Frostweed (a.k.a. white crownbeard or white wingstem [Verbesina virginica]) in the Asteraceae Family, is a stout plant with a winged stem that can be six to seven feet tall. Fleshy, longitudinal wings, a dominate feature of this and closely related plants in the genus Verbesina (collectively called “wingstems”), extend below the alternate, oval to lance-shaped leaves. Slightly toothed leaves, widely spaced on winged petioles, may be seven inches long and two or more inches wide. From August to October, flowers bloom in heads arranged in corymb fashion in many-branching clusters at the top of stems.
Each head, like in other members of the Asteraceae Family, is actually a composite of smaller flowers, supporting about five ray flowers (with the larger, white “petals”) and seven or more disc flowers. The plants favor well-drained acid or calcareous soils in open woods and thickets from Iowa to Texas and over to the East Coast. In late fall/early winter, after the first significant freezes of the season, a very noticeable feature of this plant and source of one of its common names, “frost flowers,” can be seen.
What are Frost Flowers?
Frost flowers are created on very cold days (or more typically nights) from the plant’s sap. Other names for this phenomenon include ice flowers, ice ribbons, rabbit ice and ice castles. (The term “frost flower” is also used for a blooming flower covered by frost and ice patterns on window panes.)
Frost flowers form on herbaceous (die back to the ground in winter) perennial (live through multiple growing seasons) plants that mature in late season. Their stems have porous pith that can provide for a steady supply of water and dissolved minerals from the roots. For the fragile frost flowers to form, the vascular system of a plant must be initially functioning. With the first hard freeze, expanding and freezing sap places increasing pressure on the epidermis (outer layer) of a plant so that it splits along the stem following the structure of plant fiber. When sap makes contact with frigid air, it freezes instantly into ice slivers along the stem. As the sap (ice) exits the plant, additional sap is drawn up the plant’s stem from the roots via capillary action.
As long as the sap is in motion toward the fissures in the stem (formed when the stem split), it remains liquid, but becoming ice when contacting the earlier-formed ice slivers and frigid air. This “new” ice forces the earlier-formed ice outward, thereby creating ice ribbons.
The ice ribbons curl unpredictably due to variables such as total sap flow, changes in rate of sap flow out any particular fissure, changes in air temperature and wind. The frost flowers are usually located at ground level or extend slightly up the stem, decreasing in size the higher they occur. The formation of new ice stops when roots can no longer draw in water (ground frozen) or warming temperatures causes the ice to melt or sublimate. Ice formation could also be arrested by the plant as it forms subsurface buds for spring growth, blocking off the old stem.
Frost flowers can form with subsequent freezes as water from the roots continue to move up the damaged stem, but these later ice ribbons generally appear lower on the plant and in lesser quantity.
The intricate and unique patterns of the delicate ice ribbons can form stunning shapes and structures, some appearing as elaborate “blooming” flowers, thus the name “frost flowers.”
Occurrence of Frost Flowers
Frost flowers occur in many parts of the world; however, very few species produce frost flowers. In Arkansas, in addition to frostweed, other species that are know for their frost flowers are dittany (Cunila origanoides) and sweetscent (Pluchea odorata). Dittany is more cold-tolerant than frostweed, such that dittany’s frost flowers form later in the season. (Although reported by some, frost flowers do not seem to form on yellow ironweed [Verbesina alternifolia], a relative of frostweed in the wingstem group. At the time of frost flowers on frostweed, the author has observed stems of yellow ironweed being dead without any sign of having split.)
Article and photos by Arkansas Native Plant Society member Sid Vogelpohl