1) SEEDS PERMITTED BY THE "KORBAN HA'OMER"
OPINIONS: The Mishnah states that if seeds took root before the Korban ha'Omer was offered, the Omer removes the prohibition of Chadash from the grain that grows from those seeds. If, however, the seeds took root after the Korban ha'Omer was offered, the grain that grows from those seeds is permitted only after next year's Korban ha'Omer is offered.
At what point is a seed considered to have taken root? The Mishnah in Shevi'is (2:6) records three opinions. The Tana Kama says that it takes thirty days for trees to take root. Rebbi Yehudah says that it takes three days. Rebbi Yosi and Rebbi Shimon say that it takes two weeks. Although the Mishnah there is discussing trees and not grain, the Gemara in Pesachim (55a) understands that the three opinions mentioned there apply to grain as well.
What is the Halachah?
(a) The RAMBAM (Hilchos Neta Reva'i 9:8) rules in accordance with the opinion of Rebbi Yosi and Rebbi Shimon who say that it takes two weeks for trees to take root. This is also the ruling of the SHULCHAN ARUCH (YD 294:5). Based on the comparison made by the Gemara in Pesachim (55a), the YAD BINYAMIN comments that the Rambam and Shulchan Aruch also maintain that this is the amount of time that it takes for grain seeds to take root. The ME'IRI in Pesachim explicitly says that this is the amount of time that it takes for grain seeds to take root.
(b) The SHACH (YD 293:2) quotes the TERUMAS HA'DESHEN (1:191) who rules that the amount of time necessary for grain to take root is only three days, as Rebbi Yehudah in Shevi'is states with regard to trees. The Terumas ha'Deshen says that this is apparent from the Gemara in Pesachim, which records the opinion of Rebbi Yehudah.
The Shach himself in NEKUDAS HA'KESEF says that the Gemara in Pesachim provides no proof at all. It is true that the Gemara there says that, according to Rebbi Yehudah, this is the amount of time in which a plant takes root. However, the Gemara there is not ruling like the opinion of Rebbi Yehudah, but rather it merely is discussing his opinion. Since Rebbi Yosi and Rebbi Shimon disagree with Rebbi Yehudah in the Mishnah in Shevi'is, and their opinion is recorded by the Gemara in Rosh Hashanah (10a), the Halachah should follow their opinion (as the Rambam and Shulchan Aruch indeed rule) and not the opinion of Rebbi Yehudah.
The BI'UR HA'GRA (YD 293, end of #3) suggests an answer for the Terumas ha'Deshen. He explains that it is possible that the Gemara in Pesachim records Rebbi Yehudah's opinion only because it wants to discuss the period needed for grain to take root, which everyone agrees is three days. Rebbi Yehudah happens to say this with regard to trees, and therefore the Gemara "borrows" Rebbi Yehudah's statement when it discusses grain. However, the Gemara is not ruling like Rebbi Yehudah with regard to trees, and neither does the Terumas ha'Deshen. Even the Terumas ha'Deshen agrees that the Halachah follows Rebbi Yosi and Rebbi Shimon who say that trees take two weeks to be considered (by Halachah) to have taken root. This approach is also expressed by the MISHKENOS YAKOV (end of #67), who cites many proofs that everyone agrees that the amount of time necessary for grain to take root is three days.
The CHAZON ISH (Hilchos Shevi'is 17:28) also understands that everyone agrees that grain seeds take three days to take root. Based on the words of the Chazon Ish, the DERECH EMUNAH (Hilchos Neta Reva'i 9:83) learns that the Rambam himself rules like Rebbi Yosi and Rebbi Shimon only with regard to trees. He also understands that, according to everyone, grain seeds take root in three days, as the Bi'ur ha'Gra explains. (Y. MONTROSE)
2) GRAIN THAT TOOK ROOT BETWEEN THE HARVESTING AND THE BRINGING OF THE "KORBAN HA'OMER"
QUESTION: The Gemara explains the Mishnah (70a) that states that the Korban ha'Omer permits grain which took root "before the Omer." What does the Mishnah mean when it says "before the Omer"? Rebbi Yonah (according to the emendation of the Shitah Mekubetzes) says that it means before the offering of the Korban ha'Omer. Rebbi Yosi bar Zavda says that it means before the harvesting of the Omer. The difference between the two explanations seems to be whether the Omer permits grain which took root late at night after the harvesting of the Omer (on the night of the sixteenth of Nisan) but before the early morning when the Omer was offered.
The TUREI EVEN in Rosh Hashanah (10b) asks that such a case cannot exist. The Gemara in Pesachim (55a) says that Rebbi Yehudah, who maintains that it takes seeds of grain three days to take root (see previous Insight), rules that a partial day counts as an entire day, "Miktzas ha'Yom k'Kulo." According to this concept, if seeds were planted on the fourteenth of Nisan, that day is considered one day, and the grain seeds need two more full days to take root. They would take root only after sixteenth of Nisan passes! If, on the other hand, the seeds were planted the moment before the fourteenth of Nisan, then the fourteenth and fifteenth would be two full days, and the seeds would be considered to have taken root at the beginning of the night of the sixteenth before the harvesting of the Omer! It is not possible for seeds to take root in the middle of the night on the sixteenth of Nisan according to any opinion. Why, then, is there an argument in the Gemara about the meaning of the Mishnah, when the argument has no practical relevance?
(a) The MINCHAS CHINUCH (246:19) explains that the Gemara here does not agree with the Gemara in Pesachim (55a) that says that a partial day counts as a full day towards the three day period. He says that this also explains the words of the RAMBAM (Hilchos Neta Reva'i 9:10) and the SHULCHAN ARUCH (YD 294:5) who seem to rule that a partial day does not count towards the number of days necessary for a tree to take root. Consequently, according to the opinions in the Gemara here, 72 full hours are needed in order for seeds of grain to take root, and the practical difference of the two opinions in the Gemara indeed involves grain that was planted at midnight on the thirteenth of Nisan.
(b) However, the Minchas Chinuch (303:4) later concludes that the Gemara here agrees with the concept of "Miktzas ha'Yom k'Kulo." He answers instead that there is another case which is the practical difference between the opinions of Rebbi Yonah and Rebbi Yosi. The Mishnah later (71a) says that if the grain for the Omer was harvested during the day, it is valid. It is apparent that this Tana maintains that even if the grain was harvested long before Pesach, it is valid. According to this Tana, there is an obvious difference between the opinions of Rebbi Yonah and Rebbi Yosi, since the Omer may be brought on an entirely different day from the day on which it was harvested.
The YAD BINYAMIN points out that this answer is problematic. Since the argument is relevant only according to the opinion of the Tana of the Mishnah later, and there are other opinions that maintain that the Omer is not valid if harvested earlier (see 72a), the Gemara should have said that the argument between Rebbi Yonah and Rebbi Yosi is relevant only according to this opinion.
(c) The SEFAS EMES answers that when Rebbi Yehudah says that seeds of grain take three days to take root, he means that the seeds are able to take root at any time during the first three days after they are planted. Rebbi Yehudah agrees that they can take root even on the first day. When there is uncertainty about when the seeds took root, it is assumed that they took root three days after they were planted. If an agricultural expert would determine that this particular plant took root at a specific time within those three days, then his determination would establish that this is the time at which it took root. The Gemara's argument is relevant for such grain -- grain that an expert determines took root between the harvesting and the bringing of the Omer. (For additional discussion of this question, see SHA'AGAS ARYEH HA'CHADASHOS #7, and CHAZON ISH in Hilchos Kil'ayim 4:30, Hilchos Shevi'is 25:37.) (Y. MONTROSE)