For good results, a window box ought to be at least three to four feet long but not more than six feet. If larger, it is too heavy to suspend and secure properly, and it cannot be lifted easily, even by two people. Boxes resting on broad window ledges and on firm porch railings might be eight feet long, but hardly more since moving them becomes too hazardous. Keep to a minimum depth of eight to nine inches, with a width of ten to twelve inches across the top. Of course, lengths must vary according to the window, or series of windows, or railing to be decorated.
The most common material for window boxes is wood. California redwood, which becomes a neutral gray if not painted, and cypress will last for years. Cedar is recommended, as is a good grade of white pine. Other materials
include metals, which are attractive and, for the most part, light in weight. However, they have the disadvantage of conducting heat, thus overheating the soil. Other suitable and durable lightweight materials are plastic, fiberglass, spun glass, and Gardenglas.
Instead of window boxes, shelves-wide boards with holes to support pots at the rims-can be attached to windows. Here plants are easily changed to keep up a colorful appearance. Consider though that potted plants on shelves dry out quickly.
If you are handy with tools, you can make your own boxes of wood, following instructions in pamphlets from your agricultural experiment station. Whatever plan you follow, get boards one to one and a quarter inches thick. (Thinner boards will warp and offer little insulation against summer heat.) To fasten, rely on brass screws rather than nails, which in a few years may push out and cause a box to fall apart. To make corners secure, reinforce with angle irons. Be sure to provide enough drainage holes in the bottom for water to pass through freely. Space half-inch holes six to eight inches apart.
When boxes are completed, treat the insides with a preservative to prevent rotting. Cuprinol or some other non-toxic material is excellent, but avoid creosote which
is poisonous to plants. After the preservative has dried, apply at least two coats of good paint or stain.
Painting the Window Box
Select a color which will not detract from the plants. Traditional dark green is satisfactory, though commonplace, unless you use a tint like apple green. Have in mind the colors of the flowers, especially of plants that trail over the sides. Dark flowers do not show up against dark paint, as blue browallia or lobelia against dark green or black. The same is true of white flowers against light surfaces, as white petunias against white or pale yellow boxes.