משנה על שביעית 7:1

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. 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.