How to overwinter achillea ptarmica 'the pearl'

Achillea ptarmica 'The Pearl' is hardy to roughly USDA zone 3 — winter lows around -40°C. That means it can stay outside all winter in most temperate gardens. Achillea ptarmica 'The Pearl' is properly hardy and overwinters outside without fuss. Tidy away anything dead or diseased, apply a mulch over the crown to steady soil temperatures, and otherwise leave it alone — many hardy plants actually need a cold period to flower and grow well the following year. Save hard pruning for the right season rather than doing it in autumn.

Tidy and mulch in late autumn; leave the plant undisturbed until spring.

No winter watering needed in open ground. Containers only need water in long dry, frost-free spells.

Cutting achillea ptarmica 'the pearl' hard back in autumn "to tidy up". That removes the growth that shelters the crown, and on some plants removes next season's flower buds entirely.