In message , Justin Thyme
writes

"

I would not regard a row of Leylandi type trees as a hedge.

It certainly is considered so within the 'high hedges' legislation (but
that doesn't apply AFAIK here, as the OP is in Scotland)
--

When does a group of trees become a hedge?

The legislation defines what is considered a hedge for it's purposes,
basically a line of 2 or more (or maybe more than 2) trees spaced
closely together. You can look it up easily enough if you are
interested.

Even if they are clearly set
back from the boundary and maybe mixed species, they will still have the
same light blocking effect. If you have a copse, orchard or arboretum could
the same restrictions apply?

No.
--
Chris French