I believe plants are already in a sort of ''half here half there'' connection to the spirit realm . And may be even closer to the source than other souls , they are less personalized . Perhaps they incarnate in other things , or go directly to the other side . I think it depends on the soul .

But with their deaths I think it's just a cut from being already very connected to it to being completely in it .
They go to the same place maybe on a different level .