Heavy winter road salt use may cause summer browning along Danish roads

Danish trees and plants along roads could face severe stress or death this summer due to record-high salt use during the winter, a researcher warns.

Morten Ingerslev, an associate professor at the University of Copenhagen who has studied road salt since the 1990s, said the excessive salt applied to roads this winter will linger in the soil, potentially damaging vegetation when dry summer conditions concentrate the salt near plant roots.

“Instead of the tree absorbing water, the salt draws water out of the plant,” Ingerslev explained. “We call this osmotic stress or salt drought.”

Salt damage can extend up to 50 metres from motorways, though in cities, the affected zone is typically just 0.5 to 2 metres. Newly planted trees, especially those on street corners exposed to salt from multiple directions, are most vulnerable due to their underdeveloped root systems.

This winter, the Danish Road Directorate used 70% more salt than the 10-year average, with around 350,000 tonnes expected nationwide. While older trees may survive, urban trees—valued for their aesthetic and recreational benefits—could suffer significant harm.

Ingerslev advised that extra watering during late-summer droughts might help dilute soil salt concentrations, but noted that sodium binds tightly to soil, making removal difficult. A wet summer could mitigate damage, while a hot, dry one would worsen it.

“It would be a real shame if some of our valuable urban trees started to suffer,” he said. “These are trees we enjoy having and looking at—they hold great value in the urban environment.”