Pruning Mature White Cedar Trees

(Question)

I live in Toronto. My back yard has a hedge of 40-50 ft. white cedar trees. The branches have spread 6-8 feet from the truck. I would like to prune the branches back to 4 feet. Will this harm the trees? What are your recommendations?

(Answer)

Cedar trees, Thuja occidentalis, must be pruned with care and a certain amount of restraint. Cedar trees produce new growth only from last year’s growth, so if too much is trimmed it is possible to prevent any new growth on a branch.  Care should be given to retaining some greenery on the cut branches for future growth, otherwise there will be a  ‘hole’  in the foliage which may not get filled in.   When cutting the branches try to balance the cuts on the trees.   A tree whose weight is out of balance can eventually fall over or split under the strain.

Based on your description, you have a very  tall and and wide hedge, as I cant see it, I assume that the hedge is getting scraggy and sparse in certain areas. If the hedge has not been trimmed for some time.  Mark Cullen in his blog, see link below, says a cedar hedge can be pruned anytime . The Toronto Master Gardeners have a fact sheet “EVERGREENS SUITABLE FOR HEDGING” which advises trimming a cedar hedge in June. What is important is to  proceed cautiously and give it a light trimming of new growth to start and maybe even a test prune in an inconspicuous spot to see how it does before going ahead and drastically pruning an excessive amount.   In fact, for a hedge this old and obviously overgrown, I would recommend a consultation with an arborist before doing anything- It could be the whole hedge needs reshaping to keep proper proportions, and likely needs a good feeding as well.

Please see this link for more information:

https://mark-cullen.blogspot.ca/2010/09/cedar-hedges-investment-that-grows.html