Low, high, or in between--the height of your hedge depends on the kind of shrubs you select and the trimming you do.  Regular pruning also encourages dense foliage and branching.

Low hedges
Hedges one to three feet high should be naturally neat, grow slowly, and tolerate regular shearing or pruning.  If you want to keep a hedge less than two feet tall, shearing is essential.  Begin training your plants the first year after planting to keep them in bounds.

Medium hedges
Hedges three to five feet high include azaleas and many of the yews.  If you don't want a formal, sheared effect, an annual or semiannual clipping will keep the plants compact and uniform.

Shrubs of medium height can be used to block out unsightly objects in the backyard--a clothesline or recreational vehicle, for example.  Evergreens are a common choice, but deciduous shrubs with dense branching are useful as well.

Tall hedges
Hedges four to eight feet high make excellent background plantings.  As they age, some begin to look like small trees.  They may develop bare, multiple trunks and spreading tops, characteristics that make them useful as semi-open partitions and screens.  These hedges allow for air circulation and take up less ground than shrubs that have foliage growing low to the ground.  They need only touch-up pruning to keep tidy.