There is a very good reason why the locals maintain such a strict perimeter around the ruins... too bad the protagonists don't learn why until it's way too late.
The plants themselves are clearly nightmare fuel for the locals. They're mobile, carnivorous, and infectious. If you touch them, well, you're boned. Anyone that touches them gets a prompt Mercy Kill - or is simply herded to the top of the ruins to wait for the plants to do their work.