Kilayim, Chapter Two, Mishnah Eight
In yesterdays mishnah we learned that it is permitted to plant one row of flax next to a field of grain. In todays mishnah we learn that one is not allowed to do so with mustard or safflower.
1) They may not sow mustard or safflower close to a field of grain, but they may sow mustard or safflower close to a vegetable field.
2) And they may sow close to fallow land or to plowed land, or to a wall made with loose stones, or to a path, or to a fence ten handbreadths high, or to a trench ten [handbreadths] deep and four wide, or to a tree forming a tent over the ground, or to a rock ten [handbreadths] high and four wide.
Section one: Mustard and safflower are dangerous to a vegetable field and therefore the owner will eventually uproot them. Since this is not a beneficial arrangement, one is allowed to sow them there. However, since these seeds are not damaging to grain, he may not sow even a single row next to a grain field.
Section two: In this section of the mishnah we learn that if there is a break between one field and another, he can plant one species next to a different species. For instance, if there was fallow land or plowed land separating two fields, he can plant two different species next to each other. A wall, even if its stones are only loosely piled on top of each other, a path, fences and ditches that are ten handbreadths high or deep and four handbreadths wide, also all serve as separators, preventing a problem of kilayim.
The tree which forms a tent over the ground refers to a tree whose branches lie low to the ground. If one species is planted under the branches, he can plant another species outside of the branches, because the branches themselves serve to separate the two species. Finally, a rock ten handbreadths high and four wide also serves as a separator.