Your going to wish you had planted these at least two or three feet apart. I have dug lots of trees and growing these to suitable bonsai size will be a big tangled mess and no room to work digging them up. Roots will grow into each other and be a tangled mess below the ground also.
Good luck with your new trees. These tridents are grown three feet apart each way and even these were a pain in the ass to dig out without first pruning them back a lot to even get in there to dig. Field growing requires much growing and pruning and cutting back constantly in an effort to keep them tidy, defeats the purpose of field growing. See how large those cut end of those canes are, those branches were about seven feet long and intertwined with the row next to it three feet or more away.
Just something to think about.
oh....and the other thing....if this is not what you intend to do by growing in the ground and saying something like, ..."well I don't intend to let my stuff get that big"...
Just grow it in a large container. You can grow trunks to 3 inches in a five gallon nursery container.
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