Maasrot, Chapter Five, Mishnah Two
1) One who uproots turnips and radishes from within his own [property] and plants [them elsewhere] within his own [property] for the purpose of seed, he is liable to tithe, since this is [equivalent to] their threshing floor.
2) If onions take root in an upper story they become clean from any impurity.
3) If some debris fell upon them and they are uncovered, they are regarded as though they were planted in the field.
Section one: In this case the person must tithe the turnips and radishes before he replants them because as soon as he uprooted them their processing has been completed. The seeds that will subsequently come from the turnips and radishes will be exempt from tithes (we will learn this in mishnah eight). Therefore, before they are replanted they must first be tithed at this early stage.
Section two: Onions that took root in debris/dirt found in an upper story of a building are treated as if they had been planted in the ground. What this means is that if they had previously been made ritually impure, they now revert to a state of purity, as do all plants when they are planted into the ground. The Tosefta notes that these onions are not liable for tithes because only plants that grow in the field are liable for tithes.
Section three: Albeck explains that this section is not a continuation of the previous section. If the onions were in the field and some debris fell on them and their leaves were exposed, they are treated as if they were planted in the field and they, unlike the onions referred to in section two, are liable for tithes.