Are you looking to dress up the rather plain front of your house? Or, maybe you just don’t have room for containers around your property? Window boxes bring color and spirit to barren areas, as well as considerable pleasure to those who tend them. Measure the width and length of your sill, then check garden shops or hardware stores for boxes in the appropriate size. (If you shop at a store with helpful clerks, you can get plenty of free advice about window-box attachment.)
Depending on your house style and budget, you can choose from wood, cast cement, molded terra cotta, plastic, or fiberglass. Avoid metal boxes, because they will very likely rust in a few seasons, and if placed on sunny sills will transmit heat, which burns roots.
Make sure the box is securely attached with wire or bolts. Don’t count on just gravity, no matter how wide the support is. Prepare for planting by covering the bottom with a layer of landscape fabric or plastic screen. This will hold the soil in place while allowing water to drain.
Fill the box about three-quarters full with any all-purpose potting mix, then stir in several trowels each of perlite and organic matter such as leaf mold, aged manure, or compost.
There are no design rules to planting, but contrasting leaf sizes should be a goal, as should contrasting plant outlines. Use bushy plants for bulk, tall plants for a vertical accent, and pendulous species for a graceful cascade over the side. Window boxes almost always look better if there is something draping over the edge, and for sheer drama, you can’t beat drapery that hangs in long streamers well below the box.
Unfortunately, although there are many summer stalwarts that will swag down nicely for 12 to 18 inches or so, not many plants are willing to dangle unsupported for much more than that. Plants that will include ivy (Hedera helix), ivy geraniums (Pelargonium peltatum), nasturtiums (Tropaeolum majus), ornamental sweet potato (Ipomoea batatas), trailing lantana (Lantana montevidensis), and vinca (Vinca spp.)
The list of likely trailers is short because most lax-stemmed plants are vines, and most vines would rather hang on than hang down. If they can’t climb straight up, they’ll climb any which way — on themselves, on the other plants in the box, on the brackets that hold the box up, etc. The end result is a tangled mass instead of graceful tresses.
That said, if you have a situation where vines can’t get a grip on anything, these are also worth a try: canary bird vine (Tropaeolum peregrinum), climbing snapdragon (Asarina spp.), grape ivy (Cissus incisa), and passionflower (Passiflora spp.)
If you have only part sun, try these plants for your window box: ageratum, basil, bay, bee balm (Monarda didyma), begonia, caladium, dwarf Chinese astilbe (Astilbe chinensis pumila), ferns, four o’clocks, fringed bleeding heart (Dicentra eximia), lady’s mantle, lantana, lobelia, and wishbone flower (Torenia); and English ivy, mint, or vinca to trail over the side.
No direct sunlight at all? That’s no excuse for not planting a window box. Assuming you get at least bright reflected light, there are quite a few plants that will endure. Many of the best are perennials with comparatively short blooming periods, but if you choose plants with handsome foliage, the box will be attractive even when there are no flowers.
The delicate, ferny foliage of Jacob’s ladder, for example, contrasts nicely with the scalloped round leaves of coral bells, and both remain fresh looking all summer. The Jacob’s ladder will give you blue flowers for a few weeks in late spring. The coral bells will bloom (at least briefly) a short time later, in red, white, or pink.
If you are determined to have flowers all summer long, you can try shade-tolerant annuals, but keep in mind that even tolerance has limits. You’ll probably have to experiment a bit to find which will perform under your conditions. Choices include wishbone flower (Torenia), with its small purple, snapdragon-like flowers; begonia, both tuberous and wax, available in white and every shade of read and yellow from pale pink to screaming orange; and the ever-faithful impatiens, in a spectrum much like begonia’s.
And don’t forget to plant a trailer. Vinca and English ivy will both do fine.