Susan A. Dudley and Amanda L. File of McMaster University in Ontario, Canada have published a paper indicating that plants compete less with sibling than with strangers of the same species.  It turns out that if two plants in the same pot do not share a mother they will produce more fine root mass in an attempt to compete for water and nutrients.  However, if they do share a mother they will develop normal fine root mass.  So then the question becomes, how do they recognize their siblings?