Strictly speaking, window boxes do not come under the category of planting or designing the grounds, but in a vital way they serve to tie the house to the lawn and gardens and thus help to produce the immediate effect of a harmonious whole. Particularly are they valuable in imparting a cozy and "lived-in" atmosphere to a new house. Many otherwise uninteresting houses have been made very attractive through the use of window boxes. A severe type of architecture demands a window-box treatment developed with the heavier kinds of foliage plants such as English ivy, geraniums, and fuchsias, while a lighter architectural design requires vincas, snapdragons, and ageratums. The selection of plants for successful window boxes must be the result of some study of the effect to be produced and the kinds of materials necessary to produce the effect.
Not all of our plants can be used in window-box planting. Plants for this purpose must retain their foliage throughout the summer, the period of bloom must continue for a number of weeks, and the normal growth of the plant should not be impaired by crowding the root development within a small area.
Two cardinal principles apply to the design and use of window boxes. Never put window boxes on a building unless the architectural composition requires them, and do not select for them plants which are out of scale with the architectural detail. When planning the window boxes the effect of the colour scheme should be considered from the inside of the various rooms in the house as well as the effect upon the aspect of the house itself.
There are numerous possibilities outside of the conventional boxes planted with periwinkle, geraniums, and daisies. Almost any of the showy dwarf-growing annuals may be used and the opportunity for various colour schemes with them is practically endless.
If bright colour is needed the dwarf, giant-flowering snapdragon, which comes in many brilliant shades and grows about twelve inches high, is good. The dwarf zinnia is perhaps even more brilliant in its various colours. It is also stiffer in its habit of growth and consequently better for a windy location. California poppies can be had in all shades of yellow and orange and could be used with nicotiana for a white and yellow box. Another good combination is blue lobelia, pink verbena, and asparagus fern.
Care should be taken to select the flowers which will bloom simultaneously. Foliage plants should be used to provide an abundance of green, and enough vines and flowers of a drooping habit should be introduced to counteract the stiffness of the box.
When planting, pack the roots in firmly on account of the wind. For an unusually windy position it is best to use a deeper box. In choosing the plants, exposure is the first important consideration. (See the following lists.) For sunny positions the more vigorous growing and flowering plants are apt to do best, while in shade ferns and foliage plants, generally speaking, are more successful. In a dusty location smoother-leaved plants such as myrtle and ivy geraniums should be used.
Inside window boxes should get sunshine and plenty of fresh air but must never be placed in a draft. The temperature for the average house plant is between 550 at night to 700 in the daytime. The plants should be watered regularly and the foliage sprayed two or three times a week, with the exception of those plants with fuzzy foliage, such as gloxinia, where moisture upon the leaves would cause decay. Hanging baskets should be lined with moss in order to retain their moisture.
The soil used in all window boxes must be rich, as the roots are so crowded and ample plant food must be available. A good soil mixture for this purpose is two parts garden loam, one part rotted leaf mold, and one part sand, mixed with one part well-rotted manure. This mixture can be procured from any florist. As the box becomes filled with roots it is necessary to furnish food to the plants by working into the soil a small amount of bone meal or well-rotted manure every week or ten days.
The box may be constructed of various materials: concrete, terra cotta, or wood. The inside measurements for a window box should be six inches to eight inches deep and ten inches to twelve inches wide. The outside measurements should be fourteen inches wide and one inch shorter than the window or space it is to occupy. A very long box can be made in sections averaging three feet to four feet in length, to facilitate the handling of it. Three-quarter inch holes should be bored in the bottom of the box every twelve inches, to provide drainage. A zinc or galvanized iron lining in a wooden box is desirable but not absolutely necessary. However, if a lining is not used it is best to have the inside of the box charred to prevent rotting of the wood. This is done by washing the inside, both bottom and sides, with kerosene and then lighting the oil and allowing it to burn until a thin charred coating is formed. The box is turned upside down to smother the flames. The most permanent types of window boxes are lined with copper. All boxes, whether or not they are lined, must be provided with holes for drainage. The absence of these holes may cause the soil to become sour from overwatering, a condition which is avoided when drainage is provided.