Add drama and bright colors to the front of your house by planting a window box. Fill it with colorful flowers and accent it with interesting shaped greenery. If you limit your window box to a couple varieties of flowers and one variety of greenery it will have a larger impact than a mish-mash of many different types of flowers and greens.
A low-growing border plant with tiny white flowers, sweet alyssum (Lobularia maritima) is an ideal plant to cascade over the edges of your window box. Alyssum grows just 4 to 6 inches high and spreads about 8 inches. The tiny white flowers are sweetly fragrant--particularly at night. Sweet alyssum has tiny seeds and it is slow to start growing from seed. Young bedding plants are widely available in spring and early summer and are the best choice when planting in a window box.
To create a lacy accent of green, plant asparagus ferns (Asparagus densiflora "Sprengeri") in your window box. Tiny branches grow out from their fronds, on which needlelike leaves grow. The fronds of asparagus fern will grow upright or arch over, depending on their size. They are widely available in spring as bedding plants. Asparagus fern can also be dug up and overwintered in pots indoors. Cut their fronds back by two-thirds when you dig them up, water well and they will soon begin active growth indoors. Grow them in an east- or west-facing window.
One of the most popular annual flowers, petunias (Petunia x hybrida) grow equally well in garden beds, hanging baskets, container gardens and window boxes. They flower continuously until killed by frost. A variety of petunia collectively known as "milliflora" is the best choice for window boxes. The plants grow 2 to 3 feet long and produce many tiny flowers 1 1/2 inches in diameter. They can be trained to fill in a window box or to trail over the edges. To create height with petunias in your window box, plant the type known as "grandiflora" petunias. They grow upright, developing into 12- to 15-inch-high mounds of flowers.