Should Buddhists only cultivate native plants?
Unwholesome actions are created when the intention/volition behind the action is unwholesome.

If one is cultivating plants with intentions of greed, anger, non-harmlessness, then one is creating unwholesome kamma, which will produce unwholesome future resultants.

If one is cultivating plants with wholesome intentions such as good-will, caring, altruism, wholesome future resultants will be produced.

What is important here, is the Mind that is behind the action. Everything starts in the Mind, with reference to the twin verses in the Dhammapada.