what type of tree is this?

Question

this seems to have popped up from nowhere, and I have no idea what it is. I live in the UK – London.

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Answer 1

It appears to be Rhamnus Cathartica, or the Common Buckthorn. Removing them before they seed will be important to control them. Then killing the roots will be in order. This article will help you to kill invasive trees and their roots: https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-kill-a-tree-killing-trees-in-your-garden.htm