Window boxes should be in proportion to the size of the windows, and they should also reflect the style of the house. Before buying or window boxes, measure the length and height of the windows, and take into consideration any external trimming or shutters. Also consider your climate and the time you have available to care for the window box plants. In warm climates, plants growing in narrow window boxes may need water twice a day.
Sizing Window Boxes
Window boxes should be sized according to the length and height of the windows. The length is measured from side to side, and the height from bottom to top. The width of the window boxes, from the front to the back, depends on your desired plants and style of your house.
Window boxes about the same length as the windows look best. Most windows are about 3 feet long, and most window boxes are 30 or 36 inches long, to match the length of the windows. An exception is when the window have shutters or wide exterior trim. Window boxes under these windows look best when they are as long as the window plus the shutters or trim, or 3 or 4 inches shorter on both sides.
To look proportional to the windows, window boxes should be 25 percent as tall as the height of the windows. So if a window's height is 4 feet, the window box should be 1 foot tall. If the windows are very tall, the window boxes look best about 20 percent as tall.
Wide window boxes give you options for selecting plants, but narrow window boxes may look best, depending on the style of the house. Compact perennial plants grow well in wide window boxes with plenty of potting mix, and there's no need to grow new plants every year. Plants in wide window boxes also don't need to be drought tolerant, because the large amount of potting mix retains water well.
Wide window boxes can look chunky, however, and need large brackets to support their weight. When houses have an elegant, delicate style, narrow window boxes may look better.
Window box materials are just as important as their size. Copper, iron, tin, ceramic, terra-cotta, stone, wood, wire, plastic and fiberglass are some of the available materials.
Metal window boxes can look contemporary or traditional, but they heat up quickly and should are not a good idea if the windows are in the sun for most of the day. Dark-colored window boxes also heat up quickly.
Ceramic, terra-cotta and stone window boxes lend a rustic, Mediterranean, natural effect, but are heavier than window boxes made of other materials and require large, securely fitted brackets.
Wood is a traditional material for window boxes, and it can be stained or painted to match your house colors. Wood rots over time, however.
Wire, plastic and fiberglass window boxes are light, and are available in many colors and styles. Wire window boxes must be lined with sphagnum moss or a similar material to prevent the potting mix from escaping.
Planting in Window Boxes
Use soilless potting mix and plants with similar growing needs and growth rates for the best results in window boxes. Select plants that prefer the same growing conditions, such as sunny or shady spots, and that grow at similar speeds. Fast-growing plants can quickly swamp slow-growing plants in window boxes.
Annual plants, which grow for one season and don't return, often grow well in the restricted root area and exposed conditions in window boxes, and they keep blooming for long periods. Plants with long, trailing stems look good when they send out cascades of blooms.
Place pieces of screen or coffee filters over the window box drainage holes.
Mix store-bought soilless potting mix with 20 percent fine or screened garden compost. Or mix 2 parts peat or coir, 2 parts perlite or vermiculite, and 1 part fine or sieved garden compost.
Fill the window boxes to 1 inch below the rims with potting mix.
Remove plants from their containers and plant them in the potting mix at their original growing depths and final growing widths. For example, if a plant grows 6 inches wide, plant its central stem 6 inches from the central stem of the next plant.
Water the plants using a garden hose fitted with a soft spray attachment or a watering can and fine rose spray. Apply the water evenly until it flows freely through the drainage holes.
Water window boxes when the potting mix surface is dry, which may be every day or more often in hot, windy weather.