In the same way all children cease to exist at 18 years old

They grow in soil, sometimes pots and sometimes in tree farms. but when a sapling is delivered for landscaping it is planted at the final location where it will be. They are referring to the point where a 4-12 tree sapling raised for landscaping is planted in the final location.

The terms seedling and sapling are used in almost all contexts similar to how puppy and kitten are used for that stage of their lives.