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The tree that appears above these lines surely sounds familiar to you because you have come across it many times, either because it is in your city or because while walking on vacation through any municipality on the Mediterranean coast it was there, in its most stately gardens. A clue in case you don’t recognize it at first: its seeds have a kind of wings, which helps them spread further and better. It is no coincidence: the ailanthus (Ailanthus altissima) arrive from Southeast Asia two centuries ago for its size and presence, but also because it grew quickly in practically any soil. Today is prohibited by law: It is an invasive species that kills other plants and loads the sewage. Ailanto the colonizer. It can measure up to 30 meters, has gray bark and the leaves are made up of many elongated leaves placed in a row. Its fruits are like seeds with wings, hanging in clusters. One way to recognize it is the (bad) smell: if you tear off a leaf and rub it, it stinks. Since its arrival, ailanthus has spread throughout much of the peninsula, with special intensity in Catalonia and the Valencian Community. This tree colonizes ditches, slopes, degraded banks and abandoned gardens, wherever there is a free piece of land, there is the ailanthus: a single adult specimen It is capable of producing up to 350,000 seeds winged per year and thanks to their wings and the wind, the ailanthus continues to expand. The environmental impact of ailanthus. According to the official MITECO fileailanthus alters the functioning of the forest ecosystem and modifies the chemistry of the soil where it settles, making life difficult for other species. Thanks to its rapid growth and dispersed shape, it competes for space and light in areas close to rivers (rich in vegetation), releasing toxic substances that prevent other plants from growing around it. This environmental study by the Junta de Andalucía It confirms that where there is ailanthus, there is up to 35-40% less vegetation than in areas without it. The impact is not limited to vegetation: the Ministry for the Ecological Transition and the Demographic Challenge points out which can also cause damage to urban infrastructure for one reason: its long and reticulated roots, capable of extending up to 15 meters from the foot. These roots look for cracks and gaps to spread, cracking pipes, sewers, foundations, buildings, and other structures. Context. The ailanthus comes from China, and although there is evidence that it was cultivated as an ornamental tree since the 18th century, the Ministry of Agriculture of Spain gives it as naturalized at the beginning of the 19th century. In the middle of that century, Marcelino Sáez de Sautuola, discoverer of the Altamira caves, wrote a letter to the French Acclimatization Society in which he commented on the success of planting ailanthus, at that time highly appreciated for its ornamental and medicinal qualities. Ailanthus is not only “a fugitive” in Spain, wherever it goes, it has problems. It is present and is classified as an invasive species in almost all the world. In the EU it appears in the list of invasive alien specieswhich forces member states to take coordinated control measures and is common in Germany, Austria, Switzerland, the Czech Republic, the Danube basin and practically all Mediterranean countries. The ban and fines. That is included within the Spanish Catalog of Invasive Exotic Species regulated by Royal Decree 630/2013 means that possession, transport, trafficking and trade are prohibited by law throughout Spanish territory. The fines are quite large: from 100 to 3,000 euros for minor infractions, from 3,001 to 200,000 euros for serious ones and up to 2 million euros in the most serious cases. That is state regulation, because each community may have its own additional regulations. What happens then with the ailanthus that there are? This Royal Decree contemplates an exception: those specimens that were already planted in private gardens, urban parks or botanical gardens before the entry into force of the rule. As for those planted in public gardens, administrations must progressively eliminate them. How to keep them at bay. Eradicating them is difficult and expensive. The MITECO report explains Cutting the tree is useless, as it stimulates the appearance of new shoots: the most effective way is to combine the mechanical removal of young seedlings with specific chemical treatments applied to the leaves or stumps. It is what is being done in the Montes de Málaga and the Sierra de Cazorla. Looking to the future, research points to biological control as a more sustainable alternative: the mite Aculus taihangensis has shown be an effective specific agent. In the United States, USDA investigates a soil fungusVerticillium nonalfalfae) that is offering promising results. In Xataka | Spain planted millions of eucalyptus trees to have cheap wood. 90 years later, we have confirmed that they are a green desert In Xataka | The Iberian Peninsula is being invaded: more than 1,200 exotic species have come to stay Cover | Marina Torres