Sorry, just saw this now. My notifications don't seem to always work. :?

Thank you! I've never done this so it's nice to have an idea on pot sizes.

I read the first thing Gleditsia triacanthos does is send a dominant tap root down. When I tipped the cup out, only after a week or so, there was a tap root following the bottom of the cup already.

I used some pots that were given to me by some nice, plant nursery people. :-)

7" deep x 6-1/2" @ top x 5" @ bottom. Also used some 1/2 gal milk jugs with the tops cut off. Fast draining bark, peat & perlite with dolomite (about 4:1:1).

Most will be in the ground by fall because in my cold z3, I feel they have as good a chance there as they would in a pot. What's not in the ground will go into a cold frame by the house during the colder months.

I just confirmed that these pots are about 3 liter in size. And only 6" tall, my bad.

Update:

The little G. Triacanthos got planted out in the wild woods soon after last post here. Anywhere from 12" to 16" tall. Didn't get picks but the one I planted in the front yard is similar. Top of the round white part of the tube is 12" long. If the milk carton top was transparent, you could see the leaves peeking over the top. :-)

Here is the one in my front yard. Growing nice over 20" now but it gets watered regularly. We've had a dry spell the last few weeks and haven't checked on the ones out in the country.

I've heard these can be bad for not hardening off completely in the fall so some winter damage on the branch tips is not out of the ordinary. Guess we'll see. I see them all over town, some fairly large guessing 10" diameter around the lower trunk.

Over 2 ft. tall today. Probably be all for this season. Looks to be hardening off.