Kerria japonica 'Pleniflora' (also called Bachelor's buttons and Japanese rose).
Note:Centaurea cyanus is also known as Batchelor's buttons, which shows why botanical names are useful for avoiding confusion.
Hardy, deciduous, suckering shrub grown for bright yellow flowers that bloom in spring. Suitable for an open border or can be wall/fence trained. Originates in the woodlands of China and Japan.
I was never very keen on Kerria japonica until I saw it grown against the latticed trellis, backed by bamboo matting, shown the photo above. That background showed it in a whole new light to me.
Flowers and leaves in spring. Leaves from spring to autumn.
Habit - arching, upright, slightly lax. S: 2.5m, H: up to 3m.
Stem - arching, green, round.
Leaves - lanceolate with serrated edges, bright green.
Flowers - double flowered in a cup shape, bright yellow, up to 3cm across.
Full sun to partial shade.
Fertile, well drained.
After flowering has finished, remove the older stems to make space for new shoots to grow.
Cuttings in summer or layer for 12 months and then remove the cutting and pot up.
Garden Plant Information list of plant care info by botanical name