May: The Chelsea Chop
To create sturdier plants that need less staking and to delay or extend the flowering period, cut some of your perennials back at the end of May (when the Chelsea Flower Show happens). Simply cut the plants back by half. To stagger the flowering period, if you have several of the same plants, cut only some of them back. This technique works well for Phlox, Sedum, Rudbeckia and Echinacea.