Trees love mulch and if you want them to thrive then spread some around the base of your tree after planting.
A good guideline is a layer around 50mm to 100mm thick being careful not to touch the trunk. Make it a regular habit to mulch around 2-3 times per year.
Not all mulch is created equal but nutrient dense and fertile organic materials include compost mulch, bark mulch, sawdust, decaying leaves, grass clippings and straw hay.
The benefits of mulching include:
Keeping the roots moist
Encouraging worms to burrow and recycle their waste as rich fertiliser
Insulating the soil to provide a buffer from hot and cold temperatures
Inibiting weed growth and preventing competition for valuable nutrients
Providing a barrier to reduce damage from lawn-mowers