The height that a shrub will grow is dictated by its species and to a lesser extent by its variety. No matter how much fertilizer you give a barberry shrub, for instance, it will never grow much taller than 2 feet in height. When choosing a shrub to plant, look for species that are naturally tall like the Siberian pea shrub that reaches 15 feet at maturity. Or look for species like Forsythia, which reaches 9 feet in height and can be grown as a tall shrub or a small tree.
Plant the shrub. Some shrubs are easy to plant by seed. But by starting with a sapling, your plant will reach your desired height much sooner. Use a shovel to dig a hole that is twice the circumference of the plant's original container or root ball and equal to its depth. Then plant it at the height that it was growing in its container. Do not cover the roots with more than 1 inch of soil.
Water the tree or shrub. Keep the soil moist to the depth of the root ball until the shrub establishes itself and produces new growth. Feel the soil with your hands and water whenever the top few inches are only slightly damp. After the shrub is established, water it deeply at the rate recommended for the species. Species of shrub with average water needs should be watered to the depth of their roots once weekly or whenever the top 2 to 3 inches of the soil dry out. The best way to water shrubs is with a slow-running hose placed at the bottom of the plant.
Spread a 3-inch layer of organic mulch around the shrub's planting area. But keep it 3 to 4 inches away from the base of the shrub. Replenish the mulch every few years or so to keep it at a consistent height. And as the shrub grows, extend the mulched area so that it extends at least as far as the farthest branches of the shrub.
Fertilize the shrub during the growing season if that is recommended for the species. Use a balanced (16-4-8, 12-6-6 or 12-4-8) commercial slow-release fertilizer designed for use on shrubs or woody plants. Follow the manufacturer's instructions for application rates and amounts. Water with 3 inches of water after each fertilizer application to help the fertilizer reach the shrub's roots. In addition to a regular fertilization schedule, add any amendments dictated by the results of a soil test.
Prune your shrub. To encourage fuller growth in certain species, use pruning shears to prune 2 inches off the end of the shrub's new spring stems during its first year of growth. Prune away dead and diseased wood back to its point of origin, or two inches into the healthy tissue. Certain species need more specific types of pruning to encourage healthy, full growth. See the Resources section below for pruning guides for certain types of ornamental shrubs.