By Ron Honig, Horticulture and Agronomy Agent
The goal of pruning is to maintain or enhance the beauty of a shrub without harming its health. The methods used to attain this vary depending on the type and condition of the shrub and the desired look.
Following is information on pruning shrubs compiled by K-State horticulturists Ward Upham, Cheryl Boyer and Jason Griffin.
First, make sure tools are clean and sharp and take appropriate safety precautions.
When to Prune
Light pruning (where no more than 10 percent of the shrub is removed) can be done any time of year. Timing for more severe pruning is determined by when the shrub flowers. Shrubs that bloom early in the spring such as lilac, forsythia, and mock orange are often pruned immediately after flowering to preserve enjoyment of the flowers.
Shrubs that flower in summer or those grown for foliage are normally pruned early in the spring before growth begins. It is important to note whether the plant is a conifer (juniper, pine, spruce). If you cut a conifer, it may not grow back as conifers do not have dormant buds on older wood, so they do not resprout behind pruning cuts. Most broadleaved shrubs, however, resprout vigorously due to the many dormant buds along the stem.
Light Pruning
Light pruning is done to improve the attractiveness of a shrub or to control its height and can be done any time of year on most shrubs. Start by removing all dead and diseased branches. Branches that cross or are out of place are the next to go. A few older canes or branches that have lost vigor can also be removed. Do not take out more than 10 percent of a shrub when light pruning. Dead wood does not count in this 10 percent.
Shearing
Shearing is used in formal landscapes but takes a great deal of time to maintain. Homeowners are usually better served by using another pruning method. Though some shrubs tolerate shearing, most react by producing thick, new growth along the sheared edges leaving the rest of the plant looking thin and sparse. Even shrubs adapted to shearing will have lower branches die or thin unless the lower portions of the shrub are maintained wider at the base than the top to avoid shading. Examples of shrubs that are often sheared include boxwood, juniper, and arborvitae.
Heading Back
When heading back, excessively long canes are cut back to a side branch or bud to reduce overall height. This method encourages new growth to appear just inside the pruning cut and gives the shrub a fuller, more pleasing effect. Examples of plants that can benefit from heading back include azaleas, roses, and junipers.
Thinning
Thinning is when canes are removed to reduce height and open the plant to encourage more interior growth. Older, less vigorous canes are removed first. Cut canes to be removed at ground level. Examples include forsythia, lilac, crape myrtle, and bayberry. Many shrubs benefit from a combination of heading back and thinning cuts.
Renewal (Rejuvenation) Pruning
Shrubs will sometimes become so overgrown that thinning will not be adequate. Some shrubs tolerate renewal pruning in which all growth is taken back to about 6-inch stubs. This type of pruning is a stress to the plant and should be done in very early spring before the new growth starts. Regrowth from spring pruning will be rapid and will soon cover the stubs.
Flowering shrubs normally do not flower the year following renewal pruning. Conifer shrubs such as juniper and pines do not respond well to renewal pruning and are best removed and replaced if they outgrow their space.
Shrubs that tolerate the severe rejuvenation pruning include lilac, forsythia, barberry, sweet mock orange, scarlet firethorn, spirea, viburnums, weigela, and yew.
