Centaurea macrocephala (Giant knapweed):
Huge, bright yellow almost fluffy flowers in mid-summer follow attractive papery buds. Attractive to bees. Height to 1.5m. Hardy.
Cephalaria gigantea (Giant scabious):
Very tall wavy stems topped with large lemon yellow scabious-like flowers.Easy to grow. May need staking or support. Hardy. Up to 2m.
Helianthus 'Monarch':
A perennial sunflower with tall, semi-double bright yellow flowers. Prefers a sunny spot. Flowering late summer. Hardy. Up to 2.5m.