From Wikipedia:

I'd have a hard time classifying a shrub or a tree as intelligent, unless you define intelligence as "the ability to grow".  Many organisms perform complex actions, but often these are the result of hardwired reactions to stimuli (like bacteria) rather than a problem-solving application of abstract thought.

As far as what intelligence would be like in other kingdoms, I have no idea.  An intelligent plant would be so far removed from the ones we know that it might preclude it from that kingdom all together.  I know there are plants that can react to touch, but that's about it.

I found a random site talking about what an intelligent plant might be like had they become the dominant form of life on Earth.  You'll have to download the .PDF to read it, but there are several sites out there discussing the idea.

http://hissa.nist.gov/~black/Papers/lifUniEvery.html