In article , Martin Sykes
writes

I'm planting a native hedge which isn't going to be a problem height for a
few years. When it is though, how is the height llimit enforced with respect
to the natural growth of hedges through the year? I'm thinking of something
I heard that trimming native hedges every year is damaging to the wildlife
and it's better to do it every three years instead.

Many of the rose family (which includes hawthorns, blackthorn) fruit opn
second year wood, so you have less fruit if you trim every year.

Something to do with
production of fruits etc. being better that way. If I do that though the
hedge will necessarily fluctuate in height between about 6ft and 8-9ft or
more.

No - not necessarily - it could fluctuate between 3 and 6 ft ;-)

I imagine trimming a hedge to exactly 6ft every couple of months for
legal reasons would be bad for the hedge for wildlife reasons?

Not particularly. Cutting in spring would disturb birds' nests. But
otherwise regular trimming would encourage the hedge to become bushier -
arguably a good thing for wildlife. And not everything lives on berries!
- if you want to encourage wildlife, avoid pesticides and encourage
insects - that way you get a diversity of insects and attract the insect
eating birds.

A word of caution - if you miss cutting a hawthorn hedge for a year, you
can no longer do it with a hedge trimmer, you need a saw as well.
--
Kay Easton

Edward's earthworm page:
http://www.scarboro.demon.co.uk/garden/