Kilayim, Chapter One, Mishnah Nine
1) 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.
2) One who plants a [grain of] wheat and [a grain of] barley at one time, behold this is kilayim.
3) 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.
Section one: The first section deals with someone who buries turnips or radishes in the ground not so that they will take root but so that they will stay cool. This was an ancient form of refrigeration. The mishnah rules that as long as some of their leaves are still exposed, we dont consider the turnips or radishes to have been planted. Thus the person who buried them does not have to worry about having transgressed the laws of kilayim, or the prohibition of planting on the sabbatical year. If the plant grew more in the ground then he doesnt have to tithe for the extra growth. On Shabbat he can take them out of the ground, and this is not considered harvesting because they werent really attached to the ground.
Section two: According to the first opinion in the mishnah, planting two different grains together is already a violation of the laws of kilayim.
Section three: In contrast, Rabbi Judah holds that it is not a prohibition unless he plants three grains, two of one kind and one of another, or three different kinds.