Choosing the Right Container
Wood, plastic, copper, iron, tin, ceramic, terra-cotta, stone, wire, and fiberglass window boxes are all available. If your windows are in the sun all day, avoid a solid metal or darkly colored box, because they can heat up and toast plant roots. If you choose terra-cotta, be sure to immerse the boxes completely in water for a half hour before planting so that the clay doesn't absorb all the water meant for the plants.
A standard size window box is about 3 feet long. Don't get a window box less than 4 inches wide—it won't look lush and will require constant watering. Windowbox.com recommends considering the proportion of your windows before buying a box. A window box should be about 25 percent of the height of the window; 20 percent if the window is very tall. Make sure the box has drainage holes (each about 1/2 inch in diameter, spaced about 6 inches apart). If it doesn't have any, you'll need to drill some. Cover the holes with screening or coffee filters to hold in the soil.
Designing with Plants
If you're unsure how to combine plants attractively, follow this easy formula from the late Kathy Pufahl, horticulturist and owner of Beds and Borders nursery: Pick one type of plant that grows up (grasses, spiky plants, geraniums, or other flowers on long, straight stems), one that trails down (ivy, sweet potato vine, thyme, lobelia), and one bushy type to fill in the middle (impatiens, petunia, dwarf ornamental pepper.) If you have room, you can add a fourth type as filler—one that contrasts in color, texture, or foliage size with the others in the box.