Natural rot flare
Trees in the forest (below) naturally form a swelling where roots join the trunk that is visible at the soil surface. This root flare (also called trunk flare, root crown, or root collar) should also be visible on trees planted in landscapes (right). If trees are planted too deep, the normal root flare does not develop at the surface and trees can begin to decline. Decline may begin at planting or it may not show up for many years.
See also: more photo examples