Reference for Kilayim 3:1
עֲרוּגָה שֶׁהִיא שִׁשָּׁה טְפָחִים עַל שִׁשָּׁה טְפָחִים, זוֹרְעִים בְּתוֹכָהּ חֲמִשָּׁה זֵרְעוֹנִים, אַרְבָּעָה בְּאַרְבַּע רוּחוֹת הָעֲרוּגָה, וְאֶחָד בָּאֶמְצַע. הָיָה לָהּ גְּבוּל גָּבוֹהַּ טֶפַח, זוֹרְעִין בְּתוֹכָהּ שְׁלֹשָׁה עָשָׂר, שְׁלֹשָׁה עַל כָּל גְּבוּל וּגְבוּל, וְאֶחָד בָּאֶמְצַע. לֹא יִטַּע רֹאשׁ הַלֶּפֶת בְּתוֹךְ הַגְּבוּל, מִפְּנֵי שֶׁהוּא מְמַלְאֵהוּ. רַבִּי יְהוּדָה אוֹמֵר, שִׁשָּׁה בָּאֶמְצַע:
A garden bed that is six handbreadths by six handbreadths, one may sow five different kinds of seed within it, namely, four [species] one on [each of] the four sides of the bed, and one in the center [leaving the corners, one handbreadth by one handbreadth empty]. If it [the bed] has a border [on all four sides] one handbreadth high [and one handbreadth wide, thus the bed measures 8 x 8] thirteen different species may be sown in it; three on each border and one in the middle. One may not plant turnip heads in the border as they will fill it up [and it appears all intermingle]. Rabbi Yehudah says, six different species [may be sown] in the middle [for a total eighteen species].