The Tree of Heaven of the Jardin Botanico

Zaragoza protects, as a singular tree, a specimen of the species European cities spend the most money trying to get rid of. Ailanthus altissima was sent to Europe from Beijing by a Jesuit missionary in 1751 and planted as a fast curiosity and a possible host for silkworms. It escaped. It now takes over railway sidings, demolition plots and riverbanks across southern Europe, resprouts from any cut root, releases a compound that suppresses the plants around it, and sits on the European Union's list of invasive species of concern. This one earned its place by being the best of its kind here. Of the four tree of heaven specimens on the city's register, this is the one rated in very good condition at the 2004 survey: two metres fifty-seven around, fifteen metres seventy tall, standing at the entrance to the botanical garden inside the Parque Grande. It is a handsome tree when something keeps it in order. The leaves are pinnate and can reach a metre in length, very large for anything hardy in Europe, and in a good summer the female trees hang thick with papery winged seeds that turn from green to a dull red before they go brown. The catalogue is the only source describing this tree, and it gives no age. What it gives instead is a judgement, and the judgement is that of four candidates this was the one worth keeping.