After planting, spread an inch mulch of peatmoss or other mulch over the soil to delay drying out and keep weeds in check. In a month, give a liquid fertilizer and follow up with feedings every seven to ten days. Foliage fertilizers can also be applied, but only as a supplement to root feeding.
Kinds of Plants
The choice of plants for window boxes is limited only by size. Plants over a foot high do not look well unless
boxes are exceptionally large. Otherwise, you can grow almost anything you want. For early spring, you might start with Dutch bulbs. In cold regions, these can be purchased already grown, or you can raise your own.
Try hyacinths with pansies or early tulips or daffodils interplanted with grape hyacinths, or basket-of-gold and arabis with scillas, chionodoxas, or leucojum. Include some English daisies and sweet-smelling wall flowers, so common in window boxes in western Europe. Violas, blue phlox, aubretia, and forget-me-nots are other possibilities.
Geraniums Are Tops
The favorite window-box plant is the geranium-red or pink for white, cream, or light or dark blue boxes; white for brown, blue, or red boxes. The familiar trailing variegated vinca is excellent with them. Thriving in sun or shade, the vinca needs constant pinching to prevent it from becoming too long. English and German ivies are other trailers for sun or shade. In the sun, low annuals, dwarf marigolds, lobelias and verbenas make nice edgings as does sweet alyssum, in white, purple or lavender. Petunias vie with geraniums in popularity, and any kind can be planted, though the balcony types have the advantage of trailing gracefully over the sides of boxes.
Ageratum brings "blue" to sunny boxes. Annual phlox, cockscomb, lantana, creeping zinnia, rock rose or lam-pranthus, portulaca, dwarf snapdragons, and dwarf dahlias are also lovely for a sunny set-up.
These for Shade
In shade that is open to the sky, as on the north side of a house, coleus grows superbly, with white-and-green kinds a handsome contrast for those with red-and-pink leaves. Coleus luxuriates in a rich, humusy soil and requires plenty of moisture. Pinch to keep bushy, and to improve appearance remove the spiked blue flowers, unless you especially like them. The Trailing Queen coleus is one of the best.
Other shade-tolerant trailing plants include English ivy and its varieties, creeping jenny, Kenilworth ivy, creeping fig, German ivy, variegated gill-over-the-ground, myrtle, wandering Jew, zebrina, achimenes, chlorophytum, star of Bethlehem or Italian bellflower, and strawberry begonia.