It is been noticed many times that Pali Canon extensively use ideas and conceptions borrowed from Vedas and Upanishads. At that time the ideas and analogies were obvious to any educated student of Buddha’s lectures without any explanations. They were leveraged by Gautama to convey his ideas apprehensively. The analogies was lost with time and the meaning of them is not clear for a reader nowadays. Some of the analogies might even stay completely unnoticed.
Pali canon texts often compare an Arahant with a palm stump (MN 72, overall more than 70 times). Sometimes it is followed by the statement that the stump is not able to give growth anymore. ucchinnamūlaṁ – cut off at the root (some translations incorrectly translate it as “destroyed roots”) Some suttas applies the same idea to a forest (SN 7.17). A modern meticulous reader having a bit of gardening skills might get confused here as a tree stump normally gives new branches very soon if the roots are not removed. The answer here is that palm trees unable to grow new branches being cut off at the root. But it is not the end of the story.
At Buddha’s times the notion of a tree with its roots was a very familiar and recognizable analogy for one more reason. The ninth brahmana of third chapter of Brihadaranyaka Upanishad (dated around or little earlier than suttas of Pali Canon) compares a person to a tree with the root of a man being his Self. The trunk and branches of the tree are false personal identity, created by the Self, rooted in the true Self and all that we consider as a being (a visible part like we consider visible part of a tree to be the tree). The death of the being is compared to the withering of the trunk and branches, and rebirth is when a tree grows again from a trunk, from roots.
Therefor a tree cut-off at the root meant that any visible false Self should be abandoned, cut off, extinguished and by means of it to end rebirths in any of the samsaric worlds. Removing the root was not an option according to Buddha though…