How to divide achillea ptarmica 'the pearl'

Achillea ptarmica 'The Pearl' spreads by rhizomes — horizontal underground stems that send up new shoots as they run. That makes it straightforward to divide: lift the plant and cut the rhizome into sections, making sure every piece carries both roots and at least one growing point. Dividing also rejuvenates it, because old central rhizome tends to lose vigour.

Early spring as growth restarts is the safest window; after flowering also works for many plants.

Lift the plant, wash enough soil off to see the rhizome clearly, then cut it into sections with a clean knife — each with roots below and a visible bud or shoot above. Discard any soft, hollow or woody old sections from the middle.

Pot each division into fresh compost at the same depth it was growing before, water it in well, and keep it somewhere bright but out of direct sun for two to three weeks while new roots take hold. Do not feed straight away — damaged roots take up fertiliser poorly and it can scorch them.