It will branch out in its own good time. Does it have to stay in a pot?

It will make branches don't worry...make sure it has good light, planting it will help also.

That you for the help.

I have a nice spot for it in the garden all planned out. Just wanted it to get a bit bigger as we are in a windy area.

It's currently outside so gets plenty of light.

Is it normal for it to grow so fast as seems awfully quick to get to such a height?

Yes, trees establish better if planted small. The taller it is the greater the wind resistance. That causes movement which militates against root development.

Plant it now!

I have a young mountain Ash tree

So it's a Sorbus and not a Fraxinus? Where are you located? In much of the US most species of the former are apt to be spoiled by borers. (And with the arrival and dispersal of emerald ash borer Fraxinus are becoming plagued by borers over a wide area also).

As far as how it is growing is concerned that is how the plant ends up with a tree shape later, with a single trunk and elevated crown of branches. If you want it to do that you won't be cutting it back and making it bushy while in a small size, as though it were part of a developing hedge.

That causes movement which militates against root development

Root tips being pulled back and forth through the soil by movement of the top stimulates root growth. This is why staking is to be avoided except where a newly planted specimen would blow over otherwise. Or there is some other objective involved like straightening the trunk.

Embothrium, maybe I did not express myself clearly. If a tree is dislodged by wind, ie 'moves', or is blown right over the roots don't get a chance to develop. Thus a taller transplant is at greater risk than a shorter one. That's what I meant.

It got to be 5 feet in six months? That's almost a foot a month. No, that's not my experience with the ones I planted. Wish it were.

Lots of larger forest trees grow straight up and are lanky in their youth as they rush to reach the sun and get their terminal bud out of a deer's reach. If you are planting it outside it will sort itself out. Only reason to intervene is if you want to dwarf it or keep it in a greenhouse.

What climate are you in? In Zone 6 I find they do great in the Spring but don't handle summer as well. Depending on your zone I wonder if planting in part shade might help.

That is a lot of growth. I have watched a tupelo seedling grow in my yard. It put on three feet of growth last year, no branches just vertical growth. This year, it is growing out branches from last years growth, and has only grown about 6" taller so far. Your tree may have a similar pattern to its growth?