|Grown for its large, white, semi-double flowers. This popular, vigorous, long-lived perennial with a woody base reaches 4 to 5 feet tall. Has arching, graceful stems to 3 feet tall. Plants in areas with harsh winters will benefit from a fall mulching. Staking may also be necessary, especially to keep plants upright after a rain. Divide in fall or early spring. Slow to establish, but then spreads rapidly.