Best is a relative term and that goes for "best" flowers. Nearly all flowers will grow in window boxes but some varieties are better suited than others. Generally flowers that do well in containers don't need to be dead headed to keep blooming, have a moderate growth habit and roots that don't need more than 12 inches or so of soil.
Shade
The location of the window box is one of the most important factors in how well the flowers bloom. Most flowers need at least six to eight hours of sun per day to bloom. However, there are a number of flowers that thrive in the shade. Begonias are fleshy leaved plants that have small flowers. The leaves can be bronze, pure green, or dark green. The flowers come in pink, white and purple. Tuberous begonias have flowers the size of golf balls and arrow shaped dark green leaves. Both do well in shade. Other flowers for shaded window boxes include monkshood, dwarf columbine, lily of the valley, impatiens, violets and pansies.
Sun
Window boxes that receive sun all day long need to be watered as much as once a day during the warm summer months. Lantana is drought tolerant and loves the sun. Zinnias do well with lots of sun and will pop right back if they wilt. Salvia is another good choice.
Plant the flowers in a plastic light weight window box using a potting soil which retains moisture. Line the inside of the window box attached to the house with a layer of sphagnum moss. Place the planted plastic window box inside the attached window box. These extra steps will keep the flowers from drying out so quickly. In addition the plastic flower box can be removed for a good soaking which might be difficult with the attached window box.
Combination
Probably the best location for a window box is morning sun and afternoon shade. Geraniums come in a myriad of pinks, purples, reds and white and do very well in window boxes. They are probably the best flower for that purpose. Lobelia is a low growing flower only four to five inches tall and 12 inches wide. It's covered with small blossoms in brilliant blue, pale blue or light purple. Petunias are a window box standard. The flowers can be single, double, grandiflora or ruffled. Some flowers are edged in white.
When planting the window box, choose flowers that trail over the edges like alyssum, some that grow taller such as snapdragons for the back of the box and some that are Contrasting colors
Colors
White flowers against a white stucco facade won't do much of anything. Red snapdragons against red brick will be invisible. Contrast the flowers with the color of the building so they pop. The flowers can be contrasted against each other as well. Classic combinations include red geraniums, white petunias and blue lobelia. Or yellow coreopsis, purple larkspur and orange marigolds.
Shade
The location of the window box is one of the most important factors in how well the flowers bloom. Most flowers need at least six to eight hours of sun per day to bloom. However, there are a number of flowers that thrive in the shade. Begonias are fleshy leaved plants that have small flowers. The leaves can be bronze, pure green, or dark green. The flowers come in pink, white and purple. Tuberous begonias have flowers the size of golf balls and arrow shaped dark green leaves. Both do well in shade. Other flowers for shaded window boxes include monkshood, dwarf columbine, lily of the valley, impatiens, violets and pansies.
Sun
Window boxes that receive sun all day long need to be watered as much as once a day during the warm summer months. Lantana is drought tolerant and loves the sun. Zinnias do well with lots of sun and will pop right back if they wilt. Salvia is another good choice.
Plant the flowers in a plastic light weight window box using a potting soil which retains moisture. Line the inside of the window box attached to the house with a layer of sphagnum moss. Place the planted plastic window box inside the attached window box. These extra steps will keep the flowers from drying out so quickly. In addition the plastic flower box can be removed for a good soaking which might be difficult with the attached window box.
Combination
Probably the best location for a window box is morning sun and afternoon shade. Geraniums come in a myriad of pinks, purples, reds and white and do very well in window boxes. They are probably the best flower for that purpose. Lobelia is a low growing flower only four to five inches tall and 12 inches wide. It's covered with small blossoms in brilliant blue, pale blue or light purple. Petunias are a window box standard. The flowers can be single, double, grandiflora or ruffled. Some flowers are edged in white.
When planting the window box, choose flowers that trail over the edges like alyssum, some that grow taller such as snapdragons for the back of the box and some that are Contrasting colors
Colors
White flowers against a white stucco facade won't do much of anything. Red snapdragons against red brick will be invisible. Contrast the flowers with the color of the building so they pop. The flowers can be contrasted against each other as well. Classic combinations include red geraniums, white petunias and blue lobelia. Or yellow coreopsis, purple larkspur and orange marigolds.