Susan Dudley, associate professor of biology at McMaster University in Hamilton, Canada, explains that "when plants share their pots, they get competitive and start growing more roots, which allows them to grab water and mineral nutrients before their neighbours get them. It appears, though, that they only do this when sharing a pot with unrelated plants; when they share a pot with family they don't increase their root growth. Because differences between groups of strangers and groups of siblings only occurred when they shared a pot, the root interactions may provide a cue for kin recognition."
The article in ScienceDaily goes on to say,
"Though they lack cognition and memory, the study shows plants are capable of complex social behaviors such as altruism towards relatives, says Dudley. Like humans, the most interesting behaviors occur beneath the surface.
Dudley and her student, Amanda File, observed the behavior in sea rocket (Cakile edentula), a member of the mustard family native to beaches throughout North America, including the Great Lakes."