Answer: Weeping cherry trees are grafted onto prunus rootstock. Anything that grows below the graft will be the standard prunus (cherry), anything at or slightly above the graft will be the desired (weeping) cherry. So, what you have is a continuation of the root stock which is a standard cherry tree. If allowed to grow, it will eventually overtake the grafted weeping portion of the tree. What you need to do is remove the errant growth by cutting it off the main trunk. Try not to damage the collar (slightly swollen area on the trunk where the branch emerges). The collar will close over the wound and seal it up. Be sure to look carefully along the trunk for additional sprouts from the rootstock. Try to keep everything removed except for the weeping branches at the top of the trunk.
Best wishes with your weeping cherry tree!
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