I just planted a 2m Thuja occidentalis 'Holmstrup' and 2.75m Thuja
plicata atrovirens. After they were in, I suddenly thought they might
need staking (which is what I'd do with deciduous trees). But, of
course, it is very difficult to stake conifers without damaging them.

There was precious little information on the internet about this, and
what I found was mainly on US sites. The best comment I found was the
final paragraph he
https://www.gardenerscorner.co.uk/threads/how-do-i-protect-new-planted-conifers-against-wind-damage.115945/#post-1049487
(see last paragraph).

What I find particularly strange about this is that confers have shallow
root systems and a lot of top growth which adds to wind resistance, so
newly-planted confers are particularly susceptible to the wind. In the
end I decided to not stake them, but put some heavy paving stones almost
up to the trunk on the windward side of the tree.

--

Jeff