At a recently attended DBG class called “Fearless Pruning of Shrubs,” I learned lots of tips on what I should do this spring. The instructor suggested that March was a great month to prune since the harsh weather will be behind us but the structure of the shrubs will be easier to assess. 
If your shrubs are spring flowering ones that bloom on last year’s new wood (ie, forsythia, Nanking cherry, virburnum, lilacs), you might want to wait until just after they have bloomed. Summer-flowering shrubs (ie, mockorange, rose of Sharon, Annabelle and Peegee hydrangea), that bloom on new wood grown that season, can be pruned in early spring.
I learned that the primary objective in pruning flowering shrubs is to encourage new (thus, flowering) growth from the base. One should remove one third of the oldest wood to the ground each year. Trace the cane up to the top to see the impact of its removal. In addition to more flowers, thinning increases air circulation, which can reduce pests and disease.
If a shrub is overgrown, it is best to do rejuvenation pruning. The shrub is cut entirely to the ground in the early spring before growth starts. Over the next few years, thinning new canes to several strong ones creates a youthful plant with many blooms. In fact, I learned that flowering shrubs should be rejuvenated once a decade.
If more than one-third of the branches are woody without leaves or if the shrub has many dead branches, rejuvenation might not work. Some shrubs with one or few primary trunks, such as several Viburnum and Euonymus species, should not be cut to the ground. You can thin branches back to side branches. Also, lilacs grafted on a common lilac rootstock should not be cut to the ground.
Evergreen shrubs require little pruning. New needles will not grow from branches without needles. If the branch must be pruned back past the needles, remove it back to the trunk, but just outside the branch collar. If multiple trunks occur, remove all but one. On large trees, growth occurs at the top with minimal growth at the bottom. Do not do fall pruning on evergreens as it causes winter burn. Very slow growing spruce species do not tolerate pruning.
Pine trees only retain needles on the last two years of growth, so if you want to control the size of your pine, it is suggested that each year, you snap off one-third of the new growing tips while in the candle stage (before the new needles have fully grown). That is exactly what DBG’s Japanese garden does every spring to maintain the size and shape of all their pine specimens. 
Juniper and arborvitae can be pruned by cutting individual branches back to a side branch. Again, do not prune back into wood without needles. The base of the shrub should be wider than the top. This type of pruning is very time consuming but shearing causes dense exterior growth, browning and dieback. Shrubs grown in the shade may not tolerate pruning because of their slow growth rate.An excellent online resource for additional information is Garden Notes at www.cmg.colostate.edu

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