Romans gardened with window boxes when urban habitation afforded them little growing space. Some Europeans decorate windows with containers of flowers beneath. Gardeners in the United States later adopted the window box as an outdoor decor option.
As in other methods for container gardening, consider choosing the window box transplant with function in mind. At Plant Exchange, they are beneath windows as accents to other more colorful containers at the front door patio entrance.
Readily available annuals are selected that continue to bloom for the spring, summer, and early fall. After the growing season, the boxes are cleaned and near the holidays, are filled with Eastern red cedar boughs that are plentiful in this area.
In container gardening with window boxes, the containers need to be deep enough to hold an adequate amount of potting soil for starter plant roots and enough potting soil to lessen the need for frequent watering in the heat of summer.
This window box area receives little direct sun, so shade-tolerant plants are needed here. Another consideration for these window boxes is that some windows are opened outwards on mild days, so the plants need to be flexible and not too tall to hinder opening the window. These were planted about a month ago in the cool spring but will fill in more in summer growing days.
From inside the house looking out, the plants are also visible, so keeping them trimmed in the second half of summer, like other container plants, will allow them to stay proportional to their planters. They continue to accent other plants for the rest of the growing season.
Have you tried container gardening with window boxes? Please share comments if you wish.
Plant Exchange Blog 
Venetians liked window boxes for aromatic plants that were thought to repel mosquitoes from their windows. Cascading plants were particularly practical. Ivy geranium, trailing rosemary and nasturtium are still very popular in window boxes.