University of Vermont Extension 
Department of Plant and Soil Science

Winter News Article

 
PRUNING DORMANT PLANTS
 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
           
Eager to get back in the garden?  If you have trees and shrubs, those "warmer" days of winter are a good time to prune many of them while they are still dormant.

The rule of thumb is to prune those woody landscape plants that bloom on current season's wood while dormant.  Those that bloom on the previous season's wood, prune after they bloom in late spring on early summer.  Examples of those that you should prune after bloom are lilacs, forsythia, rhododendrons, and early viburnums.  Prune these right after they bloom, as they will then start forming flower buds for the following year.  This is one answer to a question I often get, "Why didn't my shrubs bloom this year?"  Pruning these during summer or fall will cut off next year's flowers.  I often see this with forsythia, which grow rampant and gardeners "shape" during the growing season. Of course broken branches can be pruned off now in winter.

So all the rest you can prune now while still dormant and resting for winter.  Some of the common shrubs you prune while dormant include glossy abelia, barberries, blue mist (Caryopteris), summersweet, smokebush, spirea, cotoneasters, and late viburnums (such as blackhaw and American cranberry bush).  Depending on how high you want these to grow, you can prune them back to as much as above the first pair of buds above the soil.

A drastic version of cutting back shrubs in winter is known as "renewal" pruning.  This involves cutting a shrub back to about six inches above the ground, and only is suitable for some plants.  This is a good practice for these shrubs if they didn't bloom as in the past, or look overgrown with weak and straggly stems.  Renewal pruning invigorates these shrubs, as drastic as it sounds and looks.  Candidates for this include glossy abelia, barberry, blue mist, forsythia, honeysuckle, ninebark, shrubby cinquefoil (Potentilla), hibiscus, lilac, spirea, and weigela.  

If you can't bear to cut these shrubs totally back, or don't want the sight of an empty spot in the garden or landscape while they regrow, cut back in stages over a three year period.  With this plan, only cut back to the ground about one third of the oldest stems each year.  Some rampant growers, such as forsythia and ninebark, you may want to continue renewal pruning each year.  Some gardeners practice this regularly even with shrubs such as lilacs, in order to keep them lower.

A variation of renewal pruning is known as "coppicing".  This cutting to the ground during winter stimulates vigorous new growth each year.  In the case of shrubby dogwoods and some shrub willows, it can stimulate more brilliant stem colors.  The sacrifice, though, is cutting off flowers and fruits.  If you want these, then only coppice every other year.  If butterfly bush survives in your area, this too can be coppiced.  A favorite large tree to coppice is the Princess tree (Paulownia), creating tall bushy plants with large leaves each year.

A type of winter pruning of trees, seen more in Europe than America, is called "pollarding".  This involves cutting trees back to the same point each year to control growth and shape.  The result is a knobby fist where shoots grow from each year, a rather odd appearance if you are not used to this.  Purple smokebush is one small tree you might try this with.  I have seen it quite commonly on street

A group of trees generally not pruned in winter are known as "bleeders".  These should be pruned in summer after they leaf out if possible.  During late winter and early spring the sap is rising, and will "bleed" from open wounds.  This does little harm to the tree except perhaps providing a site for disease infection, even though we equate this to our own bleeding.  Maple is the most common and famous in this group of trees, which also includes birch, beech, oaks, lindens, and elms. 

Trees that you should prune in winter or late winter include apples, flowering crabapples, mountain ash, hawthorn, and honey locust.  These may get bacterial and other diseases (stem canker in the case of the latter) if pruned in summer.

Pruning in late winter is good in the sense that diseases are not active then to invade the open wounds.  When the plants resume growth in the spring shortly after pruning, the wounds will heal rapidly.  Pruning in winter, especially if severe pruning, will stimulate the plant to replace leaves and shoots first, perhaps at the expense of flowering, so is a fact to keep in mind if flowering is important. Keep in mind too that a tree or shrub, even after pruning, will grow back to its natural shape.  If you want a certain shape in the landscape, it is easier and saves much pruning to choose a plant with that shape naturally.

There are many online resources and books on pruning, including when and diagrams showing how, including The Homeowner's Complete Tree and Shrub Handbook by Penelope O'Sullivan.
             

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