Today I pricked out the last of this year’s black pine seedlings. Correct treatment of the roots at this early stage pays dividends later with better rootsystems to give superior nebari on your bonsai. These photos show my methods. These are quite advanced seedlings. Another post shows my technique with much younger seedlings but it is possible to pot up seedlings at any stage. the important thing for future nebari is to restrain vertical roots and promote well branched radial roots so cut downward roots hard.
Seedlings with strong rootsystems are easy to work with.
Some have sparse roots but should still be cut back to encourage more lateral roots.
I usually trim the tops of strong seedlings when pruning the roots. I am not sure whether this is strictly necessary (another research project?) but has been generally recommended by bonsai experts as a way of taking the stress off the plant while it has a reduced root system. I am now using the shoots I prune off as cuttings. see previous post for more about this.
Spread the roots as best you can when potting the seedlings but they can be rearranged next year if necessary then water them in well and place the pots in a sheltered location for a few days for the plants to adjust and recover.