Several scientists write in Biology Letters that plants are racist, preferring those of their own cultivar to others from the same species:
Here we show in the annual plant Cakile edentula, allocation to roots increased when groups of strangers shared a common pot, but not when groups of siblings shared a pot. Our results demonstrate that plants can discriminate kin in competitive interactions and indicate that the root interactions may provide the cue for kin recognition. Because greater root allocation is argued to increase below-ground competitive ability, the results are consistent with kin selection.
While some waffle on about genetics, a shorter path to understanding kinism is this: it helps to have people who work like you, mentally and physically, because then you can save on the resources required otherwise by default for dealing with strangers, who must be assumed to be enemies if you are sane and realist.
In this way we see racism not as a psychology of genetic victory, but as a defensive move against the insanity of the world. You need people on your tribe. Who is your tribe? The only inalienable uniform is genetic relationship, and so plants, like humans, choose to be with those like them.