Some tree-planting facts

A 2010 report for the City of Toronto found that a tree with a trunk 75 centimetres in diameter intercepts 10 times more air pollution and stores up to 90 times more carbon than a tree with a trunk measuring 15cm.
Any subsoil that is excavated as part of digging will need to be removed completely. Once digging has finished, the hole should be filled with irrigation water to ensure that the pit will drain freely after planting.
At the time of planting, make a hole that is large enough to take your tree's root ball, and ensure that the tree's collar line (the line of contact between the soil and stem) is at the same level as the surrounding ground.
Form a circular watering basin slightly larger than the planting hole. Water the tree thoroughly during and after planting.
Mulch the surface of the tree pit to a depth of at least 50mm with a product such as chipped pine bark. This helps to retain moisture in the soil by reducing the amount of water that is lost through evaporation.
Source: The National