Planting tomatoes when one is planting seeds.
Thank you for your time and assistance.
I agree with you that “planting tomatoes when one is planting seeds” is an example of synecdoche. The main difference between the two devices is that synecdoche refers to a whole by calling it the name of any of it’s parts, while metonymy refers to description of a thing that is closely linked to that thing, but is NOT a part of it. Since seeds are indeed a part of tomatoes, this saying is an example of synecdoche.