How to grow rosemary in UK clay (without losing it)

Why does clay kill so much rosemary?

Rosemary evolved on dry, stony Mediterranean hillsides, where rain drains straight through gravel in minutes flat. British clay does the opposite. It holds onto water for days. That's not much of a problem in summer; the roots rather enjoy the extra moisture while the plant's flowering. The trouble starts in November and runs through to February: the soil sits saturated, the roots can't breathe, and rot creeps up into the crown. The plant often looks perfectly fine right through to March. Then May arrives and it just collapses.

Everyone blames the cold, but the cold is rarely the real culprit. An established rosemary shrugs off -8°C without blinking. It's -8°C on top of two weeks of waterlogged roots that finishes it off.

The mound technique

There's one change that fixes almost everything: plant it proud, on a low mound. Even 5cm of extra height is enough to shed winter water clear of the crown.

1. 1

   Dig the hole twice the rootball width

   Loosen the sides as you go rather than smoothing them flat with the spade. You want roots pushing outward, not circling a slick clay wall.

2. 2

   Mix the spoil with grit

   Take the soil you've dug out and work in two large fistfuls of 2–6mm horticultural grit plus a handful of coarse sharp sand. Mix it properly. Don't just sprinkle it on top.

3. 3

   Build a mound, then plant proud

   Fill most of the hole with your gritty spoil, then sit the rootball so its top sits 5–8cm proud of the surrounding ground. Backfill around the sides.

4. 4

   Top-dress with grit

   A 2cm collar of grit around the stem stops rain splashing soil back up onto the crown and keeps that all-important root zone dry.

5. 5

   Water once, then walk away

   One good deep soak at planting. After that, only water in a proper drought. Three-plus weeks with no summer rain. Established rosemary genuinely doesn't need irrigation in the UK.

Choosing the right plant

Not every rosemary is built for British conditions. Rosmarinus officinalis 'Miss Jessopp's Upright' is the safest choice for UK gardens. Hardy, vigorous, and it holds its shape well without much fuss. 'Severn Sea' and 'Tuscan Blue' are prettier but more tender; only plant these south of the Midlands, and give them a wall to lean on.

Skip creeping rosemary (R. officinalis 'Prostratus') if you're on clay. Its horizontal habit keeps the crown low and close to wet ground, so the rot risk shoots up. It's a Mediterranean wall-trailer at heart, and it just doesn't suit a British clay bed.

The late-spring cutback

Rosemary needs a proper haircut to stay productive. Skip it and the plant goes leggy, bare at the base, and flowers itself into exhaustion.

The window is late May, once that first flush of flowers has faded and you can see fresh green growth on the soft new stems. Take secateurs to about a third of that new growth, cutting just above a pair of leaves. Never cut into the brown woody stem underneath. Rosemary won't push new growth from bare wood, and you'll leave a bald patch that's there for good.

A second, lighter trim after the summer flowering. Around early August. Keeps the plant tight going into autumn.

A rosemary that never gets pruned is a rosemary you'll be replacing in three years.

Harvesting through the year

From May right through to October, take small cuttings constantly, the more you cut, the more new growth the plant pushes out. Through winter, go easy: once a week, just a small sprig, because the plant isn't replacing what you take until temperatures pick up again in April.

For drying, cut whole stems on a dry midsummer morning, bundle four or five together, and hang them upside down in a dark, airy room for ten days. Strip the dried needles into a jar with a tight lid, and properly dried rosemary will hold its oils for well over a year.