These prolific plants bloom so hard and full for the entire season that they soon fade away. As a perennial, these only last three to five years in good cultivation. Wallflowers can be cut back after blooming, or you can wait until the end of winter or very early spring. Cut them to nearly the ground, and the plant will reward you with a new flush of growth when temperatures warm up.

Depends which one - if you've got Erysimum, best not to prune it hard, just clip it over lightly - they can get die back in winter, and if you've already cut it, its more likely to die.

Shear or clip off dead flower stems, and lightly shear the whole thing in spring, taking a few leaves if its getting a bit leggy.