There is no reason your window boxes need to stay bare and lifeless in the winter. With just a little planning, and very little care, you can have living plants growing in your window boxes throughout autumn and winter. In milder climates you can have blooms throughout winter, and in colder climates you can at least have green foliage. Either way, you'll be adding colour and interest to what may be an otherwise bleak garden.
Most often, window boxes are made from wood. These should stand up to winter weather just fine. However, thin plastic liners or pots may crack, so it makes sense to replace these with thick plastic pots. Terra cotta pots placed in a window box may not survive frost and snow unless you spray them first with clay and stone sealant, sold in statuary and craft shops.
When choosing plants for autumn and winter window boxes or other containers, Fine Gardening writer Muffin Evander suggests looking for varieties that are hardy two zones below your garden. For example, if your USDA hardiness zone is 8, choose only plants hardy in gardening zones 5 and below.
While it's fine to have some plants that will die back in winter, try to find some plants that won't die back. Evergreen plants will provide some colour throughout the winter months, whereas deciduous plants will offer only brown stems.
With their shiny leaves that turn deep red, bergenias are excellent for autumn containers. Pansies will survive the autumn, if not the winter. Evergreen arbor vitae is also an excellent choice for winter survival, as are some sedums, like "Angelina." In addition, English ivy is hardy in many gardening zones. Other good choices include cora bells, sweet potato vine, coleus, rock daisy, prairie verbena, flowering kale or cabbage, and damianita.
Cold weather care
Once it is autumn, stop fertilising plants -- even those grown in window boxes. It's true that continued applications of fertiliser will encourage new growth, but it will also make plants more susceptible to damage from frost and snow. On the other hand, you should continue watering window boxes until the soil in them freezes. Plant roots require moisture until the cold weather makes them dormant.
Unusually cold weather
Sometimes winter is colder than you expected. In such cases, if you want to preserve perennials in your window boxes, the best bet is to transfer them to a warmer location. Remove the pots or heavy plastic liner in your window planter and move them to the garage or a cool location in your home.