Halakhah sur Tohorot 8:10

From the laws of the commandment is that which they, may their memory be blessed, said (Sifra, Shemini, Chapter 9:1) that the understanding of that which is stated in the Torah, "From any food that is eaten," is that it is eaten by man. And therefore they, may their memory be blessed, said (Mishnah Tahorot 8:6) that any food that is not designated for man does not contract impurity at all. And they also explained that it is not called food regarding the contracting of impurity until it is detached from the ground. But the whole time that it is connected - even by a small root from which it can be sustained - it is not called food regarding the contraction of impurity. And even if all the impurities touched it, it is [still] pure.