, which otherwise creep across the surface of troughs and smother small plants. Refresh it annually, and top up the mix beneath if it has slumped. Repot container-grown alpines every two to three years in late summer or early autumn, teasing out the old compost and trimming any roots that have circled the pot.

Watering without drowning the crown

Water in the morning, at the base of the plant, and never over the foliage if you can help it. Wet leaves overnight in a cool British autumn are an open invitation to fungal trouble. In summer, water thoroughly and then let the mix dry slightly before watering again — a soak-and-dry cycle suits alpines far better than a constant trickle.

Rainwater is preferable, particularly for lime-haters, since tap water in hard-water areas will gradually raise the pH of a small trough. In winter, most established alpines in open ground need no water at all. In containers, keep the compost barely moist and make sure the pot is raised on feet so it never stands in a saucer of water. A sheet of glass or a cloche over a trough from November to March, propped open at the ends for airflow, will bring tricky species through the wet months far more reliably than any amount of careful watering.

Watch for the warning signs: a plant that rocks slightly in the ground, a brown patch spreading from the centre, or a soft, discoloured crown. Lift it, cut away the rot with a clean blade, dust the wound with sulphur, and replant into fresh, grittier mix in a better-drained spot. Alpines are forgiving of a great deal — but never of a wet, heavy root run.