The most important criteria for selecting plants for outdoor containers are: (i) hardiness for site: (ii) suitability of growth habit for container size and shape; and (iii) features that give the plant particular year-round or seasonal interest or usefulness.
Trees and shrubs that are kept outdoors in containers are exposed to a wide - and rapidly changing - range of environmental extremes. In comparison to in-ground plantings, the root systems of container plants are more exposed to winter freezing - thawing cycles, hot summer temperatures, and drying out of the rootball soil. Therefore, only very hardy trees and shrubs should be used for year-round, outdoor container plantings. In addition, larger containers 18" minimum, preferably larger, provide more soil mass, water retention, and temperature insulation; therefore, provide more soil mass, water conditions for plants.
The most suitable trees and shrubs for container plantings also have growth rates and habits that allow them to be kept reasonably "under control" and in scale with their containers and surroundings. In addition, plant features such as branch structure leaf texture and color, and seasonal flowers or fruit should all be taken into account in deciding whether the plant is appropriate for the location.
The following list is not exhaustive, but does include many of the best and most dependable trees and shrubs for container planting. Plants that prefer sun are marked with a single asterisk (*); plants that prefer shade are marked with a double asterisk (**). Most plant varieties can be planted safely in spring, summer or early fall; plants that can also be safely planted later in the fall are marked with an "F'. For a more complete description of hardy woody plants and perennials suitable for containers, please refer to Linda Yang's excellent boot, The City Gardener's Handbook (Random House 1990) (esp. Chapters 4 and 7).
The following list of suggested container plants is organized by plant category, and varieties are listed within each category by alphabetical order of botanical name:
- Trumpet Vine * F (Campsis radicans)
- Hardy English Ivy Cultivers ** (Hedera helix cvs.)
- Honeysuckle * (Lonicera Heckrotti)
- Wisteria varieties * F (W. sinensis and floribunda)
Need support and regular pruning to restrict size.