I missed the forest for the trees in citing Tosfos Shabbos 4 and not the gemara itself. The gemara asks why the person who placed the dough in the oven would need to remove it and violate the derabbanan of rediyas hapas – if he remembers before the baking happens that baking is assur, then he would not be liable a chatas even if the dough is not removed because the melacha was not completed with the “ha’alama” of issur. QED (maybe) that the issur of ofeh is not merely placing the bread in the oven, but must include the state of it being baked (Afikei Yam II:4). I will concede that one can argue that this is not so – the ma’aseh afiya perhaps concludes when the bread is placed in the oven, but one is retroactively liable only if the baking takes place in a state of ha’alama of the issur (see Eglei Tal, meleches zore’a s”k 8). Why should this condition be necessary if the act of issur has already been done? Perhaps there is a distinction between the completion of the ma’aseh aveira qua human action and the completion of the melacha as a total act.
I argued in the comments that one can distinguish between zoreah and ofeh. By zoreah, the act can never be completed on Shabbos as the plant will take root only later, therefore, the definition of the aveira must be the act of placing the seeds alone. The same does not hold true by ofeh where the entire act may have to occur on Shabbos. The Afikei Yam adopts this sevara as well. One is tempted to argue here as well that the rooting of the plant (hashrasha) may be part of the melacha and one becomes liable retroactively days later - nafka minah if one can uproot the plant in the interim. We once discussed a complex yerushalmi which my brother-in-law raised a kasha on here but he did not quote the punchline that relates to our discussion. According to Reish Lakish, even though one is chayav for kilayim merely by tossing seed (even before they land), in hilchos shabbos one is chayav only once the seed hits the ground. Clearly, according to the Yerushalmi, taking root is not required. The Ohr Sameich proves the Bavli agrees from Rav Papa’s kashe (Menachos 71) that if we learn from “asher tizra” that the omer permits new wheat, then it should permit even wheat which has not even taken root (i.e. because that is also called “zeriya”!) I am not sure if I have muddled the waters too much, or clarified things a bit : )