Trees look very attractive growing out of a lawn, especially in spring and summer if bulbs have been planted around them.
But grass can be a danger to young trees, so when you first plant, leave a 12in ring of bare soil around it because if the grass grows too close it will steal food and moisture from the soil and your young tree could fail to thrive.
Long grass growing against a tree trunk can provide a haven for pests and you risk damaging the tree when mowing right against it.