Winter: A Time to Prune Shrubs

By Bruce Crawford – Manager of Horticulture, Morris County Parks Commission

Winter is the season when many gardeners wish to hibernate indoors, thumbing through seed catalogues that endlessly appear at the doorstep.  Granted, there are inhospitable days, yet there remain many very pleasurable days for tending to the Garden. One of the more important tasks for winter is pruning, a task professional gardeners know all too well! It is essential for maintaining the integrity of the garden and pruning your shrubs is a very rewarding task for the gardener.

First, let’s consider the tools. One tool not needed are hedge shears unless a formal hedge is desired!  For most pruning needs consider a pair of bypass hand pruners. Bypass pruners allow the blade to travel past the anvil, much like a pair of scissors as it cuts the stem. Bypass pruners are effective for stems up to 3/8″ in diameter and for larger cuts, a small handsaw is preferred. I favor a tri-edge saw, providing an exceptionally smooth cut which reduces the potential for decay. The drawback to tri-edge saws is the difficulty of sharpening the blade, making it wise to purchase several spare blades when buying the saw.  I prefer models with a 6-8″ folding blade which are ideal for stems up to 3-4″ in diameter. Loppers are also handy to have on hand for where an extended reach is necessary, such as into a rose bush or a multistemmed shrub.

It is also important to qualify the major approaches to pruning. Rejuvenation pruning involves the removal of dead and non-vigorous older stems of multistemmed shrubs. This pruning encourages vigorous new growth from the base that will yield better flowering. Renewal pruning involves cutting the entire plant back to ‘stubs’, allowing the plant to essentially start anew. January through March is the optimum pruning period, while late summer is the worst pruning period as it encourages new growth likely to be killed by frost.

Without foliage, winter also allows you to evaluate a plant’s branching habit. Shrubs like Seven Son Flower (Heptacodium) will produce vigorous upright shoots from branches called water sprouts, detracting from the form. Water sprouts are in response to the plants’ need for more photosynthetic sugars. It took me 30 years to understand how thinning these sprouts, not removing them entirely is best for the plant! Similarly, vigorous growth or suckers from the base of grafted shrubs like Witch Hazels (Hamamelis x intermedia) indicates the graft is not 100% compatible. These shoots should be entirely removed close to the point of growth, regardless of the season.    

Rejuvenation pruning is best for mature, multistemmed flowering shrubs where stems or canes are losing vigor. Cutting a few 6 to 20-year-old stems back to 4-10″ result in the production of new stems that are more floriferous and provide an overall improved appearance! Flowering shrubs such as Forsythia, Mock Orange, Lilac, Deutzia and Weigelia benefit from this thinning and even though you are removing flower buds with the stems, the flowering impact come spring still looks great.

Hydrangea pruning is always surrounded by much controversy, since some species bloom on the previous season’s growth while others on current season’s growth. Those species native to colder areas, such as H. arborescens and H. paniculata bloom on current growth and can be pruned moderately to harshly in winter. For H. arborescens, I usually pursue drastic pruning measures and cut the entire plant to the ground in February as with the resulting June bloom! For H. paniculata, I typically prune the plant back to a 3-5′ tall structure, although cultivars with more open flower panicles like ‘Unique’ can be pruned more drastically. These open flower panicles hold less rainwater and can be supported by the willowy 5′ stems! Species native to warmer climates, such as H. macrophylla and H. quercifolia produce flowers on the previous year’s wood and care should be exercised in the amount and which wood is removed. For H. macrophylla, the current season’s growth is usually mahogany brown in color and should not be touched as the terminal bud is the flower bud! Older stems are grey and should be cut to the ground after 6-8 years as they lose vigor. For H. quercifolia, the pruning is more to shape the plant, not to rejuvenate the plant.

For plants with colorful stems during the winter months, such as Red-stemmed Dogwoods (Cornus sanguinea) and Willows (Salix alba ‘Britzensis’), only the younger stems are colorful while the older wood develops grey bark. In areas populated by deer, it is best to rejuvenate these plants by only removing those canes 3 years or older, stimulating new growth as seen in the last image. The remaining stems provide the newly emerging stems protection from deer browse. If deer are not an issue, the plants can be renewal pruned to a height of 3-6″ every 3 years!  This results in a very uniform development of new shoots.  Butterfly Bushes, as well as purple or golden foliaged forms of Smoke Bush (Cotinus coggygria) also benefit from a renewal pruning to 6-12″ annually.

For those old and leggy Rhododendrons, Azaleas and Mountain Laurels, March is the time for renewal pruning by cutting the plants back to 12-24″ in height. The plants will break bud along the stems and within 2 years a more compact plant will result! Overgrown plants of Holly (Ilex), Tea Holly (Osmanthus), Boxwood (Buxus) and Yew (Taxus) will also respond in a similar manner and can be pruned severely. This is an effective treatment for overgrown hedges as seen at right, although new growth may not appear until June and all the plants may not respond with equal vigor.   

It should be noted that you don’t need to prune every plant! Virginia Sweetspire (Itea) and Southern Bush Honeysuckle (Diervilla) will rarely need pruning. Yet for most shrubs, pruning is beneficial as it literally preserves a garden’s past while shaping its future. What better a task for the gardener on a nice winter’s day!