Mishná sur Orlah 3:11

The following neither defile nor can they be defiled and they do not join together: The roots of cabbage-stalks, Young shoots of beet growing out of the root, and [similar] such turnip-heads, [And produce whose roots] that are ordinarily cut off but in this case were pulled up [with their roots]. Rabbi Yose declares them all susceptible to contract uncleanness, but he declares insusceptible cabbage-stalks and turnip-heads.