"when I cut back my summer garden, can I use the leaves as mulch or should I trash them?"

You can use any organic matter for mulch. When the stuff disintigrates, dig it into the ground and use newer leaves/stems. Mulch is good for keeping the water in the soil so it doesn't evaporate so quickly, which is also good water conservation. The mulch is great to keep the ground around your rose bushes and other perennials from freezing so the cold doesn't kill the roots.

Bleeding heart greens are fragile and have very little "heft:" they'll burn up with repeated frosts. Ferns have long been used as bedding for livestock and should be okay as mulch, but check them and the hostas for egg sacs from garden pests, especially if you had problems with slugs during the summer.

Rose gardeners usually shun old garden cuttings for mulch as these often carry bacteria, mold and plant diseases. My own preference is to use oak leaves or hay for mulch: they don't pack into a mat under moisture and have pockets of air that loft up and provide better protection to your mulched plants.

Hope that helps.