R. Levi ben Biray citing R. Yehoshua ben Levi: Yehoshua made four stipulations with the nation upon their entering the land...

They may gather grasses anywhere except from a fenugreek field, which would involve theft (since their growth there is beneficial to the fenugreek).

Mishnah (Maseches Kila'im): If grasses grew amongst fenugreek, he is not obligated to uproot them.

This means that they do not require him to uproot them since the owner does not want them to be there.

Question: But one of Yehoshua's stipulations was that one may collect grasses from any field, except for a fenugreek field, where there is a prohibition of theft to take them (meaning that the owner wants them to be there); whereas here, we say that he does not want them?

Answer: There, he planted the grasses to use the straw to tie his produce (the fenugreek); but here, he planted it for food.

Question: If it is prohibited to plant fenugreek with grasses, why would Yehoshua make it prohibited for a person to pull the grasses from a fenugreek field?

R. Shmuel bar Nachman citing R. Yonasan: As he wants them to be detached to use for tying (rather than wanting them to grow) and it is as if they are already uprooted and resting in front of him.

Question: Why didn't Yehoshua also say this about a vineyard, (that you should be permitted to leave the grasses growing since you will use them for tying)?

Answer: All aftergrowths (grasses that grew in Shemita) are prohibited (to prevent people from planting them and claiming that they grew naturally) except for those that grow in a fallow field, a ploughed field, a vineyard and a field used for planting seeds (as in all of these cases, there is no concern that a person intentionally planted them)... (Note: This entry follows the explanation of Rav Chaim Kanievsky in Maseches Kilayim.)

A fallow field - as he isn't using the field; a plowed field - as he wants to retain the current state of the field; a vineyard - because it will prohibit his grapes there; and a seed field, as he does not want them to grow there. (Note: See Rambam Hilchos Shemita V'Yovel Perek 4 Halacha 4.)

And even if a person does want them to grow, he only needs to use them to tie (but not for the seeds to grow) so it is as if they are already detached.