Andrea is a garden writer and horticulture extension agent. She is a formally educated ornamental horticulturalist, but has a personal passion deeply rooted in edible gardening.

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PRUNE PERFECT: Pruning Landscape Shrubs with Perfection
by Andrea Dee    

Gardeners often forget the magic of how quickly shrubs can fill an empty space in the landscape. While controlling size is the most common need for pruning, other reasons include encouraging bounties of bloom, artistic shaping such as when designing topiaries, and removal of dead and diseased wood. 

Ultimately, choosing the “right plant for the right place” is your best option for controlling shrub size and supporting plant health. The days of seeing only boxwood with predictable growth habits are gone. Now there are hundreds of cultivars available of some species. It is important to learn specific cultivars and to read the plant information highlighted on the tag. A commonly favored boxwood is ‘Green Velvet’. Its mature size is 2 feet high by 2½ feet wide. This makes it a perfect choice for most foundation plantings and alleviates the need for annual pruning to manipulate size. If you are looking for something with a more pyramidal form, you may prefer ‘Green Mountain’, which matures into a 4-foot-high by 3-foot-wide, pyramidal-shaped evergreen that requires minimal pruning to keep shape. These are examples of how to pick the right plant for the right place, but sometimes there is no variety to match your desires and you must rely on pruning. 

Pruning Flowering Shrubs
Shrubs that flower before June should be pruned during or immediately after flowering. These shrubs bloom on “old wood,” which was formed the previous summer. When pruned just after blooming you are allowing ample time before winter to develop wood for next spring’s bloom. If you are less concerned with a spring show of flowers and more concerned with controlling the size of an unruly shrub, you can forfeit the blossoming buds and prune in late winter instead. Examples of these shrub varieties include: barberry (Berberis spp.), quince (Chaenomeles speciosa), smoke tree (Cotinus coggygria), Cornelian cherry (Cornus mas), DeutziaForsythia, holly (Ilex spp.), mountain laurel (Kalmia latifolia), Magnolia, mock orange (Philadelphus coronarius), PierisPyracanthaRhododendronSpiraea, lilac (Syringa spp.) and Viburnum.

Shrubs that flower after June or have insignificant blooms should be pruned in the winter or early spring before new growth appears. These shrubs bloom on new wood formed during the current spring or summer. Examples of these shrubs include Abelia, butterfly bush (Buddleia spp.), beautybush (Kolkwitzia amabilis), Clematis, Clutha, rose of Sharon (Hibiscus syriacus), Hydrangea, crapemyrtle (Lagerstroemia indica), ninebark (Physocarpus opulifolius) and rose.

Some shrubs may benefit from light pruning before and after flowering. Double pruning can increase flowering and may result in a second bloom during the growing season for certain shrubs. Examples of these varieties include: Abelia, butterfly bush, red twig dogwood (Cornus sericea), Cotoneaster, Oregon grapeholly (Mahonia aquifolium), Spiraea and Wiegela.

Pruning Evergreen and Conifer Shrubs
Evergreen shrubs can be pruned in the spring when new growth pushes out. Simply pinch or prune new growth with hand pruners to control size. Avoid hedging evergreens with electric shears, which leads to tattered branch tips and a top-heavy shape shading the lower canopy contributing to dieback. Instead select an appropriately sized cultivar for the landscape and thin the canopy through hand pruning to allow maximum light infiltration and airflow. 

Prune outer “green wood” and avoid pruning brown “dead wood” within the inner shrub canopy. You can check to see if wood is green and actively budding new growth by simply scratching the bark with your fingernail or a dull knife. Active tissues will show green cambium beneath the outer bark, and when pruned will rejuvenate by pushing new growth. If a shrub is pruned into the brown “dead wood” the shrub will not likely push new growth and that branch will remain bare. Yew (Taxus spp.) is the exception to this rule; yew shrubs will regenerate new growth even when pruned deeply into the brown cambium wood. 

Conifers fall into two categories of growth, whorled-branching and random-branching. Spruce (Picea spp.), fir (Abies spp.) and most pines (Pinus spp.) are considered whorled-branching types and should be pruned back to another branch or bud. Additionally, special care should be taken to not remove their central leader. Their central leader stem is the most upright pointing middle branch in the center of the specimen. If this stem is damaged or accidentally broken, train a new central leader to take its place through bracing a side branch upward. Juniper (Juniperus spp.), hemlock (Tsuga spp.), yew and arborvitae (Thuja spp.) are considered random-branching. Their undesirable branches can be removed altogether. Branches can also be shortened by pruning back to a bud, making sure to not prune beyond the last green growth unless removing a branch entirely.

Rejuvenation Through Pruning
If you have missed a few years of annual pruning it may be time to reclaim the garden and get aggressive with a pair of loppers. The good news is that some shrubs can be severely cut to the ground and will regenerate a healthy new canopy. Rose-of-Sharon, forsythia, hydrangea, butterfly bush, lilac, and spirea all benefit from this type of rejuvenation. Many gardeners consistently control the size of their Knock Out shrub roses by pruning stems to a height of 8-10 inches from the ground in late winter before new leaves emerge.

For some species, pruning the entire canopy to the base is too drastic. You can implement a three-year pruning sequence if that is of concern. Typically, plants can survive losing up to one-third of their canopy in one growing season. You can re-shape and reinvigorate a waning shrub, like an overly mature viburnum, through a three-year process of removing one-third of the canopy each year. It is recommended this pruning be done in late winter before leaves emerge. This method is most useful when a shrub has matured to be top heavy and lacks internal foliage. After the third year of rejuvenation, and removal of all old branches, you will have created a much more compact and vibrant shrub. 

Tools to Get the Job Done
There is a wide range of pruning tools available. Using bypass pruners and loppers is recommended to encourage proper healing of cut wounds. Bypass pruners and loppers work similarly, with a blade passing through a branch like scissors while the hook blade holds onto the branch. Pruners work best when the branch is smaller than 1 inch in diameter, and loppers are recommended when the branch is larger. If loppers won’t make the cut, you may consider using a pruning saw. Pruning saws are very effective on larger branches; but keep in mind they are designed to cut on the pull stroke, since pulling is the most natural motion for cutting overhead branches.

 

 

A version of this article appeared in a previous print edition of State-by-State Gardening.
Photography courtesy of Andrea Dee.

 

Posted: 02/06/19   RSS | Print

 

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