How to Prune yarrow 'Hella Glashoff'?
Cut back untidy foliage in spring

yarrow 'Hella Glashoff' · Achillea 'Hella Glashoff'
An open sunny position in a moist but well-drained soil is best, but it will tolerate most situations apart from heavy, wet clay in winter
Cut back untidy foliage in spring