At a fixed or doublehung window, or on a rail, use tall and mounded plants so the mini garden may be seen from inside, as part of the view.
HISTORICAL Plants have always thrived at the window, but the heyday of the window box was during gardening’s golden age in the early 20th century. Their popularity matched the bungalow’s natural aesthetic; it also coincided with smaller lots, which encouraged container gardening. Boxes are ubiquitous on Historical Revival styles: Cotswold cottages, Dutch Colonials, and cozy, informal houses with sloping roofs and wood shutters. Some Spanish and Mediterranean houses built in the 1920s have wrought-iron windowbox cages; a box slips inside. The iron is securely bolted to the house, and water easily drains away.
DESIGN POINTS Size and proportion are critical for large boxes on a primary façade. The box should extend several inches beyond corbels or brackets and windowsills (approximately the width of the window or group of windows), and rise approximately 1/4 the height of short windows and 1/ 5 the height of tall windows. You might make a cardboard cutout of the proposed box, to check size and proportion. You can even paint the cardboard to check whether the box will look best in the house’s body, sash, trim, or accent color.
Where there are three windows, say, in a group, you may use three matching boxes close together, or one long one. As to height of the box, a rule of thumb is 20 to 25% of the height of the windows. If windows are very large and tall, adding brackets can help with proportion.
Long and large window boxes will be very heavy once filled with soil and plants. You can plant in the liner (either at the potting bench or with the liner already