Evergreen plants are the ultimate in low-maintenance window boxes. They will survive through the year, eliminating the need to replant as the seasons change. Use evergreen plants as a durable, all-season backdrop to your other window-box elements or as the main attraction in your window planter.
Also known as green boxleaf, Japanese boxlea Euonymus and Euonymus japonicus, green Euonymus is an all-year shrub that holds its color through the winter. Rather than the thin needles of some evergreens, the boxleaf has narrow leaves densely clustered along its branches. The glossy leaves, lined with attractive pale borders, start off as new pale-green shoots and grow darker as they mature.
Dwarf English Ivy
Whether you view English ivy as a wonderfully hardy decorative vine or a frustratingly stubborn pest, there's no denying that it is a durable evergreen plant adept at growing just about anywhere. According to Mo Plants, miniature strains of Hedera helix are hardy to U.S. Department of Agriculture Zone 5 and have elegant, delicate leaves that look much nicer than full-sized ivy in the limited space of a window box.
Dwarf Needle-Leaved Conifers
According to Mo Plants, dwarf conifers look like miniature Christmas trees and are "remarkably cold hardy." Dwarf conifers such as Juniperus communis are normally small or slow growing, and the limited root space afforded by a window box keeps them this way for a long time. Buy your dwarf trees as small as possible and carefully shear new growth to keep them small enough to grow in a window box.