Shrubs include a diverse range of species and forms. For our purposes here, we classify shrubs as evergreen, deciduous, and coniferous, similar to how our trees are classified. Another similarity is when shrubs reach sizes and forms that can classify them as trees. We classify shrubs as plants no larger than 13' with multi-trunks and low branching, making them difficult to establish usable space like seating underneath their canopies. By contrast, trees are larger than 13' with single or multi-trunks and are pruned or naturally formed to allow sufficient clearances underneath for usable space, such as seating, patios, walkways, canopies, and even streets.
Some shrubs listed here can reach sizes larger then 13'. For designers, the challenge is to decided how the shrubs will perform under the conditions of your design. For example, if a shrub such as Nerium oleander can reach 20' in height, designers have the options to consider specifying a shrub, certainly, or a specimen trained as a tree. Either way, the second decision is to determine if ample space has been provided for its mature size. "The right place for the right shrub" applies here. If the space is inadequate but the larger specimen is chosen, then more maintenance will be required by clients and their gardeners/landscapers to keep them pruned to an appropriate size. If the designated space is just too small, designers have the option to consider more appropriately sized shrubs. If we look at our oleander example, designers may choose a dwarf cultivar more suitable for the available space.
Their desired form is another decision designer will need to determine. Will a line of shrubs become a hedge, and if so, formal or informal? Will they be a part of a mass planting that is not a single line, or is one accent or specimen shrub sufficient? For hedges, formal or informal merits further exploration to determine the best choices. Formal hedges require lots of pruning, and it is the pruning that may cause the hedges to look sharp or terrible depending upon the selected species. Rule of thumb for formal hedges: choose species with small leaves and most likely evergreen (there are always exceptions). Small leaves can be sheared with little impact to the overall aesthetic of the plants. Larger leaves when sheared look unsightly, mangled, and torn unless limbs are manually removed (read: more maintenance). So the mantra about the right plant for the right space can be modified to the right plant for the right purpose and maintenance practice.