Panel One: Garfield is giving us gardening tips, one amateur horticulturist to another, though with his trademark dearth of enthusiasm. As usual, the strip flows smooth as honey, but after staring at it for a few minutes, subtle cracks emerge in the surface: is Garfield telling us about the benefits of talking to plants because he knows Jon is doing it, and is leading us to Jon for a demonstration? Or do the events happen to coincide? Garfield's surprise in panel two is partly because of the nonsense Jon is spouting, but couldn't Garfield see Jon a foot away down the table when he started walking and talk/thinking with the reader?
Answer: The rules of how light and sound travel between off-panel and on-panel spaces is variable depending entirely on the requirements of joke mechanics. We understand this intuitively about other laws of physics in comics, but the relationship between the on-panel and off-panel is so subtle the bending reality may not even register.
Panel Two: Jon's fear is funny for any number of reasons (e.g. - how much are elbows supposed to match? How can you tell when wearing a long-sleeved shirt? Why is this inspiring terror?), not least of which is that he seems to be asking for a second opinion from the plant. Since he usually gets a response from his housepets, maybe this isn't such ridiculous behavior.
Panel Three: Garfield: Actually mad that Jon hurt a plant, because he cares about plants? Disgusted that Jon has upset his gardening lesson for the reader? Or looking for any excuse to smack Jon? You decide.