1 litre sized pot.  This Achillea (Fern-leaf yarrow) is a clump forming perennial which has grey-green aromatic, soft fern-like foliage and gives rise to flowers held 50cm+ on strong stems.  The flower umbels are a brilliant yellow, made up of many tiny flowers loved by bees and beneficial insects.  They like a sunny, well-drained position and are tolerant of dry conditions.