Imaged by Jesper Grønne (Photojesper.dk).
©Jesper Grønne, shown with permission.
The frosty spikes are rime.
When the air temperature is below freezing, suspended fog or mist droplets do not necessarily freeze. They can remain as supercooled liquid at quite low temperatures for lack of nuclei to initiate crystallisation.
All that changes when they hit a suitable surface - here the points on a holly leaf. They freeze and the ice surface then provides a centre for further droplet crystallisation to give these unusual formations.