Fox21Online 18 cork trees in total will need to be removed. July 9, 2021 18 cork trees in total will need to be removed. They were planted after the completion of the 4th street reconstruction. Afterwards, the Minnesota Department of Agriculture conducted research on the cork trees and found that they could be invasive. “A few of the trees have been switching their sex to female and so they are bearing fruit which is not supposed to happen. That leaves us open to the potential of the tree spreading beyond cultivation,” City of Duluth Forrester, Clark Christensen says. The city of Duluth will cut the trees at the stump, treat them with herbicide so they don’t sprout back and then plant a variety of new trees in the future.