This is an interesting thought. Plants are actually very animated at their own timescale, adapting to their environment through phenotypic plasticity. However this happens far too slow for humans to notice without a time lapse.
This is why it takes so long to hold a meeting of the Ents.
That's a moot point.
But plants are really obviously complex life forms when we examine them at human timescale. I don't see how a complex life from escapes our noticing it by moving really slowly.