Why prune?

Pruning has a major influence on a shrub’s flowering habit, size, shape, and pest problems.

Prune to Encourage Flowering

Pruning has a major influence on shrub flowering.  Over time, an unpruned flowering shrub becomes woody with little new growth, which supports flower bud development.

Spring-flowering shrubs bloom on one-year-old wood (twigs that grew new the previous summer).  Buds develop mid-summer through fall for the following spring.  Pruning in the fall to spring removes flowering wood with buds.  However, the old adage “head back spring flowering shrubs after blooming” may lead to an incorrect understanding of proper pruning techniques.  Spring-flowering shrubs should be thinned or renewed (as described below) in early spring before flowering or growth starts. [Figures 1 and 2]

On spring-flowering shrubs it is recommended to “deadhead” spent blooms (remove flowers after they fade).  While time-consuming, it conserves the plant’s energy, which would otherwise be spent on seedpod and seed development.  On many shrubs, the spent flowers and seedpods are not attractive.

Summer-flowering shrubs bloom on new wood that grew earlier this growing season.  Summer-flowering shrubs are also pruned by thinning or renewal in the early spring before growth starts.  [Figure 3]

On shrubs noted for their bark color (like Red-Twig Dogwood) the new growth has more brilliant color.  Routine pruning encourages new shoots, which have the desired red color. 

Removing older canes of flowering shrubs such as lilac also allow better sunlight penetration into the shrub.  This results in better flowering throughout the shrub, instead of flowers just at the top where sunlight is sufficient.

Prune to Direct Shape

Shaping is another reason for pruning shrubs.  Shape can be managed to some degree by pruning to side buds or branches growing in the desired direction.  While pruning has some control over size, it is not an effective method to keep a large shrub in a small space.  Where shrubs have overgrown their space, consider replacing the plants with smaller cultivars or other species.  [Figures 4 and 5]

Prune to Manage Pests

Pruning is a management technique for some insect or disease problems.  For example, removing the older wood in lilac reduces the incidence of oystershell scale and borers.  Thinning a shrub to increase air circulation reduces the incidence of powdery mildew and leaf spot diseases. 

Pruning Methods for Flowering Shrubs

The primary objective in pruning flowering shrubs is to encourage new (flowering) growth from the base.  This is best accomplished by rejuvenation. 

Branch by Branch Shaping

With branch to branch shaping, shorten the length of excessively long branches by cutting them back one-by-one.  Cuts are made back in the shrub leaving branches at varying lengths.  Avoid making cuts at a uniform “edge” creating a rounded ball.  Make cuts at appropriate branch unions (crotch) or buds.  [Figure 6]

This method maintains a more natural shape to the shrub.  It does not however significantly encourage new growth of flowering wood for maximum bloom.  Branch by branch shaping is a slow process. 

Shearing to Shape

Shearing shrubs to a round ball or other desired shape is a common pruning technique because it is quick and easy.  However, sheared shrubs lose their natural shape and the rounded “balls” may detract from a more natural informal landscape design.  Shaping spring flowering shrubs after mid-summer removes the new wood with next year’s blossoms.  Frequent shearing does not encourage new growth from the base, which is needed to promote flowering. 

With frequent shearing, the plant becomes bushier on the exterior.  The thick outer foliage may shade out the interior and lower foliage and the plant becomes a thin shell of foliage with a woody interior and base.  The thin shell of foliage is prone to browning and burning from wind and cold weather.  Over time, shrubs become woody with lots of dead branches and few flowers.  When shrubs become overly woody from routine shearing, replacement is the best option to refresh the landscape design.  [Figures 7, 8 and 9]

Thinning
One method to encourage shrub flowering is with annual thinning.  The objective is to remove one-third of the oldest wood to the ground each year, which in turn stimulates new (flowering) growth from the base of the shrub.  This is done in the spring before growth starts.  When done correctly, this method becomes rather time consuming and is difficult to nearly impossible to do on multi-stem, twiggy-type shrubs.  [Figure 10]

Cutting back and thinning an over-grown shrub will not restore its natural informal form.  It will simply look like an over-grown shrub that has been pruned. 

Rejuvenation Pruning

Many shrubs can be easily renewed with rejuvenation pruning.  The shrub is cut entirely to the ground in the early spring before growth starts.  The shrub simply regrows from roots, giving a compact youthful plant with maximum bloom.  This is the method preferred for most flowering shrubs, being quick and easy with great results. 

Rejuvenation is typically done every three to five years, basically when the shrub begins to look gangly and woody.  It works superbly well on multi-stem twiggy-type shrubs such as Hydrangeas, Spiraea, Caryopteris (Bluebeard), Potentilla, Red-twig Dogwood, and Sumac.  (Note: Caryopteris flowers best if renewed each spring.) 

Limitations:

• Spring flowering shrubs will not bloom the year of rejuvenation. 
• On shrubs with a rock and weed fabric mulch, rejuvenation may not be successful due to decreased root vigor and interference of the mulch with growth from the base.
• Extremely overgrown shrubs with a large woody base may not respond well to rejuvenation pruning.
• Shrubs with a lot of dead branches will not respond well to rejuvenation pruning.  As a rule of thumb, if more than one-third of the branches are woody, without healthy foliage, the shrub will probably not respond.
• Some shrubs are structurally more like a small tree with a few primary trunks.  This includes several of the Viburnum and Euonymus species, and shrubby forms of Rhamnus (buckthorn).  Don’t cut these shrubs to the ground.  Prune by thinning branches back to a side branch.
• Lilac cultivars budded onto common lilac root stocks should not be cut to the ground. Regrowth will be common lilac rather than the selected cultivar.

Replacement

When shrubs that have been repeatedly sheared become woody with dead twigs, the best option is to replace them.  In many commercial sites, labor issues prohibit routine pruning.  When shrubs become overgrown they are simply replaced as a low maintenance alternative.

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