Hi,

Thanks for your question.

Your instincts are right about the reason to mulch. The perennials that we grow in the Twin Cities are winter hardy, but freeze-thaw cycles during the winter can cause the ground to heave and push a plant's tender crown up to the surface where it may then be exposed to warm daytime sun followed by freezing nighttime temperatures. 

You can help most of your perennials, and in some cases pollinators, by leaving at least 6" of vertical plant material in the fall. Stalks that you leave sticking up will naturally collect leaves. You can also add a few inches of leaves from your yard around your plants. Now is a good time. (Iowa State Extension has a detailed article about mulching.) 

For asparagus, cut down the ferns after they've turned brown. As with other perennial plants, you can mulch the asparagus with leaves (or straw) to protect the crowns. Pull the mulch away from the asparagus crowns in the spring, when danger of frost has passed, to facilitate the emergence of asparagus stalks. (University of Maine Extension has information about winter care for asparagus.)

By the time spring and warmer temperatures have arrived, much of the leaf mulch around your perennials will have broken down. If you find heavy layers of wet leaves, remove/compost them to reduce the risk of plant diseases that thrive in wet conditions.

Another thing to think about is protecting any shrubs or woody ornamental perennials such as hydrangea from rabbit browsing. The surest way to do this is by using fabric cloth (36"-48", in case there is a lot of snow acculumation) to create fences that you sink 2-3" into the ground or anchor with u-pins. 

Good luck!