Form

• In some cases, the line between shrubs and upright trees is finely drawn. Depending on pruning and growing strategies, you can train certain species to grow as either trees or shrubs. As a rule of thumb, to distinguish between the two forms, a shrub has multiple stems, whereas a tree grows upright from a single main trunk. Within this definition, the form of the shrub can vary tremendously. A broad spreading shrub has a wide and low profile, while a spreading shrub is slightly less wide. A globose shrub is nearly spherical. The upright shrub shape is slightly taller than it is wide, and a columnar shrub resembles a pillar of foliage.
  
  

Size

• Small shrubs may measure 2 to 4 feet tall at full maturity, often with woody stems, unlike deciduous herbs and low-growing plants. For example, the grow-low sumac (Rhus aromatica) is a low-growing ornamental shrub. In some cases, extremely small plants with shrub-like forms are termed sub-shrubs. Medium-sized shrubs, such as the butterfly bush (Buddleia davidii), reach heights between 4 and 6 feet. Larger shrubs reach the same heights as small trees but retain the multi-branched form. For example, the Bougainvillea produces a spreading form around 10 to 12 feet tall, dotted with showy flowers.
  
  

Flowers, Berries, Foliage and Form

• An ornamental shrub usually has at least one feature notable for its aesthetics, whether eye-catching flowers, colorful berries, handsome leaves or a form that is graceful or easy to prune into set shapes. For example, the Rose-of-Sharon or shrub althea (Hibiscus syriacus) produces showy flowers. The pussywillow (Salix discolor) is notable for its soft, gray catkins. The Isanti dogwood (Cornus sericea) produces unusual reddish branches. The Chinese holly (Ilex cornuta) produces scarlet red berries.
  
  

Types of Shrubs

• While you can select from hundreds of shrubs for any given climate, you can narrow your focus by selecting a particular type of shrub that best suits your space. For example, if you have extremely acidic soil, opt for an ericaceous shrub. These shrubs include several popular flowering species, such as azaleas, rhododendrons, and pieris, as well as some broadleaved evergreens. These shrubs rely on acidity to absorb iron from the soil. If you're looking for a shrub to add color and interest year-round, opt for an evergreen shrub. Among your many choices you have the low-growing creeping juniper (Juniperus horizontalis) and the oleander (Nerium oleander), well-known for its fragrant blossoms.