Jean, unless you have a perennial wallflower you don't prune them. When flowering is over, late spring, dig them up. In their place plant summer subjects. If you want a perennial wallflower look for Erysimum Bowles Mauve.....this has glaucous blue foliage and purplish flowers the whole summer long. Wallflowers naturally grow in poor soil but I grow mine on in the veg patch so that they make bushy plants some 2' high that make impressive impact