I would appreciate your help please. I live in Toronto and have a Chinese elm hedge which faces a busy road – it’s been there for over 20 years. Suddenly 3 separate sections of the hedge have died back. Leaves are all brown and dried but when I snip one of the branches I don’t think the wood is dead. Do you have any idea what could cause this – or a solution to fix it. I have attached a picture showing one of the sections.
Knowing when a plant has died is not as simple as it seems! There are many conditions that may cause part of a plant to die off, including, for example, pests, diseases, or drought. The question often comes down to whether your shrub is damaged to such a degree that it is not worth salvaging. If there is some green foliage left on your three Chinese elm sections, it may be worth pruning back all the browned areas and leaving what is obviously still alive. The inner layers of the branch (especially the main trunk) are, as you suggest, evidence that the plant still has some life. You should certainly check for signs of pests and disease, such as bumps or scales on branches, holes in leaves or branches, webs or insects on the undersides of leaves, evidence of insects at the base of the plant. Chinese elms are susceptible to a variety of fungal diseases that cause wood rot. If there are signs of disease, you should remove the affected branches. If there are no visible signs of disease, and you can see that portions of the affected plants are still alive, pruning the dead branches is one of your options, keeping in mind, though, that enough living foliage must remain for photosynthesis to take place. This may be enough to keep the plant viable throughout the winter and it may branch out anew in the spring.
Another option to consider is to remove the three sections altogether. From your photo it appears that your Chinese elms are quite closely spaced and are growing together thickly. Removing one of the plants may not leave as large a hole as you might fear, and opening up the hedge to more light and air may result in renewed vigour from the adjacent plants next year. The fact that Chinese elms are fast-growing and respond very well to pruning is in your favour with this approach: any gaps might be quickly filled. Likewise, if you do decide to remove and replace these three sections, the newly planted Chinese elms should quickly fill in any open spaces.
You may find it interesting to read this earlier Toronto Master Gardeners post which contains some good advice on pruning a Chinese Elm hedge with a view to regeneration and renewal: https://www.torontomastergardeners.ca/askagardener/lowering-a-chinese-elm-hedge/