Craig and Sara recognized our mystery seeds from last week as buckthorn. In fact they are from the common buckthorn, Rhamnus cathartica.
Common buckthorn is a small weedy tree. It is originally from the Old World, and can form dense stands that shade out indigenous plants here in North America.
Fortunately buckthorns are not long-lived trees.
They are also carry, and are susceptible to, rust diseases.
(Illustration from Wikimedia is in the public domain.)
Common buckthorns have inconspicous white flowers that ripen into black berries after pollination.
It is said that birds eat the berries, but they are toxic to mammals, including humans.
A related plant, Rhamnus frangula or glossy buckthorn, is also from the Old World and looks similar to the common buckthorn. The margins of the glossy buckthorn leaves are entire, which means they lack the serrations or teeth along the edge you can see in the common buckthorn leaves in this photograph (click on center "fruit" photograph here for a clear look at a glossy buckthorn leaf).
Both common and glossy buckthorns have a negative reputation. Do common buckthorns have any redeeming qualities? What do you think?