As with ground-level beds, light conditions will determine what you can grow. Full sun accommodates blooming annuals, while shade best suits foliage plants, like coleus and caladium. To properly show off these displays, select a box that's the same width as the window.Use sturdy brackets to attach the box to your house, and invest in a high-quality potting mix.
Arrange plants on top of the soil until you're happy with how the design looks from inside and out. Then ease them out of the nursery pots and settle them in. Some crowding is fine, as long as you keep pruners handy to rein in rampant growers. You can also just drop potted plants directly into boxes and surround them with soil. This makes swapping out poor performers and popping in seasonal selections a snap.
Shown: The silver leaves of this euphorbia tone down a boisterous mix of striped petunias, purple lobelia, and hot-pink nicotiana.