Suggests/Advice How big a air pruning pot for a 5 inch trunk patio tree?

[Antony82]

I am seeking advice on cultivating a Northern Catalpa tree as a patio plant within a whiskey barrel. My goal is to develop a curved trunk approximately 5 inches in diameter and 4 feet in height, which may be excessive, but is intended to support a 4-foot canopy above it. Is there a general guideline regarding the volume of soil needed to grow an inch of trunk diameter?

I mainly need to accommodate a trunk up to 5 inches in diameter. I plan to initially use slip potting for a few years and then transfer the tree to a large air-pruning pot once it starts vigorous growth. I intend to prevent the tree from rooting into the ground to avoid disrupting my urban landscape.

 

[Shibui]

I don't think pot size is directly related to trunk size but growth rates and thickening rates will be influenced by the pot size. Basically the larger the pot the more the tree can grow and therefore the quicker the trunk will thicken. Note that to thicken the trunk quick the tree is usually allowed to grow way more than intended and then cut back to the final size when trunk has reached expected size.
Ability to water and water requirements of the tree will also be a factor in how big the pot needs to be. Smaller pots may need water 2-3 times a day. Larger pots may only need once a day.

Final pot size is also a visual balance with the size of the canopy and a size that will hold the tree in winds, etc. I would guess you'd want a pot around 18"-25" diameter to balance a 5" trunk.

Definitely do not go straight to a huge pot for a small seedling as that can cause problems. As you already mentioned, potting on a few sizes as needed helps reduce the potential for root issues.

 

[ShadyStump]

You are technically making a bonsai, though I'd be careful who you actually say that to.
How are you starting? From seed; a sapling dug from the yard; a tree bought from the garden center? This will influence how long a project you're looking at, best practices in the beginning.

Once you're beyond sapling stage container size becomes less of a concern as long as it's not too small. One practice you could adapt from the nursery industry is to plant it in a burlap sack filled with soil. The sack itself will air prune the roots, and this could be done as soon as your trunk is stiff enough to support itself under weight.