Tuesday Nedarim 58

At the end of last Blatt the Gemara brings a proof that what grows afterwords Mivatels what you planted. R' Yochanan says if you plant an onion that all the Trumas and Maasaros were removed, what grows afterwards is tevel and you must take off Trumos and Maaseros on the whole onion. So we see that even what was planted becomes tevel because it's batul to what grows.

The Gemara says that really it's not Batul, but the Rabanan were Machmir

Ran asks: Why don't we say that since really what you planted is not Tevel you might take off Trumah from what you planted on what grew which is obligated. Thus it would come out that it wasn't separated properly.

Ran answers: since what you planted doesn't become Batul that grows, ther reason for that is what grows becomes Batul to what you planted as it grows. Therefore, really none of theis onion is Tevel from the Torah.The whole onion is only Tevel Midarabanan, and has the same obligation. So you can separate for any part of the onion from any other part.

The Gemara wants to bring a proof from a sixth year onion that you plant on Shvious, we say that what grows Assurs the main part (especially before Biur, then even the smallest amount grows Assurs the planted part since its a Davar Sheyeish Lo Matirin, since you can eat it with the Hilchos Shvious.) So this proves that we don't say what grows is Batul to what you planted.

The Gemara also answers that it's only a Chumra.

Ran asks why does the Gemara ask this, if we already answered this in the first case. So why should we think this case is different to ask it again.

Ran answers: I would have thought that we wouldn't say that this Braisa is only a Chumra, since it already has a Chumra to say that even the smallest amount is not Batul to what's planted because of Davar Sheyesh Lo Matirin, i would assume they wouldn't impose another Chumra that the growth is not Batul to the original. Therefore i would assume that the reason it's not Batul because that;s the actual halacha. However, the Gemara answers that it's a Chumra.