Sometimes they do it in their own species, but much more commonly they do it across species. Eucalyptus will kill all but eucalyptus. Redwood trees will form networks and help each other; even an albino redwood tree (no chlorophyll https://education.nationalgeographic.org/resource/white-wond...) can survive.

A plant that killed all offshoots of itself would not survive. But plants much more often make perfect genetic copies than animals do, so the selfish gene can explain this behavior