Lappeenranta cuts back giant hogweed after decades of control

Long-term efforts to eradicate giant hogweed have reduced the plant’s spread in Lappeenranta, Yle reports. The city has worked to control the dangerous invasive plant for almost 20 years, and it remains the top priority in its invasive species programme.

About 170 locations are being monitored in Lappeenranta this summer. The number of plants at those sites ranges from single specimens to hundreds. The city originally had more than 200 control locations.

“Giant hogweed is the most important one for us because it is dangerous,” said Maisa Hautala, who is responsible for maintaining the city’s built green areas.

The plant’s sap can cause burn-like, potentially permanent skin damage when it reacts with sunlight. It can also cause severe allergic symptoms and breathing difficulties in particularly sensitive people.

Control work begins in spring and continues into autumn. Two employees have been working almost daily across Lappeenranta, and the largest infestations have already been removed.

Eradicating giant hogweed takes time because its seeds can remain in the soil and produce new seedlings for more than 10 years. This seed bank must therefore be tackled for at least a decade, said Taina Lahtela, an environmental inspector with the Lappeenranta region’s environmental services.

Property owners are responsible for removing giant hogweed from their own land. The law prohibits allowing the plant to spread into the environment or growing it.

Recommended control methods include pulling the plants, repeated mowing, removing flower heads and covering infestations with plastic. Herbicides can also be used on large infestations, but protective equipment is required because of the plant’s harmful sap.

A plant introduced from abroad

Giant hogweed has been controlled across Finland for decades. The plants can grow up to five metres tall, suppressing other vegetation and reducing biodiversity.

Large infestations can also prevent recreational use of an area and reduce the value of plots and properties, said Erja Huusela, a senior researcher at the Natural Resources Institute Finland.

Giant hogweed was originally brought to Finland as a garden plant because of its large and striking flowers. The plant tolerates northern conditions well and has spread across almost the entire country, apart from the northernmost parts of Lapland, Huusela said.

In South Karelia, the plant has spread at least partly from Russia. Seeds may have become attached to car tyres when the border was crossed, Lahtela said.

Lahtela believes Lappeenranta could eventually bring giant hogweed completely under control, but this will require many more years of determined work. The plant’s numbers are expected to decline over the course of decades, she said.

European Union and Finnish invasive species legislation aims to reduce the spread of harmful invasive species and the damage they cause. Huusela said that raising public awareness is even more important than legislation, alongside resources for practical control work and monitoring.