This grower is sure of this plant identification.
Plants supplied by us shown flowering at Aberglasney Garden, Carmarthenshire, Wales
Goat's beard spiraea
Aruncus dioicus (Goat's beard spiraea). This is a clump-forming perennial species. It will produce large fern-like leaves that are easy to grow. The flowers are small white and will bloom in the summer.
Under shrubs, border, mixed bed. Summer interest.