Window boxes add warmth and cheer to any home, and those positive effects do not have to stop at the end of summer. Hardy flowering plants, ornamental cabbages and kales, textural grasses, and plants with evergreen foliage can bring vibrant color and interest to your window boxes all winter in milder climates, and late into the fall in areas that experience harsh winters.
Kales and Cabbages
Ornamental kales provide rich foliage color in your window box all winter, according to the Clemson University Cooperative Extension. Available in ruffled versions as well as in more traditional cabbage-head varieties, ornamental kales and cabbages come in brilliant shades of white, hot pink, and burgundy. Other varieties of kale sold for edible, rather than ornamental purposes, are also attractive and extremely winter-hardy. Russian Red kale has gray-green leaves with magenta stems; Red Winterbor has deeply ruffled burgundy leaves; and Dwarf Scotch kale has frilly leaves that remain dark green through all but the harshest sub-zero winter temperatures. Interplant kales and cabbages in your window box with pansies, which will keep blooming late into the fall, and the earliest spring crocus bulbs.
Ivy geraniums (Pelargonium peltatum) are common window box plants in Europe, and they thrive in a cool, damp winter climate in many coastal regions of the United States. The Marin County Cooperative Extension suggests making sure your window boxes are well drained,and using a light potting soil, as ivy geraniums like a humid environment but do not like having their roots stay wet. They make superb window box specimens because they bloom in profusion when pot-bound. Keep the blossoms dead-headed to ensure year-round flowering.
Evergreens and Grasses
The Washington State Nursery and Landscape Association suggests filling your window box with small conifer and evergreen shrubs sold in 1-gallon nursery containers in the fall. These plants maintain deep green textural interest all winter, then they can be transplanted to the yard in spring. Grasses are also recommended for winter texture. The association recommends autumn sedges, which maintain a green-gold color through the winter, or blue or red ornamental grasses for vertical color. Red osier twigs and white paperbirch branches can be inserted vertically into a window box along with small evergreen plantings for winter-resistant visual interest and holiday cheer, especially with draped with small white lights.