OK,..never had one of these before. I was planning on purchasing a full sized tree and cutting back but when I got to the nursery all the trees had horrendous graft scars. So I settled on thiS prunus mume kobai seedling. All I have done is repot, cut the tap, and remove any suckers or dead branches. It seems to be a good candidate to develop into a broom. Thoughts?
It's good that you have one on its own roots, so you don't have the graft scar that can be ugly forever; though you may be in for a wait to see blooms. That's fine, spend the time growing it into an interesting tree. Grow it hard this year, then plan a
low chop in a couple years to add movement and taper. (Remember the recent threads about the expensive, fat little umes? Grow hard, chop low!)
I have been taught by several pros to go very easy on ume roots; replace the soil, but try to avoid pruning many roots. Since yours is fresh out of a nursery can and you did work the roots hard, hopefully you did all that needed to be done in one shot. I have repotted mine each year for the last 4 years and have not needed to remove more than a small handful of roots.
Since it's too early to repot, keep it above freezing, and add a bit more soil to cover those surface roots. They will bulk up faster underground, and you definitely don't need them drying out. My ume is finishing it's bloom period, and it will rest for a couple months, so I don't plan to repot it until at least mid-March or maybe later.
They are thirsty trees, so I'd keep it on the damp side for now too, especially if you reduced the roots considerably...it won't have access to water roots aren't in contact with.