Perennials that bloom in early spring need to be cut as soon as they finish blooming. This way they will be mildew free and have new clean leaves.
Spring bloomers need to be able to flower and leaf freely. Cutting back after blooming helps prevent early spring disease.
They can be cut back hard, or to the ground, if necessary. Nurseries do this all the time, but it is not recommended.
Late summer or early fall blooming is different. If you let them get too big and cut them back, they will produce multiple shoots. Fall perennials must be cut back if you want a great show.
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