Railing boxes, or flower boxes, are long, typically shallow planters designed specifically to be hung from a railing or windowsill. They are a great way to add color to a deck, terrace, or window ledge. Even those living in a high floor of an apartment building can enjoy growing plants in this type of planter. The key is to choose the right kind of plants.
Because of the small and shallow nature of these boxes, not all flowers are suited to them, and it is important to consider location, hardiness, and drought tolerance when deciding what will work best in any specific spot. Tropical annuals work well, because their lifespans are limited and they won't have time to get large. Native perennials also work well, because of their adaptability in small spaces and their preference for less rich soils.
Flowers for Full Sun
Begonias (Begonia x semperflorens-cultorum) are popular annuals and grow well in small spaces. They come in a variety of colors and enjoy each other's company. Marigolds (Tagetes spp), are another annual that like it hot and dry, and unlike begonias, seed like crazy and grow from seed easily. Purple Coneflower (Echinacea purpurea), U.S. Department of Agriculture plant hardiness zone 4 through 10, is a native perennial of the taller variety, but takes to small spaces well and will come back year after year.
Flowers for Part Shade to Shade
Pansies (Violax wittrockiana), USDA zones 8 to 11, are the ultimate cool weather, shade loving flower. They are biannual, and can be found in a variety of whites, blues, and purples. Nasturtium (Tropaeolum Sp), is an annual vine that will tolerate some shade, loves dry and poor soil, and flowers in all sorts of colors. Nasturtiums can even be eaten in salads. Foamflower (Tiarella cordifolia), is a short native perennial, USDA zone 3-9, with tall white foamy flowers, and will easily survive cold winters.
Basil (Ocimum basilicum), is one of the most popular culinary herbs. It is a full-sun annual that comes in a variety of colors and grows well in hot and dry areas. Mint (Mentha Spp) is a useful perennial culinary herb, and blooms profusely with white flowers. It is better grown in pots, and likes USDA zones 3 through 8, depending on type. Rosemary (Rosmarinus officinalis) is a small shrub-like herb that produces blue flowers and will stay green for years in USDA zones 8 through 10.