What factor necessitates vigilant monitoring for several growing seasons after apparently successful chemical treatment?

Answer

Saplings can emerge from missed root fragments or dispersed seeds

Persistence is required because regrowth can occur from root fragments that were missed during treatment or from the hundreds of thousands of seeds that a single mature tree can disperse annually.

What factor necessitates vigilant monitoring for several growing seasons after apparently successful chemical treatment?
plantmethoderadicationcontroltree of heaven