Achillea ptarmica The Pearl
This hardy herbaceous perennial makes a great groundcover. Like Achillea ptarmica 'Double Pearl', its bright green foliage emerges early spring, followed by long stems topped with multiple small white pompom flowers but 'The Pearl' has more singular flowers than the doubles frequently seen with 'Double Pearl'. The stems tend to flop but the masses of white flowers make up for this. Likes full sun, well-drained soil.