When using impatiens in a large planter, it's important to use plants with different textures than the impatients, to add interest. Impatiens are colorful but have a medium texture, which in itself is rather unremarkable. Alone, they can seem a bit boring, but add a variety of other plants and they shine. Mix at least three textures whenever possible.
With any large planter, you have the option of using a small tree or shrub as the focal point of the design. Dwarf Japanese maples make excellent container specimens, as do the smaller camellias such as C. sasanqua, a graceful shrub that blooms in fall or winter. Permanent plants such as these allow you to create year-around interest by planting bulbs for spring and impatiens for summer and fall.
Other shrubs and small trees with either flowers or interesting textures to consider are: azaleas and rhododendrons, paperbark maple (Acer griseum), Stewartia and mountain laurel (Kalmia latifolia).
Hostas are an ideal choice for contrast, being just as shade and moisture loving as impatiens and having large leaves, sometimes striped or splotched with white or yellow. Another possibility is bergenia, which has clusters of pink or rose flowers in spring. Acanthus, with very large shiny leaves and spikes of white or lilac flowers in early summer, makes substantial clumps and is capable of acting as a focal point on its own.
Other bold textured plants for shade include lady's mantle (Alchemilla,) Hellebore and Rodgersia.
Ferns and Other Fine Textures
Maidenhair ferns are some of the most elegant partners for Impatiens, with delicate, airy stems up to several feet high. Other ferns are similarly effective, setting off the bright flowers with intricate shadings of green.
Other small textured plants for shade include yellow Corydalis (C. lutea). Astilbe, bleeding heart (Dicentra) and meadow rue (Thalictrum).
Linear and Sword-like
Linear, sword-like plants can act as focal points when placed at the center of your planter, or as simple texture contrasts when three or more are mixed with other plants. Short plants with this texture include Liriope, crested Iris and sedge (Carex,) Several kinds of Iris such as Siberian Iris (I. sibirica) and Gladwyn Iris (I. foetidissima) are taller possibilities. Don't forget Dracaea, a tender plant often grown indoors or used in mixed containers of flowers during summer.