Nothing To Be Afraid Of: Pruning Shrubs

Shrub pruning often can instill fear in contractors, but following simple guidelines can calm nerves and lead to successful shrub care.

Here’s a quick poll idea: At your next neighborhood gathering, ask 10 people if they remember the last time they pruned their lilac, Viburnum or dogwood. Then ask them how they went about performing that task. If they answer, "Uh, yeah, I think I whacked a few branches off of my shrubs a couple of years ago. I guess now is the time to prune them again." Then you know they have no clue.

Most folks simply don’t know how to prune correctly. Believe it or not, some landscape maintenance crews don’t either. This lack of knowledge leads to fear. Fear that you’ll prune incorrectly and the shrub will look awful afterwards. Fear that the shrub will never bloom again. Fear that the customer will complain about the way the shrub looks. Fear that this will lead to a sudden need to peruse the "help wanted" section of the local newspaper. Relax. Shrub pruning is quite simple, actually.

RENEWAL PRUNING. There are three methods of shrub pruning that merit mentioning. The first one is "renewal pruning." This is very simple. Just cut off all the stems at the ground level. Renewal pruning works well for overgrown, heavily diseased, damaged or winter-killed shrubs where the objective is to start over with new growth.

The types of shrubs that respond well to renewal pruning are Spiraea, dogwood, Viburnum, coralberry, Cotoneaster, privet, burning bush, butterfly bush, hydrangea and barberry. In general, most evergreen shrubs don’t respond well to this approach because they don’t have apical meristems or new growing points at the ground level of the plant. So, avoid this method of pruning for yew, juniper, holly, Rhododendron, boxwood and muhgo pine.

Renewal pruning should be done in early spring or mid- to late fall for best results. The key is to allow for several months of good growing conditions for the shrub to encourage new, healthy stems to form. Early fall is not an ideal time for renewal pruning, as the new stems need a few months to harden off before experiencing winter’s cold temperatures. Early fall renewal pruning lets the shrub develop ample new, succulent growth to make it susceptible to winter kill. This is especially true of landscapes in the transition zone of Illinois, Missouri, Iowa, Nebraska, Kansas and Indiana, where extreme winter temperature fluctuations are common. Meanwhile, pruning too late in the year doesn’t allow enough time for significant shoot growth.

Other entomologists and plant pathologists that I have discussed renewal pruning with have commented on how much they like this method. The benefits from their point of view are twofold. First, renewal pruning offers a great opportunity to remove diseased stems and insect-infested tissue from the plant. In many cases, fungal cankers develop on a plant. There are no effective control methods for these diseases other than to cut out the affected areas of the plant. Crown gall of Euonymus, black knot of ornamental plums and cherries, anthracnose of Spiraea, and cytospora canker of corkscrew willow are good examples. Likewise, lilac borers and various scales on Cotoneaster, privet or Euonymus can be partially or completely controlled by removing infested stems.

A second advantage that pest specialists highlight is the improved shrub architecture that renewal pruning creates. As new stems develop and replace those that were removed there is less stem mass, which presents better opportunities for air flow through the stems. This leads to leaves that stay drier and are less likely to develop foliar diseases such as leaf spot and rust. The new plant growth allows landscape maintenance crews to work with the new shoots as they develop to produce the proper spacing and orientation, instead of dealing with a solid mass of ungangly stems.

A disadvantage of renewal pruning is the delay of future blooms. Most customers are probably used to seeing their lilacs and dogwoods bloom each year. Because this method removes wood of flowering age and replaces it with new shoots, no blooms are likely to be formed the first year after pruning. Regardless of that fact, convince clients that renewal pruning is an investment in the health of their landscape and should be considered a viable procedure. Avoid renewal pruning on all of a client’s landscape shrubs in any given year, however. This may lead to client dissatisfaction. After all, the client is paying the landscape contractor to improve the look of the landscape, not worsten it.

Clear communication is a key element to successfully adopt this method, especially with new accounts. Take some time to explain to the customer that in most situations the growth will be about 12 to 18 inches high in a few months, and after a growing season the shrub will return as a vibrant part of the landscape. If you don’t warn clients in advance, they might accuse you of killing the plant by cutting it back too far. "Before" and "After" photos will help to explain this rapid regrowth potential.

The Pruning

    Pruning Method

    Good Candidates

    Bad Candidates

    Renewal pruning

    Spiraea, dogwood, Viburnum, coralberry, Cotoneaster, privet burning bush, butterfly bush, hydrangea, barberry

    yew, juniper, holly, boxwood, rhododendron, muhgo pine

    Thinning out

    Spiraea, Viburnum, dogwood, holly, yews, juniper, azalea, boxwood, rhododendron

    pine, spruce, fir

THINNING OUT. Another common shrub pruning method is "thinning out." Each year, after blooming, remove about one-third of the oldest stems at the ground level. This method produces a balance of young, middle-aged and mature wood in the shrub. This means getting down on your hands and knees, pushing aside the stems to identify them and then cutting them at the crown of the plant. In most cases, a by-pass hand pruner, lopping shears and a small pruning saw will be the only tools needed.

When uncertain about how many stems to remove from a shrub, do some quick math. In general, removing one-fourth to one-third of the stems produces the desired results, allowing greater air flow through the shrub and benefiting the shrub’s health. For example, if a Spiraea has 30 stems, remove seven to nine of them. When a shrub has just a few stems, like some black haw Viburnums do, then removing only one or two stems may be sufficient. Remember that the goal is to produce a shrub that has new and mature wood.

Older stems are easy to identify as removal targets. They are generally darker in color and thicker than newer stems. In many shrub species, older stems tend to produce a naturally exfoliating or peeling bark. This is a cue that the stem must be removed. Other targets for removal include stems with suspicious abnormalities or holes with fine sawdust surrounding them. As previously discussed, these are common homes for various pest species.

After the stems have been selectively removed, finish by removing a few more straggly stems to give the shrub some shape. In most situations, a few stems that are growing at undesirable angles or are much taller/longer than the rest of the plant will remain. Give the shrub a finished look by removing them entirely or clipping the objectionable part of the stem. This lets customers know that contractors are concerned with aesthetic appeal as well as overall plant health.

Similar to renewal pruning, shrub varieties that respond well to thinning out are deciduous species such as Spiraea, Viburnum and dogwood. A few evergreen shrubs also respond well to this technique, and holly, yews, junipers, azaleas, rhododendrons and boxwood will benefit from periodic thinning. Conversely, pines, spruce and fir do not produce regrowth when this method is used, so double checking the plant’s identity is an important step that should be taken before pruning.

Thinning out should be done in early spring or mid- to late fall for best results.

HEADING BACK. A third pruning method, "heading back," is appropriate only for new hedges, and rarely should be used. Ironically, this is the technique that most homeowners and some novice landscape contractors use. This approach involves using a hedge trimmer to make arbitrary, indiscriminate cuts in the shrub. Heading back produces an attractive plant the first few times it is practiced. The shape is nice and clean, the lines are sharp and the plants can be manipulated to appear identical.

However, in a few years, an area of knotty growth develops near the area cut by hedge trimmers. This area becomes unsightly in a short period of time. The worst consequence of this pruning method is that when mechanical injury occurs from sunburn, vandalism or routine wear and tear, the shrub has no mechanism to replace the affected area. In most cases, the shrub developes large voids or dead looking areas with no leaves at all.

A few special situations exist where heading back is recommended. If a client desires a high- maintenance landscape and requests evergreens be cut into funny-shaped balls or depictions of elephants and animals, then this is the way to do it.

More reasonably, heading back can be exercised to keep an evergreen shrub or tree from growing into a sidewalk area or over a driveway to maintain clearance for traffic. In this case, cut the new growth in half in midsummer. This allows the plant to thrive in the landscape for many years before lateral growth shadows activity on the path or patio. Actually, Christmas tree growers use this technique each year when they prepare and shape their crop for sale.

By using a thinning out or a renewal pruning technique, customers will be well served and landscape contractors will be satisfied from producing an aesthetically pleasing, healthy landscape for them.

The author is an extension educator at the University of Nebraska, Lincoln, Neb.

October 2000
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