A frost flower is a name commonly given to a condition in which thin layers of ice are extruded from long-stemmed plants in autumn or early winter. The thin layers of ice are often formed into exquisite patterns that curl into “petals” that resemble flowers.
Frost flower formations are also referred to as frost castles, ice castles, ice blossoms, or crystallofolia.
Types of frost flowers include needle ice, frost pillars or frost columns, extruded from pores in the soil, and ice ribbons, rabbit frost or rabbit ice, extruded from linear fissures in plant stems. While the term ice flower is also used as synonym to ice ribbons, it may be used to describe the unrelatedphenomenon of window frost as well.
Hair ice, frost beard, ice wool, or feather frost describe a hairy, sometimes silky variant of frost flowers, extruded from openings of histological rays in the wood, which also requires the presence of fungus metabolism.
The formation of frost flowers is dependent on a freezing weather condition occurring when the ground is not already frozen. The sap in the stem of the plants will expand (water expands when frozen), causing long, thin cracks to form along the length of the stem. Water is then drawn through these cracks via capillary action and freezes upon contact with the air. As more water is drawn through the cracks it pushes the thin ice layers further from the stem, causing a thin “petal” to form. In the case of woody plants and (living or dead) tree branches the freezing water is squeezed through the pores of the plant forming long thin strings of ice that look uncannily like hair i.e. “hair ice” or “frost beard”.
The petals of frost flowers are very delicate and will break when touched. They usually melt or sublimate when exposed to sunlight and are usually visible in the early morning or in shaded areas.
Examples of plants that often form frost flowers are white crownbeard (Verbesina virginica), commonly called frostweed, yellow ironweed (Verbesina alternifolia), and Helianthemum canadense. They have also been observed growing from fallen branches of conifers and contain enough hydraulic power to strip the bark off.