The Curious Case of the Conjoined Walnuts

Staff at Matthaei Botanical Gardens & Nichols Arboretum discovered over a dozen double walnuts on an individual black walnut tree at Matthaei. Experts from U-M and Clemson University were consulted but the jury is still out as to the cause of the unusual double fruits. 

Matthaei-Nichols horticulture manager Mike Palmer asked U-M Professor Emeritus Burton V. Barnes for his opinion. Barnes taught the popular Woody Plants class for years at U-M and co-wrote the definitive Michigan Trees with Warren Wagner. Barnes noted that it was a good idea to check out the crown of the tree to see if there were additional double fruits, and told Palmer that this probably suggests a particular characteristic of this tree, with a likely genetic cause. "It might be a classic 3:1 ratio of a recessive gene," Barnes added, "that is if you have 3 singles to every double."

Palmer cut the conjoined walnut in half with a coping saw to reveal two separate nuts inside. See picture for detail.

A rare double black walnut from a tree at Matthaei Botanical Gardens. Over a dozen fruits were discovered to be conjoined. Experts from U-M and Clemson were consulted but still don't know the cause.