Are trees ever weeds? How can I find out if a particular tree is a weed?

A weed is a plant out of place or not desired in the given context. By that definition, a tree can be a weed if it is planted in the wrong place, if it seeds on its own in an undesirable location, or if it is a species that has the potential to be invasive. Invasive trees, once established, tend to reproduce rapidly and out-compete other trees and plants. 

You can check with your local Extension office for help determining whether a particular tree might be considered a weed or an invasive species in your region.  Another good resource is