Some may mourn the loss of colorful autumn flowers now that nighttime temperatures are falling below freezing. However, with this change of season, other types of beauty are visible in the early morning hours, such as "ice flowers" (Figure 1). These transient "flowers" are actually layers of ice crystals that form when moisture in certain herbaceous plants freezes and splits stems vertically. With adequate moisture and continuing sub-freezing temperatures crystallization continues, forming layers of ice form much like ribbon candy on the surface the plant stem. When temperatures begin to warm, the ribbon will curl inward and eventually melt.
Ice flowers are not a new phenomenon. In 1833, John F.W. Herschel published in Philosophical Magazine that ice "seemed to emanate in a kind of riband-or frill-shaped wavy excrescence-as if protruded in a soft state from the interior of the stem, from longitudinal fissures in its sides." In 1850, John LeConte observed that the "striking and beautiful" ice formation on plants, resembling "cotton-wool, varying from four to five inches in diameter." In 1892, William H. Gibson described "a flower of ice crystal of purest white which shoots from the stem, bursting the bark asunder, and fashioned into all sorts of whimsical feathery curls and flanges and ridges." He also speculated that the source of the moisture for these ice flowers was from the uptake of soil water by plant roots.
Other names for ice formation on plant stems are ice fringes, ice filaments, rabbit ice, and frost flowers. Although ice flowers can occur anywhere there are sub-freezing air temperatures and adequate soil moisture, not all herbaceous plant species produce them. Some of the native Missouri wildflowers on which spectacular ice flowers form, include Verbesina alternifolia (known as yellow ironweed or wingstem) (Figure 2) and Verbesina virginica (white crownbeard, frostweed, or tickweed). Yellow ironweed is found in 71 of the 114 counties in Missouri, whereas frostweed is found only in the southern half of the state. To find ice flowers, search at the base of plants in low-lying areas, weedy gardens or fencerows, and along streams at dawn or shortly thereafter, when temperatures are still in the lower 20's (°F). In Missouri, the most likely time for favorable conditions for ice flower formation is from October through early December.
Other types of ice formation occur on rocks and soil. Needle ice occurs on silty to clay-type soils with optimum porosity. Rods of ice, known as pebble ice, can form on porous sedimentary rocks that are contact with moist soil. Some types of pottery and brick are also known to form pebble ice. Both needle and pebble ice can increase in size over successive nights when sub-freezing temperatures occur. For more images of ice formation on rocks, soils, and plants, see https://www.americanscientist.org/article/flowers-and-ribbons-of-ice.
REVISED: February 21, 2017