6inches is too deep. By a few I'd say 2 or 3 max. You need a bit of a rim so water doesn't overflow when irrigating. You also need some space for top dressing between repots. But growing medium tends to settle so if you start too deep it ends up even deeper.

You also need some space to put the gravel to keep the cats from using it as a cat box. Without the stones the squirrel and other small birds and animals will dig in the pot.

I know not to bury the root ball.

==>>> the root flare should be at media level ... the roots should be covered ..

a good ... proper .. draining media .... really makes a difference ... especially with irrigating.. as flora notes ...

trees prefer a media different than water loving perennials and annuals ...

ken

I grow ALL my collection of Japanese maples in containers and have for many years. As has been mentioned, the potting soil or media is really of primary concern, as that will govern how well the plant's roots develop and are able to access nutrients and moisture and so determines how well the tree will do.

I don't find the planting depth in the container to be much of a concern as long as you do leave a sufficient lip or space between the soil level and the top of the pot. With some of my containers, the depth is 2-3 inches below the pot rim; with others it may be as much as 6". The deeper space is more common with my larger pots and trees. You just want to make sure that the top of the current rootball at planting is not covered by the potting soil and that the root flare is exposed, just as you would with any inground planting.

I also have found little need to topdress my pots with anything, although I have included a ground cover in some. But no rocks or gravel and no compost or mulch. Now and again a squirrel has buried a nut in a container but birds, cats or other creatures have never bothered them.

The top dressing I'm referring to is with fresh media, not anything else. It's standard practice here for long term pot culture of shrubs and trees. I do it, when I remember. It defers having to repot.

Perhaps a different practice in your area floral, but repotting of J. maples here is a standard practice every 3-5 years simply to address root pruning. The soil is changed out at that time. Even the best quality potting media deteriorates in time and needs to be replaced. Fresh media added to the top of of the container soil generally doesn't accomplish much of anything. YMMV

I'm with Gardengal about the top dressing and need for repotting. I don't see how it would do anything to address the deteriorating potting-media structure. And, by the time the media starts breaking down (significantly) it's time for root pruning anyway. I do wish there was a way I could keep putting repotting off, but I don't know of a way that's good for my plants.

Just doing what the RHS advises ... Last sentence under 'repotting'. Works for me.

Removing two inches of the existing soil and replacing with fresh is going to be complicated with any containerized JM. Their roots will be right at the existing soil surface and disturbing them to any degree is likely to create a lot of problems! It is not a practice I would follow or recommend but if it works for you, great!

What ground cover would work with a tree in a pot? Would blue fescue be fine around a tree?

Blue fescue is not really a groundcover as it doesn't spread......that quality is pretty much SOP for any sort of groundcover :-)) I have used Veronica 'Georgia Blue', wire vine and stonecrops. But only in the largest of my containers.

Whatever you use with a JM, it needs to be something that can be planted easily and with a shallow root system and not require any ongoing attention other than maybe trimming back occasionally. You want to avoid much cultivation in the root zone so I only include when newly potting or up potting.

How about creeping thyme?

The irrigation of containered JM maybe a lot more water than a creeping thyme would appreciate. But it might work.