Take it in steps. First remove the nursery soil and get it into your chosen substrate.
Start by knocking off the plastic nursery pot. Cut off the bottom third to half of the plug. Then wash and/or gently comb the roots to get rid of the soil. Then do as I described, using the nursery pot it was in. If (based on experience) this seems like removing far too much root, find and larger diameter pot.
If reusing the nursery pot it came in, you should fill the bottom 2/3rds or so with bonsai substrate and put your plant on top and cover the roots with substrate. But I've glossed over two problems.
- substrate will just pour right out the nursery pot drain holes
- I put a bit of sphagnum or green moss in the drain holes
- OR a big piece of screen that will cover the bottom of the pot and all the drain holes
- the plant is not constrained in the pot
- I thread a long piece of wire through one or two drain holes, bending the end up on the outside of the pot, then place the plant and feed the wire down the other side and out an opposite drain hole, pulling to secure the tree and again bending the end up on the outside.
- secure the trunk/branches with guys to the pot rim so that it won't move when the pot is jostled.
Trees grow as the temperature increases up to about 95F/35C. The rate of photosynthesis is and remains maxed out, but the other metabolic processes continue to increase with temperature, so the tree doesn't grow - it burns carbon faster than it can be fixed. Other and deleterious effects occur above this temperature - certainly by 115F/40C tissues are dying. Conversely, even though you may feel uncomfortable,
trees are growing fast and thriving at temperatures below 95F/35C. Below 40F/5C metabolic activity is very very low, and there is virtually no growth.
The issue with 'heat waves' is that they arise because of the incursion of a dry air mass. Like my climate, the relative humidity (rH) of yours rarely drops below 50%. When you had the heat wave, it dropped as low as 30% or so when the temperature of the day maxed-out in the afternoons. I went over this in some detail years ago - maybe reading through
one such old post will help you to understand, if you want to take the dive.