Timing Is Everything
Shaping your shrubs may be counterproductive if you prune them at the wrong time of year. Shrubs grown for their foliage, such as heavenly bamboo (Nandina domestica), a perennial in U.S. Department of Agriculture plant hardiness zones 6 through 9, benefit from early spring pruning, before new growth appears. A general rule of thumb for pruning spring-flowering shrubs that bloom before May, such as forsythia (Forsythia spp.), which grows in USDA zones 4 through 8, is to prune them immediately after they finish blooming. Prune summer-blooming shrubs that flower after May, such as beautyberry (Callicarpa dichotoma), which grows in USDA zones 5 through 8, in spring before the new growth appears.
Hedges define borders, provide privacy screens and create windbreaks that protect other plants in your yard. Shaping shrubs to create a dense hedge may require frequent pruning, sometimes twice yearly -- once in spring and again in midsummer. Some shrubs, such as cleyera (Ternstroemia gymnanthera), a perennial in USDA zones 8 through 10, have new growth in a contrasting color to older growth. If you prune cleyera in spring, you remove its attractive reddish new growth, so you may want to wait until midsummer when the red turns green before you shape the hedge. Instead of shearing a hedge to shape it, prune the plants in a vase shape so they are slightly wider at the base than the top.
Older and Overgrown Shrubs
Renewal pruning is a three-year process that can restore an untidy shrub, such as French or bigleaf hydrangea (Hydrangea macrophylla), which grows in USDA zones 6 through 10, to a thicker, healthier shape. The first year, remove all dead and crossing branches and one-third of a shrub’s oldest stems. The second year, remove one-half of the remaining old stems. The third year, remove the rest of the old stems. Rejuvenation pruning is a more drastic shaping technique that cuts all of a deciduous shrub’s branches close to the ground. Plants that can withstand the severity of this technique include red twig dogwood (Cornus sericea), a perennial in USDA zones 3 through 7, and butterfly bush (Buddleja davidii), which grows in USDA zones 5 through 10.
Topiaries and Espaliers
Shrubs formed into topiary or espalier shapes can add an eyecatching focal point to your garden. Espaliered shrubs, such as scarlet firethorn (Pyracantha coccinea), a perennial in USDA zones 6 through 9, grow against walls or fences. As a shrub grows, tie its branches to supports that form the desired shape and remove all other branches. Topiaries form three-dimensional shapes. Common boxwood (Buxus sempervirens), a perennial in USDA zones 5 through 8, is the quintessential topiary choice. If you have an artistic flair, you can snip shrubs freehand into the desired shape. Alternatively, plant shrubs inside a moss-filled wire topiary form. As the shrubs grow, they fill the form.
Sterilizing Pruning Tools
Regardless of whether you use handheld pruning shears, lopping pruners or hand saws to shape your shrubs, you may unknowingly spread diseases to your plants by using contaminated tools. Disinfect pruning tools by soaking them for five minutes in a solution of 1 part household pine oil cleaner and 3 parts water. Rinse the tools with water before using them. Do not apply the disinfectant directly to plants because it may inhibit the formation of callus tissue, which helps the shrubs heal themselves from pruning wounds.