It was a chilly day, shortly after Valentine’s Day. I walked through the Leventritt Shrub and Vine Garden, where the sun had warmed the ground, and the nearby trees of the North Woods rebuffed the assault of the day’s strong breezes. Magnolia buds were beginning to swell and the showy Hamamelis blooms stood out in bright protest against the grey and beige of February’s garb. I spotted, off in the distance, what appeared to be a bundle of arrows, point down, in one of the beds. Discarding the idea that Cupid had left them behind after playing a game of matchmaker on the Arboretum grounds over the holiday, I approached to give them closer inspection. Could they be leaves or flowers that remained on the plant, after others had senesced?

Frost ribbons on Isodon henryi in the Leventritt Shrub and Vine Garden
Frost ribbons on Isodon henryi (AA# 590-2010*A) in the Leventritt Shrub and Vine Garden.

Close up, the structures revealed themselves to be iridescent, composed of what appeared to be minute, fused filaments. I had seen these before—but not with this unique geometry, and never so high on a stem. They were ice ribbons, one of winter’s most elusive and ephemeral manifestations. I excitedly snapped a few photos with my ever-present Swiss army knife of mobile technology. I marveled at the precise ninety-degree angles of the ice’s exit point, and that the ice emerged from different stems at a similar distance from the base of the plant. Snapping a photo of the tag, I headed back to the Dana Greenhouses to learn about the biological and physical causes of ice ribbon formation.

Many details and photographs of ice ribbons of plants, and other frozen phenomena, are offered by the website of geographer and atmospheric scientist Dr. James Carter, professor emeritus at Illinois State University. He is interested in the phenomenon of ice segregation: the process by which ice crystals form following the movement of supercooled, liquid water through any porous medium, such as soil, porous stone, and plant material. He has also compiled a complete bibliographical collection of nineteenth century records of ice ribbons. He and linguist Dr. Robert Harms posed explanations for the phenomenon. Harms minted the term “crystallofolia” using Latin roots which can be interpreted to mean ice flower.

frost ribbons on Isodon henryi
Close up image of frost ribbons on Isodon henryi (AA# 590-2010*A) in the Leventritt Shrub and Vine Garden

A century of records indicate that only certain plants have a structure that facilitates ice ribbon formation. Many of these plants have specialized regions of dense vascular bundles, and the ice ribbons form when the ground temperature is warmer than the air temperature. This temperature difference causes water from the roots to be pulled into the stems, where it is secreted, through weak spots in the bark, at right angles to the stem. Some of the common names of these plants reflect their tendency to form ice ribbons. Frostweed is a common name for Verbisina virginica, Helianthemum canadense, and Helianthemum bicknellii. The ice forms that emerge from these plants seem to take shape based on the internal structure of their stems’ vascular tissue.

Many members of Lamiaceae, the mint family, produce frost flowers. Isodon henryi, the plant I encountered in the Leventritt Garden, is among them. The four protruding crystals that looked like the feathers of a fletched arrow formed due to the characteristic square stem structure for which the family is well known.

Despite yesterday’s snowfall, conditions for the formation of frost flowers (warm soil and air temperatures below freezing) may have passed for the season. Come fall 2020 and late winter 2021, look for these beauties in the Leventritt Garden.