You have not provided enough information about the site conditions for any constructive advice. And what exactly is a 'box tree'?

The details are a bit lacking :-) Did you mean trees planted in boxes or box plants (Buxus species), which are not trees but shrubs? If the boxed trees, what kind, how big? Location is very helpful and a photo of the proposed planting area will help to generate the best advice.

It’s Buxus scrubs

I have 18 x Pyrus Chanticleer and Hydrangea Annabel here too

If it is just boxwood shrubs you are planting, just dig a hole and plop them in! Nothing fancy required. Just space them out properly and don't plant too deeply. Box grows exceedingly slowly so outgrowing the planting area is unlikely any time soon and they can be pruned at will.

But you should be aware that boxwood blight is rampaging throughout the east and moving west. Box may not be the best choice.

I won't even comment on the 18 callery pears :-((

H not C😉

We might be talking to a health care professional.

Maybe should scrub the walls instead of the planter.

I do intent to power wash the walls as soon as weather permits thanks for advice

I'd do it before planting the boxwoods in front of the wall. And planting them where the wash water will run off into.