Oslo - Many of the world's plants are turning "alien", spread by people into new areas where they choke out native vegetation in a worsening trend that causes billions of dollars in damage, scientists said on Wednesday.The invaders include water hyacinth from the Amazon, which has spread to about 50 nations where it crowds out local plants, while Japanese knotweed has fast-growing roots that have destabilised buildings in North America and Europe.Citing a new global database, an international team of scientists wrote in the journal Nature that 13 168 plant species - 3.9% of the global total - "have become naturalised somewhere on the globe as a result of human activity".The spread of alien plants was likely to increase with rising trade and travel by emerging nations led by China, it said."North America has had most - many came from Europe after Columbus because colonists brought plants with them," lead author Mark van Kleunen of the University of Konstanz in Germany told Reuters.The global numbers were higher than most earlier estimates of just one or two percent, he said. Plants can be introduced deliberately as crops, for instance, or can get accidentally carried as seeds."With continuing globalisation and increasing international traffic and trade, it is very likely that more species will be introduced outside their natural ranges and naturalise," the authors wrote.Scientists have previously estimated that all invasive species - including microbes, animals and plants - cause damage of more than $1.4 trillion a year to the world economy. One 2012 report estimated that water hyacinth cost China alone about $1.1bn a year.Piero Genovesi, who chairs a group of invasive species experts at the International Union for Conservation of Nature, welcomed the study as a step to control the spread of new plants.The European Union was drawing up a "black list" of the worst species in which all trade would be banned from January 2016, he said. Other European rules call for action to eradicate newly identified alien plants within three months."I don't think it's possible to stop [the spread of invasive plants] but we can indeed significantly reduce the impacts," he told Reuters.