The question that really needs to be asked is "How can intelligence be defined?", at least in my opinion. If you have a precise and complete definition of what is considered intelligent, then sorting every species is going to be made much easier. But in order to establish the concept of intelligence, one must carefully weigh actions and decide if they are a sign of intelligence or not, and there is the point where everything gets difficult. It is very hard to separate a conscious, thought-out decision from an impulsive or lucky one that just happened to have a result, especially if it is impossible to communicate with the animal or plant in question. For example, some scientists thought that animals were just as intelligent as humans because they were capable of opening doors and such tasks, but they failed to consider that the results of their actions may just be coincidence.
Then there is the other end of the spectrum, which is that even human action is just a series of conditioned reflexes, and is not a sign of intelligence, that every human achievement is just a result of chance. If one takes the time to think about it, then it becomes clear that a lot of human actions are, in fact, just conditioned reflexes, for example the way we act is determined by our experiences. If a human gets shocked by electricity because he touched a pole, they will likely not touch that pole again. If a human has an unpleasant experience with a particular event, they will try to avoid it instinctively. If one starts to think about it, then everything a person does is related to some stimulus, but can one call thoughtless reactions intelligence?
In conclusion, until someone comes up with an exact, and universal definition of intelligence, one can argue about the intelligence of plants endlessly.
