Deeper into the Zone there are some examples of plant life that mutate; usually the most extreme have been either taken out or burned, but there's a species of mushroom whose poison went in overdrive and became a quite strong hallucinogen.

When the militia tried to burn a patch, they ended getting a faceful of the fumes, and that was bad. Imagine when Mariette got paranoid. then imagine it's several squads of military personnel and they are all armed

These days when they find a patch of those they tend to instead pour a lot of bleach on it. a lot less chances of people getting high and paranoid.