growing for transplanting

Any tips for growing Ulmus americana with the intent to move it as a young tree?
I have an American Elm that I grew from seed and planned to keep as a bonsai of sorts. Now a friend wants it for her place outside Casper, WY. I am growing it in San Diego near the beach. I think it is two or three years old, maybe more. It is 2.5 ft tall. I went to transplant it to a larger pot now that I want a bigger tree- and didn't notice the tap root had escaped- and oops! tore it off when I lifted the pot. Well- the tree seems to be dealing with it just fine- about 3 weeks later now. I considered planting it in the ground- until I found root knot nematodes killing some of my plants plants in the ground. Guess you really are supposed to rotate the tomato crops!
Is Ulmus resistant to nematodes? Haven't been able to find any info on that. Will this plant ever grow up now that it's been in a pot for so long? Not to mention the fact I cut the tap root! Any suggestions for nematode control is of course also appreciated. I have read that adding oyster mushroom mycelium helps to control them. anyone tried this?