When buying a tree from a nursery it may have been growing in the same pot for quite a long time. This poses a few problems.
The first being that the root ball will be very dense and compacted. Luckily this problem is not too difficult to fix, as roots can be pruned at any time of the year and as much as a third of the roots can be removed without any mishaps.
The other thing to think about when repotting a tree is that it may have been planted deeper than usual to stop the tree or shrub blowing over in the wind so much. What we need to do is explore under the soil surface to find the true base of the trunk. This will be the point at which the first roots appear from the trunk, and hopefully you will get a nice flare at the trunk base. The roots above this point will need to be trimmed back. You don’t need to be too gentle with this part as long as you keep the roots below the trunk base in good condition.
Cut all the roots which are growing upwards as they definitely won’t do anything for the aesthetics of your bonsai to be!
After sorting these two things you can trim the roots at the bottom of the root ball back a bit so that it can fit in your pot. You may be suprised at how much roots you need to remove but don’t worry too much as you will be trimming branches to even out the need for water.
