Plants, by not being in a hurry, can disperse their information processing machinery in a way that makes it both resistant to damage and, anatomically, indefinitely hard to identify.

They also can get by, for the same reason, with radically less of it, because the same machinery can worry at all stages of the problem, serially, one after another, not all at once as animals must.

Finally, because slow processing demands such a small fraction of their energy budget, they can afford to support a great deal more of it than animals can.

Plants can afford to be very, very intelligent. If we have not found examples of it, we might just not be looking correctly. What a plant could usefully do with intelligence might be decidedly non-obvious to such as ourselves, but it would be idiotic to conclude no such use is likely.

(Octopus, incidentally, are able to mimic colors their eyes are believed not equipped to perceive.)