Ice-nucleating particles (INPs) aid the freezing of water droplets in clouds and thus modify cloud properties. In a campaign in a Finnish boreal forest, biological INPs were observed, despite many of their potential biological sources being snow-covered. We sampled tree-dwelling lichens that were not covered in snow and tested their ice nucleation ability in the laboratory. We found that the lichen harbours INPs, which may be important in similar snowy environments.