3,047. When a Divider is Necessary

Hilchos Kilayim 3:16

In the previous halacha we discussed a divider between crops, such as a fence or a ditch. Such a divider is necessary when a person plants two different species in his own field. However, if one person planted wheat in his field, his neighbor is allowed to plant barley in the adjacent field, as per Leviticus 19:19, "Do not plant mixed species in your field," i.e., the prohibition only applies to planting mixed species in one's own field. The Torah doesn’t say, "Mixed species shall not be planted." Not only that, if a person planted barley in his own field next to wheat and he planted the barley until it was next to his neighbor’s field of barley, this is permitted because the barley in his field appears to be the end of his neighbor’s field.

Hilchos Kilayim 3:17

If a person’s field was planted with wheat and his neighbor’s adjacent field was planted with wheat, he may plant one row of flax on the side of his wheat by the neighbor’s field. This is because people recognize that it’s not common to plant just one row of flax. Therefore, the owner of the field must just be testing his field to see if it’s good for flax and he planted it intending to ultimately destroy it. It is therefore prohibited to plant another species in between the two fields that are planted with the same species until one makes a distinction between them on his own property.