A plant called the searocket (these guys) have been shown to give preferential treatment to family members, while attacking unrelated roots.
From the NYT:
"The finding is a surprise, even a bit of a shock, in part because most animals have not even been shown to have the ability to recognize relatives, despite the huge advantages in doing so....
Since the research on sea rockets was published in August in Biology Letters, a journal of the United Kingdom’s national academy of science, Dr. Dudley and colleagues have found evidence that three other plant species can also recognize relatives."
