…your field you shall not plant with mixed species… (Leviticus 19:19)
The term for mixed species is kilayim. (That is pronounced like “key lime,” though I’d advise against eating “kilayim pie.”) While kilayim refers to all types of mixed species, including crossbreeding animals as in the previous mitzvah, most people equate the term primarily with planting different kinds of produce in the same field, which is the subject of this mitzvah.
This mitzvah prohibits us from planting such crops as wheat and barley together, or beans and lentils together, specifically in Israel. (There are some differences between which forms of kilayim apply only in Israel and which apply elsewhere. Planting mixed species in a field only applies in Israel, though planting them in a vineyard – Mitzvah 548 – applies elsewhere by rabbinic enactment as per Talmud Kiddushin 39a. Grafting and crossbreeding are Biblically prohibited everywhere.)
This mitzvah only applies to plants that are fit for human consumption. It doesn’t only forbid placing the seed in the ground; helping mixed species to grow together, such as by weeding the field, is also prohibited. The minimum distance that must be between different species in a field is one and a half handbreadths, which is about 5-6 inches. If the plants are properly placed, it doesn’t matter if the leaves mingle; what matters is that the bases of the stalks are far enough apart.
This mitzvah applies to both men and women but only in Israel. It is the subject of the tractate of Mishna entitled Kilayim and is also discussed in the Talmud in tractate Kiddushin on pages 38a-39b. This mitzvah is codified in the Shulchan Aruch in Yoreh Deah 296. It is #215 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #107 of the 194 negative mitzvos that can be fulfilled today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.