The Mishnah in Masechet Kilayim (1:9) states:
Planting wheat and barley seeds together (simultaneously) - this (act) constitutes kilayim. Rabbi Yehudah says - it is not kilayim, unless two seeds of wheat are planted together with barley, or two seeds of barley planted together with wheat, or there is wheat, barley and grape seed planted simultaneously.
There is an incident that occurs in the Gemara (Kiddushin 39a) that mentions this law:
Rav Chanan and Rav Anan were walking along a road… they saw (another) man sowing wheat and barley among grapevines. Rav Anan said to Rav Chanan- “Come Master, excommunicate him (for violating the laws of Kilayim)5.” Rav Chanan replied “You are not enlightened (educated) - do we not hold like R’ Yoshiya who states (that one is not obligated) …until he plants wheat, barley and grape seed simultaneously?”
In this Gemara, R’ Yoshiya holds that the biblical prohibition of planting kilayim in a vineyard - kil’ei hakerem - in Eretz Yisrael refers specifically to the simultaneous planting of a grape seed and two other species of seed in the same place (as in our Mishnah). Therefore this person described in the Gemara who planted wheat and barley seeds near existing grapevines was not in violation of the laws of kil’ei hakerem, and this is the reason for R’ Chanan’s harsh answer to Rav Anan in the Gemara.
The ruling in this case is also brought by the Rambam, when he explains that the Issur of kil’ei hakerem only applies when two seeds are planted simultaneously with grape seed, whereas planting one seed (of grain) with a grape seed would be permissible. The Rambam in Hilchot Kilayim (1:6) states:
“It is permissible to plant a seed (eg. wheat or barley) and a seed of a tree or vine together”.
Since the Rambam does not distinguish between a seed of a tree and a vine (grape seed) it appears that it is permissible.
This form of kilayim only occurs in a case where two seeds and a grape seed are planted together- in such a case one would be in violation of the prohibition of kil’ei hakerem. However, if only one seed was planted with a grape seed one would not be in violation of this prohibition.
The reason behind this is because of the pasuk in Devarim (22:9) which states: “And your vineyard should not be sowed with kilayim”. Some Tana’im interpret this pasuk to mean, that there should not be kilayim (i.e. two seeds) that are mixed with the vineyard (grape seed). Therefore, if there was only one seed that was planted with the grape seed, that would not be kil’ei hakerem in accordance with the interpretation of the pasuk, and one would not have transgressed the prohibition of kilayim.
This explains why one would not be obligated in kil’ei hakerem; however, there is also a prohibition of kil’ei zeraim (mixing of seeds). Why would one not be obligated for mixing the seed of a tree or vine (i.e. grape seed) with a seed of another species?
The Rambam answers that it is permitted to mix seeds of different trees together. According to the Rambam, a tree is not considered with other grains and does not come under the prohibition of kil’ei zeraim, and therefore it is permissible to mix a seed of a tree (seeming to include even a grape seed) with a grain seed.
5: [Ed. note:] This incident occurred outside . One must be aware that in chutz la’aretz, there is no prohibition of kil’ei zeraim but there is a rabbinic prohibition of kil’ei hakerem which mirrors the biblical prohibition of kil’ei hakerem in . Consequently, kil’ei hakerem is the only issue addressed in the story and Rav Anan wanted to excommunicate the farmer for transgressing this rabbinic prohibition.
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