Spoilers ahead for Stranger Things Season 2, Episodes 1-6. It's no secret that weird things are afoot in Hawkins, Indiana. While things may have settled down after the events of Season 1, a year later it seems that Hawkins is once again becoming victim to some weird happenings. The first clear sign that there's something rotten in the town of Hawkins is the decomposing pumpkin patch that Hopper investigates. Eventually he learns that it's not just one pumpkin patch — nearly all of the crops in Hawkins are dying, and it has something to do with the mysterious vines beneath Hawkins. What are the vines doing in Stranger Things, and how are they connected to all the other odd stuff happening in Hawkins?
A few vines may sound like an underwhelming threat when there is a giant shadowy abomination looming over the town in an alternate dimension, but it seems that these two things are related. The vines, the Shadow Monster, Dustin's new pet, every strange thing happening in Stranger Things comes back to The Upside Down. When Hopper starts literally digging into the pumpkin patches, he discovers a tunnel with the characteristic floating specks of the Upside Down and grimy vines running along the course of the tunnel. What Hopper doesn't realize, however, is that when he sees the vines — the vines see him, too.
As Hopper investigates the tunnels beneath Hawkins that seem to stretch throughout the town, he doesn't realize that he's not alone in these caverns. Vines are crawling and growing alongside him, following him as he explores these mysterious pathways. Hopper thinks he's alone, but soon ends up trapped in the vines, holding him down and seemingly suffocating him. These vines bear a strong resemblance to the restraints that held Barb and Will Byers down after being trapped in the Upside Down, but is it even accurate to call them vines?
These tendrils are alive, but not in the same way that vines are. Vines certainly don't have the ability to entrap others and move about as they please, but whatever the long, slimy things in the tunnel are, they seem to be sentient. These vines are one of a few things that are infecting the real world from The Upside Down, but what Hopper doesn't know is that these living vines are much more powerful than a few angry plants. Episode 6 spoilers below.
Eventually, its made clear that both the vines and the Demodogs that are terrorizing Hawkins are both being controlled through the Shadow Monster, who has eyes on the happenings of the real world thanks to a "virus" that is infecting Will Byers. These vines are an extension of the Shadow Monster, and it uses the vines, the caverns, and the Demodogs to trap and kill many members of the Hawkins Lab. It seems the members of the Hawkins Lab were too blinded by science to realize that they were inviting some especially volatile creatures into their world.
The vines beneath Hawkins may not seem like a large threat compared to cat-eating dogs and whatever the Shadow Monster is, but their presence in the real world has some horrifying implications. While a Demodog or a Shadow Monster is easily noticeable, how far could these vines grow before anyone could suspect that they weren't actually vines? If it wasn't for Hopper, would anyone have even thought to look under the ground for these tunnels? It's clear that the Shadow Monster is finding new ways for the Upside Down to infect the real world. These vines have clearly already taken root, meaning that things are never going back to normal in Hawkins.