Winter interest in the garden sounds like an impossibility in our area. But the longer I garden the more I find of interest in the winter. There are a few plants which actually bloom in the winter. We have a witch hazel tree in the medicinal garden and it blooms with small yellow flowers in February.
Aconite and some hellebores also bloom in winter.
It's also a good time to appreciate the form of plants - particularly trees and shrubs. Without leaves, it's easier to notice their shape and even the arrangement of branches. We have a red-twig dogwood growing in our side border. It's actually a multi-stemmed shrub, unlike the common dogwood tree. It has flowers in the spring, but they're not too showy. Now, however, without its leaves you can really appreciate the bright red stems-looks especially pretty with snow cover.
Woody stemmed herbs which do not die back are still attractive in winter. Lavender, thyme, rosemary,(all my second year plants which survived last winter are still going strong) santolina, sage,etc. provide interesting form and foliage. Plantings, like our knot garden and thyme walk, that depend more on their form rather than color or bloom, look just as good as in summer and stand out more against the quiet winter landscape. Echinacea flowerheads, which I let stand to feed the finches are attractive, as are sedum's dried flowers and the bare, bleached stems of russian sage. I let these stand through the winter and just cut them off in the spring as new growth emerges from the base.
Some things stay green or at least produce new growth under a top that has died back. Much of chamomile's foliage remains green, ornamental candytuft has evergreen foliage and there's new foliage underneath on burnet, costmary and St. John's wort. There's winter interest out there. It just doesn't shout at you like it does in spring.