Flowering plants grown in window boxes are exposed to wind, reflected light, fast-draining or fast-drying soils. The "best" flowers to grow depends on your climate, size of the window box container and exposure to sun and wind. Compact, trailing and ever-blooming plants are better as they soften the look of the planting box and are more resilient to wind.
Climatic or Seasonal Considerations
The best flowers for a window box depend on your climate or the season. Matching the plant type or its environmental tolerances to the conditions present in the window box is key to success. For example, in a hot, arid climate, growing plants that need lots of water will increase daily maintenance, or planting a cool season annual like a pansy in the heat of summer will disappoint you since flowering will be diminished.
Consult with gardening neighbors and plant nursery professionals for recommendations on plants that perform well in your climate. Although plants that have vividly colored foliage or produce abundant amounts of flowers are ideal, growing them in the constraints of a narrow window box that is exposed to wind and sun on a ledge can change their beauty. Florist geraniums (Pelargonium), both compact and trailing types, are popular choices as they are sun-loving, drought and wind tolerant. However, if the climate or season is cool, humid and often cloudy, the geranium may be not the best choice. In hot summer regions, pansies are best used in fall and spring; use impatiens in locations that are shaded where the soil is constantly moist.
Window boxes have a long, narrow shape with a relatively shallow basin to hold soil. Even though having a larger window box with more soil will likely yield healthier plants, it also increases the weight of the window box on the building facade. Therefore, window boxes remain small and more easy to lift, sometimes being made out of lightweight materials like vinyl or plastic. The smaller containers dry out more quickly and need more watering to sustain plant life.
Choose plants that are a size appropriate for the window box. Aggressive plants like lantana or an ornamental fountain grass may be drought tolerant, but their root systems become so widespread. Tall, large plants look out of scale in many window boxes and can block views from windows or flop or cause the window box to be unstable in windy gusts.
Heat and sunlight is intensified on plants in window boxes because of their tight location in front of glass, and light-colored wood or metal siding. Wind increases the higher up on the building, too. All these factors combine to create an environment that causes increased need for soil moisture.
Often trailing plants or those that are low and rounded, or compact in their shape are best chosen for window box containers. Unless you have easy access to the box for frequent watering, plants that handle drought are ideal selections, such as florist geranium (Pelargonium), petunia, verbena, compact coleus (Solenostemon), French marigold (Tagetes), Madagascar periwinkle (Catharanthus) or firecracker flowers (Cuphea). Lobelia, million bells (Calibrachoa) and twinspur (Diascia) are popular since they are mounding or trailing in habit and flower heavily, ever-blooming, without need for copious watering and deadheading of old blossoms.
Only in shaded, protected locations will a window box with moisture-loving plants like impatiens, tuberous begonia or caladium be a wise choice. Fewer drainage holes can be made in window boxes to slightly increase soil moisture levels, but more water increases the weight of the container and can increase risks of fungal diseases.