Larry, do you ever play games with your 2 year old where you say things in a joking manner which aren't true? Like, for example, pretending you're going to eat his fingers, or letting him hide under the duvet and then saying loudly "Oh no, I wonder where DS has gone, oh well, what a comfy bed that looks, I must sit on it!", or pretending you think that his sock goes on his head to make him laugh when you're getting him dressed.
I think that sometimes although it seems obvious to us when a situation is joking/humorous and when it is serious, using different tone of voice, warnings etc, it can be hard for children to understand the difference and with these kind of things where they know they aren't supposed to do something but they do it anyway with a cheeky grin, that can be their attempt at imitating humour.
Other times, they just want to do it and their wanting to do it is stronger than their desire not to upset you. Impulse control is pretty low at this age.
I agree with the others who say remove him and/or show him how to treat the plant properly. Do you have a garden? If so perhaps a small place where he is allowed to dig and play in the mud would be appropriate.