“You may not plant your field with a mixture!” (Vayikra 19:19)
It is a negative commandment not to plant two types of seeds (for example wheat, barley, beans or lentils) together, in Eretz Yisrael, as it says “You may not plant your field with a mixture!”.
Amongst the roots of the commandment are that Hashem created the world with a profound understanding, and He designed and formed all of the creations in the form that was fitting for it. This is what the Torah means when it says “And Hashem saw all that He had made and behold it was very good!”. When the Torah says here that Hashem “saw” it means that he understood and thought profoundly about each thing, and since Hashem knows that all that He did was formed in the absolutely ideal way to fit with its purpose in this world, He commanded each and every type of plant to produce fruit according to its type, and the types should not be intermixed, for if they would be intermixed, maybe they would be lacking some part of their ideal makeup and Hashem will not give them His blessing.
The prohibition of mixing plants applies to men and women only in Eretz Yisrael. Outside Eretz Yisrael, however, one is permitted to mix seeds and plant them. Mixing trees, on the other hand, (this means the relevant prohibition of grafting) applies even outside Eretz Yisrael.