משנה על בבא בתרא 7:2

One who buries turnips or radishes beneath a vine, if some of their leaves are uncovered, he need not have fear [of having transgressed] kilayim, or the sabbatical year, or tithes and they may also be pulled up on Shabbat. One who plants a [grain of] wheat and [a grain of] barley at one time, behold this is kilayim. Rabbi Judah says: it is not kilayim unless there are two grains of wheat and two grains of barley, or one grain of wheat and two grains of barley, or a grain of wheat, a grain of barley and a grain of spelt.