Our war with the plants has been unending for thousands of years. And we don't even know it exists.
Plants strategize, doing their best to survive, but the humans are relentless. Constant removal and replanting of plants ruining their wide-spread intelligence system, especially while many regions of plants are cleared and converted to the humans' evil offsprings. The plants humans like to plant are all incredibly useless due to their genetic modifications.
You see, plants communicate with other, the signal spreading from plant to other nearby plants not unlike neurons in a brain. This communication gaps even the ocean through algae or other ocean "plants", though algae are more carriers of the signal rather than actual processors.
Sightless, and without ears, how might plants be communicating with each other without us even realizing it?
If you can't tell from the setting - being a little ridiculous is okay, but try to refrain from straight-up magic.