Plants, even the hardiest of varieties, are prone to winter damage. It used to be the rule of thumb to cut back all plants before winter, but that school of thought has eased up. That's because some plants provide winter interest and others provide berries and seeds to birds or cover to animals. Still, there are certain instances where you should cut plants back to promote their growth and good health.
Prepare plants for winter before the snow starts falling, but after the first hard frost. Even hardy plants are susceptible to winter damage.
Cut back plants that are in water gardens, such as water lilies. Wait until the foliage turns brown and then cut it off with pruning shears. Lower the plant into the deepest part of the water, to protect it from freezing.
Trim off all the foliage on plants that had an insect infestation or disease with your shears. Bag the trimmings with your trash or, better yet, compost them.
Winterize plants that don't bloom or show color of any kind in the winter. Cut them down to the ground, leaving the rootballs in the ground. This includes geraniums and veronicas. They turn black and become mushy when it becomes cold outside.
Prune reseeding plants, such as coreopsis, down to the ground when preparing for winter. If not, they will take over the planting bed the following season.