Plants to use. Many small flowering plants that grow in the ground or are appropriate for container plantings will work in a window box. You can take advantage of the elevation of window boxes by adding plants that have a trailing habit — ivies, petunias, lobelias, impatiens, sweet potato vines — but don't forget vertical plants to balance things out. Geraniums, marigolds, tiny evergreen topiaries, low-growing ornamental grasses and even miniature roses can be tucked in to add a bit of height and keep things interesting. Be sure to use plants that do not grow taller than one-third the height of your window, or they will appear overgrown and imposing rather than charming.