Answer: Window boxes often freeze solid during the winter so it is difficult to keep plants alive in them. Some gardeners will use pansies or ornamental kale for the fall, replace them with cut winter greens for the coldest months and then use the window box as a cache-pot and put in bulbs which have been grown separately in pots for early spring. If you simply want something that looks "like something" you might try English ivy which is evergreen and can trail nicely, or you might try small juniper shrubs if your window boxes are large enough. Keep in mind however that these plants may not survive.
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