Canna is a genus of perennial subtropical flowering plants grown from underground bulbs or, more precisely, rhizomes. Cannas flower in summer atop tall, rigid stalks with thin, spear-shaped foliage. Cannas grow year round in warmer climes and need to be trimmed back each year in cooler climes when the flowers and foliage fade.
Trim back spent canna flower stalks after the flowers at the top wilt and die back only if the plant is so untidy looking that it mars the appearance of the plant or garden. Use loppers to cut the stalk down at the crown of the plant between the leaves a few inches above the soil.
Trim all of the spent flower stalks and dying foliage in the late fall or winter. Using loppers, trim the plant down to the crown just a few inches above the soil. Leaving the dying foliage in place for a few weeks or a month will allow the plant nutrients remaining in the top foliage to be redirected back into the underground rhizome to support the following seasons' bloom.
Harvest fresh canna flowers with secateurs at their peak of bloom for use in cut flower arrangements during the summer. Cut the flower stalks down to the base of the plant, trim to fit the vase and place into tepid water immediately.