It looks harmless and has been widely used in private gardens, but Japanese knotweed has caused numerous problems in many European countries and is now threatening to spread in the southern Swedish county of Halland. Biologist Nils-Gustaf Nilsson studied the expansion of the plant in the region and found several wild specimens outside Ljungby.
"When we inventoried, we had squares of 5 by 5 kilometers, and we didn't find it in all squares, but we did find it in many throughout the county," Nilsson tells Swedish Radio in Halland.
This invasive species, which has been banned in Britain, only needs a small root stub to germinate. Japanese knotweed thrives in Europe's climate and a small bush can become several meters tall in one year. As it grows, it kills everything around it.
"If you want to get rid of it, you need to chop it down to its base and cover it with black plastic for years," Nilsson explains.
Despite being a serious threat to biodiversity, it is not included in the Swedish Environmental Protection Agency's national plan for foreign species. Johan Näslund works at the environmental agency and believes it is time to step in.
"If we think of the problems it has caused in, for example, Germany, we see it's a threat, it can take over," Näslund says.
Its invasive root system and strong growth can damage concrete foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites. The European Union is currently discussing a common strategy to fight its expansion, according to Näslund.
"Sweden supports it, but other countries are against it, as the problem has gone to far and it is virtually impossible to do anything about it," he says.
Japanese knotweed isn't as widespread in Sweden as in other European countries, but the risk is still there. The environmental agency has warned against planting it private gardens and encourages everybody to inform the authorities if they find any specimens in the wild.