If I trim Leylandii or cedar hedge to much and show the brown stuff, will it ever become green again?
Thanks for your question about trimming cedar or Leylandii hedges. It would be useful to know which you have. If you live in Toronto, the likelihood is that your hedge is cedar, but some of the same principles apply whichever you have.
Cedars pruned on old wood, which is what I think you are describing, will not rejuvenate: that is, they will not produce new growth on the older branches that remain. But when newer growth is trimmed, cedars – and Leyland cypress – will respond by producing more new growth. If you have inadvertently pruned too far into the hedge, it is possible that with good irrigation, fertilization and soil amendment, your hedge may produce enough new foliage on other branches to help hide somewhat the cuts that have been made to old wood.
The timing of your pruning is important, whether your hedge is cedar or Leyland cypress. Since pruning stimulates new growth, it is best to avoid pruning in late summer or fall, because the new growth that emerges will be vulnerable to the coming cold temperatures and could easily be damaged. Save your pruning until late spring and early summer, and prune for shape on new growth only.
The Toronto Master Gardeners receive many questions about pruning evergreen hedges. Here are a couple of links to previous posts that may be helpful:
You may also find it helpful to read our guide to evergreen hedging: https://www.torontomastergardeners.ca/index.php/factsheet/evergreens-suitable-for-hedging-a-toronto-master-gardeners-guide/
Finally, for a good article on cedars and pruning: