The plant will then put its energy into producing more flowers. You will have a bushier plant with more flower blooms on it. Bonus - Some perennials will have their biggest burst of blooms in the spring but will actually bloom a second time in the fall if you cut off the first flush of flowers as they start to fade. Remember though to keep the leaves and stems intact on the plant. These will continue to soak in the sun's energy putting the good back into the plant's roots.
Nature provides the best model for mulching.
If you walk through the woods you'll notice a thick padding of leaf litter on the forest floor. Through this blanket the spring wildflowers emerge. Mulch away anytime to about 3 inches thick, but if you mulch early in the spring you'll maximize the benefits - the extra warmth will act as an incubator for newly planted seeds and seedlings giving them a boost. Mulching early also gives weeds no opportunity to take hold. Organic mulches include compost, grass clippings, leaves, straw, peat moss, sawdust, dry manure, and bark chips.