In some garden books, larger-growing herbaceous (i.e. non-woody) are grouped with the shrubs, but here "shrub" will be used to describe a plant with woody stem(s) that is bushy down to the ground.
Most shrubs in the garden range from ground-hugging (prostrate) up to 2-3m. Some species which might tend to climb or become small trees if left to their own devices can be maintained as shrubs with regular cutting back. Conversely, some shrubby plants can be trained as small trees with appropriate pruning. Shrubs of course are also the raw material for hedges and topiary.
Shrubs' Keys to Success
Shrubs occur on 40% of Earth's land surface, making them more common than trees at 28%. Theoretical modelling has shown that it is the multistemmed nature of shrubs that makes then so successful. For a given volume of above-ground wood, the greater cross-sectional area at the base means water and nutrients can be transported faster to the leaves. The larger area of bark, from which buds sprout, is also believed to be advantageous because a canopy of twigs and leaves can be developed faster. Trees can ultimately attain the height that allow them to compete, but not before the faster shrubs have flowered and spread their seed. The growth form of shrubs are also better suited to extreme environments. Source: Shrubs More Expansive Than Trees (September 2016)