Can I reduce the size of a mountain ash now
Why do you want to reduce the size?
You can thin the crown of a mature rowan, but I'd think very carefully before doing it - the shape of the rowan is one of it's chief glories and it's very easy to get these things wrong and end up with a stubby horror which will never improve.
As Welshonion says, why do you feel it needs reducing? Is there another way around the problem?
How big is it? Now would be the time if you are going to do it. I have one that I keep to about 12 - 15 ft. I usually take out any cross pieces, damaged branches and then keep the leaders cut back. About every 3 to 4 yrs. Let the birds have the berries first though.
I agree with Dove, you don't want it to look like an up turned stool which will grow into a witches broom.
Have a look at these.
Well pruned trees are not so easy to find photos of because they look like every other tree but smaller than before.
For anyone who isn't sure how to prune, those pics would be no help at all.
I think those pics are meant to be a warning, not a guide !!!
Thanks for all your views, I really hate to cut this tree or any tree, but at 20 feet it was becoming unsteady on windy days and can't risk leaving it.
I'd be very surprised if a healthy Mountain Ash were 'unsteady' even in a strong wind. They're a tree which is very much at home on windy hillsides, and as such are flexible and bendy in the wind - it is this flexibility which gives them their strength - it is more rigid types of trunk that snap in the wind.
If it is moving at the base, then it may well be rotten at the roots and needs removing.
I've successfully coppiced a mountain ash it flowered in the second year and now has berries.
I think if I did anything to a mountain ash it would be coppicing.
I just hate the look of cut back mature branches.