Specifically the root of a plant, often with a colloquial or slightly childish feel compared to 根 (ね).
雑草の根っこをしっかり抜いてください。
Please pull out the weed roots thoroughly.
Refers to the remaining stump after a tree has been cut down; the base part still in the ground.
木を切ったあと、根っこが残っている。
After cutting the tree, the stump remains.
Figurative use for the fundamental cause or origin of a problem, similar to 'root' in English.
この問題の根っこは、コミュニケーション不足だ。
The root of this problem is a lack of communication.
This tree has put down its roots deep into the ground.
This problem needs to be fixed at its very root.
What lies at the root of his anxiety is a past failure.