The Tree of Heaven (Ailanthus altissima) is a Chinese tree that I have seen featured in more than one botanical garden in Spain (in Madrid and in Malaga) but it is also, in my opinion, the most invasive and troublesome of all the non-native trees in Spain. The Tree of Heaven is not an ugly tree, in fact it has some interesting ornamental properties when in flower and when its samaras are forming. It is also a very prolific and fast growing "weed" that is fast propagating itself in virtually all parts of Spain.
One of its favorite places to spontaneously sprout is along the sides of roads and freeways. I think that this is due to the fact that the winged seeds (Samaras) often fall onto parked cars and are then transported along until they fly off the top of the car and end up along the side of the road where they then do what they seem to due best - grow!
This is one of the tree species featured in the "Around the World In 80 Trees" exhibit in the "Jardin Botanico-Hisorico" of Malaga. If you take a closer look and the areas around the tree you will discover that this tree species is making a run and taking over the garden. It seems to like the climate a great deal.
The image below is of a thicket of Ailanthus altissima along the side of a road in Southern Spain.