Despite its "heavenly" name the Tree-of-Heaven (Ailanthus altissima) is in my opinion the most pesky invasive tree species in the United States and Southern Europe. I have personally observed this tree species as a very prolific seeder in Spain, France, England, Canada (Toronto) and the States of Oregon and Washington. According to this website it is invasive in at least 30 of the 50 States. In this post I have tried to document in images just one instance of how this tree can spread. All the images are from the same yard in Eastern Washington. In the top image you can see the "mother" tree that is responsible for "seeding" the yard with new little Trees-of-heaven.
I did not stop to count them but there must have been at least 40 of the fast growing seedlings. In the image above you can see them growing all along the side of the house. This is a fast growing tree species that can form trunks up to three feet in diameter. Letting them grow this close to the house is NOT a good idea.
This next image shows more seedlings growing along the cyclone fence. I've seen these grow up to 2-3 feet tall in just one year! The image below shows a few more growing out by the garage unchecked. The owner of this house seems unaware that this tree species is capable of growing into dense thickets and can have an adverse affect on other plant species.
I mentioned in my last post that the Tree-of-Heaven can be confused with the Black Walnut. The leaves of both are quite similar but the flowers and fruit is easily identifiable.