Perennials—uncovering in spring
Winter mulches of leaves and straw should be removed when the plants they're protecting begin active growth in spring. Mulch is placed over the plants in fall as soil begins to freeze, to insulate them and prevent premature growth caused by early thawing. Once the soil warms enough for growth to start, the mulch has lost part of its function.
Timing is important for removal of winter mulch. If you wait too long, plants may be damaged by mold. Pale, weak growth that developes beneath the mulch may also be damaged by sun and wind when the cover is removed. And heavy rains at this time of year may cause some plants to "suffocate" from matted down covering material. On the other hand, if mulch is removed too soon, you may encourage early growth that will need to be covered again if frost threatens.
Removing winter mulch about mid-April in the Twin Cities Metro area is right most years, but wait another week for every hundred miles north of the Cities. You can also check for the start of new growth or mold development to help you decide. Though daytime temperatures are often in the fifties, evening temps may still be near freezing. Keep some of the mulch handy in case frost is predicted.