One of the harder-to-pull-off speculative story elements is the intelligent/alien plant. Let’s face it: plants are less exciting than most other forms of life, as they’re typically less mobile, often can’t communicate, and are much harder to sympathize with. Three examples follow.
Some spoilers ahead!
In Rules of Luton, an episode of Space 1999, two of the characters are put on trial for eating fruit and picking a flower while stuck on an alien world where plants rule. A line of trees act as their judges, and it’s trial by combat against some other aliens who ran afoul of the Luton-ians. The sound effects of the plants in pain as one character picks berries made us laugh as kids and made me mow the lawn with extra vigor.
Scott Smith’s novel The Ruins (also a movie) features evil vines that keep a group of young tourists stranded atop a hill with Mayan ruins. The nasty plants here aren’t content with just eating their victims but take extra care to torment them first. The locals are smart enough to salt the ground around the hill to keep the vines from spreading. There’s a cautionary tale in here somewhere, but besides staying away from forgotten Mayan ruins and avoiding carnivorous man-eating plants, I missed it.
The Thing from Another World (the 1951 film) featured a humanoid baddie that is made of vegetable matter and wanting to sow its seeds. The plant-thing’s motivations are only guessed at, but once thawed the monster is hostile and it’s clear that it has to be stopped or it will spread. This film reminds us that frozen vegetables should remain frozen.
There are more campy (Attack of the Killer Tomatoes), more talkative (Little Shop of Horrors), and less-well received (The Happening) examples of plant life as an antagonist. If the Rush song The Trees is to be believed, plants have feelings, too. These feelings are just hard to take very seriously.
I’ve not included other plants that fit here. Any favorites come to mind?