Companion planting was practiced in various forms by the indigenous peoples of the Americas prior to the arrival of Europeans. These peoples domesticated squash 8,000 to 10,000 years ago, then maize, then common beans, forming the Three Sisters agricultural technique. The cornstalk served as a trellis for the beans to climb, the beans fixed nitrogen, benefitting the maize, and the wide leaves of the squash plant provide ample shade for the soil keeping it moist and fertile.
Authors in classical Greece and Rome, around 2000 years ago, were aware that some plants were toxic (allelopathic) to other plants nearby. Theophrastus reported that the bay tree and the cabbage plant enfeebled grapevines. Pliny the Elder wrote that the "shade" of the walnut tree (Juglans regia) poisoned other plants.
In China, mosquito ferns (Azolla spp.) have been used for at least a thousand years as companion plants for rice crops. They host a cyanobacterium (Anabaena azollae) that fixes nitrogen from the atmosphere, and they block light from plants that would compete with the rice.