Reference for Kilayim 2:8
אֵין סוֹמְכִין לִשְׂדֵה תְבוּאָה חַרְדָּל וְחָרִיעַ, אֲבָל סוֹמְכִין לִשְׂדֵה יְרָקוֹת חַרְדָּל וְחָרִיעַ. וְסוֹמֵךְ לְבוּר, וּלְנִיר, וּלְגָפָה, וּלְדֶרֶךְ, וּלְגָדֵר גָּבוֹהַּ עֲשָׂרָה טְפָחִים, וּלְחָרִיץ שֶׁהוּא עָמֹק עֲשָׂרָה וְרָחָב אַרְבָּעָה, וּלְאִילָן שֶׁהוּא מֵסֵךְ עַל הָאָרֶץ, וּלְסֶלַע גָּבוֹהַּ עֲשָׂרָה וְרָחָב אַרְבָּעָה:
It is prohibited to sow mustard or wild saffron close to a field of grain, but mustard or wild saffron may be sown close to a vegetable field. [One may] sow [different species] close to a fallow field, a plowed field, a [loose] stone fence, a road, a fence ten handbreadths high, a ditch that is ten [handbreadths] deep and four [handbreadths] wide, a tree that overshadows the ground [its branches are less that three handbreadths off the ground] or a rock that is ten [handbreadths] high and four [handbreadths] wide.