Roses that enter winter healthy, their moisture and nutrient needs having been met through the growing season, are more likely to emerge vigorously the following spring. Wait until nights are consistently below freezing and the soil surface freezes—usually in December where I live—before applying winter mulch. This is a method I recommend:
1. Find a cardboard box that fits around the plant to be protected and is about 12 inches tall. Fold in the bottom and top flaps so they are flat against the sides. Place the box around the plant.
2. Fill the box with small bark nuggets or any other type of mulch that won’t pack down or become waterlogged.
3. Tie stout twine around the box so it won’t come apart when it gets wet.
4. For extra protection (or if you don’t like the look of cardboard), drape evergreen boughs over the plant. Long canes can be trimmed at this time, but in general it’s best to wait until spring to prune roses.
When the weather warms and the rose’s leaf buds begin to swell, remove the box and spread the mulch around the rose plant. You’ve just accomplished two tasks in one: removing the winter protection and refreshing the growing-season mulch—a double benefit you wouldn’t get by using a soil mound.