By Hopeman
United Kingdom 
This year an otherwise healthy mature Leylandii hedge is showing large brown patches. These form where the exhaust plume from my neighbour's central heating exhaust emerges from the hedge on windless frosty mornings. I am told the brown patches may be due to rapid thawing. Is this true or is there a more complex explanation?
27 Oct, 2013
Answers
It's probably not the fumes that's causing the problem as carbon monoxide does not poison plants since it quickly oxidises to form carbon dioxide which is used for photosynthesis. It may be the heat causing the leaves to dry out that's causing the problem.
27 Oct, 2013
Modern heating boilers tend to be of the condensing type and the exhaust fumes are cold. The exhaust consists of CO and water vapour. If the hedge has several patches I would be concerned about a virus which is attacking and killing leylandii.
27 Oct, 2013
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sound like fumes/heat is the problem , which you can see on
"exhaust emerges from the hedge on windless frosty mornings"
this is happening on a constant basis all through out the year, even when you cant see it, friend of mine had the same problem with a bay tree and it took us forever to think why it kept having dead leaves on one side and the pipe was 10 feet away but with the wind blowing it in the direction of the bay
27 Oct, 2013