If you are growing them on to increase size, then you want a pot that has a diameter at least one inch wider than the root ball. The depth is much less of an issue. And it IS the root growth you are accommodating rather than the topgrowth. That is always the critical issue with containerized plants - take care of the roots properly and the tree will thrive: mistreat or ignore the roots and the tree won't last long.

The reason for restricting the size of the container is to avoid excess wet soil surrounding the roots. Containers have such limited drainage compared to inground plantings, you run the risk of root rot unless using a very freely draining potting mix. With something like the gritty mix, you can get away with a larger container as it drains very rapidly so root rots are a non-issue. The excess below the rootball in a tall container is also a non-issue, as it will drain away before compromising the roots.

Up-pot them annually until you get the size you want and can best manage. For me, that's a 30G ceramic container, max. Then root prune every 2-3 years and refresh the potting soil. The trees will continue to grow but just very slowly.

Well draining is not a issue so they shall all remain then

I'm stil on the learning curve on moisture management. I think I'm -fine- during the growing season. However, I did a potential repot of a Hinoki. 20G pot. Removed it, While roots did appear to fill it up almost 2/3+, half of them were dead/dried out. After removing said tree and then the dead root mass, , I put it back in the same pot since volume was now back to half filled. It was a mild winter and I was occasionally watering the pots but even as such, looks like not enough H20 during winer months. I do believe my winter watering was on point...... Day by day I suppose

My biggest problem with trees in pots; was in the Fall when temperatures were cool, cloudy and extended periods of rain would keep the pots on the wet side. Plant performance dropped considerably, and I lost a couple (root rot?).

The above can be an issue anywhere that receives average winter rainfall. That's why a very fast draining potting mix is most desirable.

I lost a potted hydrangea this winter from just this reason. A big container, it hadn't been repotted recently and the soil had become compacted and lost aeration. When I finally discovered it earlier this spring, it had a couple of inches of water sitting on top of the soil!! Poor thing drowned :-((

Maybe this is more a seperate thread in itself but what about URBAN trees in containers - as we talk about up-potting if not potting medium/compaction..

I just passed by my old office today. Tree planted in container about a decade back,. The sidewalk planter was aout 40" or so square overall. Maybe close to 48:" but I don't think so it's that large. I don't expect someone has changed the soil in this thing but driving by, the tree is still in it's container and easily 5 stories + high and pretty decent sized canopy to match

Depends on what the base of the sidewalk planter is. Typically they are open to the ground/soil below so not really a container - just more of a raised bed.

Nay. Concrete Sidewalk. No cutout made

Don't think it was by the the city but privately done by the bldg owner.

Just two large planters flanking the edge of the sidewak and lines up with the bldg propery lines itself

I recall seeing the planters and was curious about the construction when this was put up. It was heavy gauge metal - I suppose it would be considered your typical commercial planter. They painted the interior with waterproofing sealer - similar to what the tile tradesmans use when they *paint* cement board or sheetrock with this sealer to make it impervious to water.

' That's why a very fast draining potting mix is most desirable. '

For the record, I had these in 511 since the spring, but they were pines which don't like wet feet at all, plus one was a dwarf from seed pine.

But maybe because of its unusual, quirky form it was slated to die anyways. It possibly was the only one I lost now that I think about it.

Mine was i 5-1-1 as well. It was a conscious choice on my part as I don't want to water on a daily basis in summer. Except for succulents, all my outdoor containers are in 5-1-1. This is the first time this has ever been a issue and just reinforces the concept that even a fast draining mix loses that ability if allowed to compress and collapse over time without refreshing or replacing.

* refreshing or replacing*

For the wide tapered or square straight, I'm feeling okay....

Not looking forward when the day comes for the round tapered ones.