The best way to guarantee bright, healthy spring and summer flowers is to pamper them year round, including during the winter. Pull annuals up in the fall—or pot up perennials like geraniums that may be grown as annuals in your area. In areas where the snow falls in November and a good layer stays until the spring thaw begins, little protection is needed for hardy perennials but for those who have to cope with the freeze-thaw and desiccating winds of many North American winters, a well-executed fall program will reward you with more survivors.
Water plants as usual through the first frost to give roots plenty of moisture to get through the winter. Stop fertilizing perennials, though, with a final half-strength feeding in mid to late August, depending on your growing zone.
Pot up geranium, calendula and other tender plants to be "kept over" inside. Set them in sheltered positions and water until they’ve overcome transplanting shock. Take them inside and give them the brightest window to acclimate them to indoor living for at least a week before you turn on the heat.
Trim wilted perennials after the first frost. Hardy chrysanthemums may keep blooming for several weeks but most perennials will die back and go dormant; trim them back to the ground, leaving an inch or two of stem for identification. Clean leaves, branches and other debris from around plants and pull back the summer mulch.
Winter-mulch perennial plants with a 2- to 3-inch mixture of garden soil and compost after the first freeze when the ground begins to cool below 60 degrees F; this should keep them safe for the winter. Cover the tubers and rhizomes of surface-dwelling perennials like iris and peonies grafts with a thicker covering of mulch and mound garden soil on tender herbaceous perennials like garden roses.
Wrap shrubs (like roses or tree peonies) with paper or burlap to protect them from winter winds. Most hardy shrubs, like lilacs do not need winter wrapping but very young or tender shrubs may survive better with a wrap. If insulation is needed, compost can be added from the top of the wrap.
Remove wraps and mulches before the ground begins to warm in late winter. This will evict any winter-settled critters before the first tender shoots start growing and allow perennials to “wake up” normally as spring begins.