Here is how I plant very small trees. Usually less then 12” ( 30cm)
Like you can see the plant came in a 3.5” (9 cm). I dig a hole the just a little bit bigger then the pot.
I plant it in the hole , NO fertilizer, NO compost and mostly NO Phosphate. I just fill up with the same soil I just dig. Pore lots of water and press a little.
I put a cardboard, about 12” ( 30cm). Just so the grass around the small tree will eventually die.
I put some mulch over ( what ever I have in hand ). ET VOILA.