We have some 30 year old Cedar Trees in our garden. They are 25-30ft high. We have lopped off the top 18″ of these trees. Did we make a mistake? Our intention was to encourage them to spread out rather than simply growing taller. Comments and recommendations will be appreciated.
Thank you for contacting Toronto Master Gardeners about your Cedar trees, Thuja occidentalis. From your description, I am unsure if you are trying to get last year’s new growth to branch out, or if you are hoping for new growth from areas with old wood or from the trunk
Cedar trees and hedges do not regrow from old wood. Cedar trees grow from last years growth so when trimming, one should not trim past the green growth from last year. If you have cut the top main branch past the green leafy growth, hopefully one of the other top branches will compensate and become the new leader for the tree.
To encourage new growth lower down one can thin out new growth to allow more light into the lower branches. As you are dealing with large trees it would be advisable to consult an arborist for safety reasons.
There are two detailed answers on the Toronto Master Gardener website, which discuss pruning and care for cedars. I am attaching links below for you to consult.