The Frost Below
MANY PEOPLE IN THE GREATER SOUTHEAST have never heard about, much less witnessed, one of their region’s most striking natural phenomena.
It involves Verbesina virginica, a plant commonly known as frostweed. The “weed” part of the name tells us that early English-speaking settlers didn’t respect this plant, even though it stands erect and typically attains a height of 4–7 ft. The bloom period extends from August through November, depending on latitude and altitude (frostweed has been known to grow as far from its primary region as Iowa, Ohio and even Pennsylvania).
Despite the nominal suggestion of weediness, the flowers are indisputably pretty and also serve as a good nectar source for pollinators in general and monarchs in particular (in fact, it’s one of the Center’s top five recommended fall nectar plants for supporting monarchs). As is true for most plants in the sunflower family (Asteraceae), two sets of flowers occur at the same time, small disk flowers surrounded by somewhat larger ray flowers. In this case both are white, so we can forgive someone unfamiliar with the species for assuming that the flowers’ whiteness metaphorically became the “frost” in frostweed.
Not so: Another common name, ice plant, gets us closer to the truth. After frostweed’s flowers fade, this perennial plant goes dormant and stands there gradually drying and looking rather bedraggled. Only then is the stage set for frostweed to perform its magic trick.
When the first good freeze sets in, the plant draws underground moisture up through its roots and extrudes that water sideways through the lower portions of the stalk as ribbons of ice. Those striated ribbons may curl tightly inward, suggesting scrolls of paper or parchment, or they may undulate irregularly as new ice crystals form and push the older parts of each ribbon further away from the stalk.
Dr. James R. Carter, a geography and geology professor who began investigating the phenomenon post-retirement, notes that “the dew point temperature of the air must be below freezing and the temperature must fall to the dew point, or more properly the frost point” in order for this to occur. He explains further: “What we observe … is ice segregation along the stem of a plant. When the supercooled water penetrating through the stem encounters the first crystals of frost, the supercooled water turns to ice. That process continues and new ice is added at the stem-ice boundary. This pushes the old ice out and away from the stem.”
Within each involved frostweed plant, the extrusion of ice typically ruptures the epidermis and cortex that together make up the outer portion of the stem. That covering usually gets pulled away from the hard core, which remains intact and can extrude ice again during future freezes, although the amount of ice and the maximum distance above the ground tend to diminish each time.
Among the imaginative names on record for these wintry formations are ice fringes, frost flowers, ice castles, frost freaks, ice flowers, frost beards, rabbit ice, frost castles, ice blossoms and ice leaves. To create a scientific version of that last name, Dr. Robert T. Harms (similarly, a linguistics professor who delved into botany after retiring) coined the term “crystallofolia,” which has the form of a Latin plural noun. (Other plants that exhibit crystallofolia in their native environments are Helianthemum canadense, likewise called frostweed; Pluchea odorata, known as marsh fleabane or sweetscent; and Cunila origanoides, called stone mint or common dittany.)
Whatever the shapes taken by and the names given to frostweed ribbons, they remain so delicate that the gentle touch of a finger risks breaking them. Imagine a cold, translucent version of phyllo dough. Others have followed the confectionary trail to visions of cotton candy or taffy, while less-hungry viewers have imagined tissues or spun glass.
Frostweed’s ice ribbons are sensitive not only to touch but also to light. Once the sun rises high enough for its rays to clear nearby obstacles and fall on the base of a beribboned frostweed plant, the ice may sublimate. In the relatively warm regions where most frostweed grows, rising morning temperatures soon melt frostweed’s ice.
The key to appreciating frostweed ice is to get near it. Because the majority and the best of the ice formations occur close to the ground, a view from normal eye level can’t do them justice. If bending over to have a look is good, kneeling or sitting on the ground is better. A magnifying glass reveals more texture and details than the naked eye.
And for those of us who want to photograph frostweed ice, the best view is generally the lowest one possible. To that end, it’s good to carry a mat on which to recline, aiming sideways and keeping as many parts of the mostly vertical formations in focus as possible. If Hamlet had been a nature photographer (or even just a nature lover), he might have said, “Thus frostweed doth make liers of us all.”
A Tip for Gardeners
For those of you who live (primarily) in the Southeast who’d like to witness frostweed’s magic ice trick without traveling farther than your own property, here’s some gardening advice from the Center’s director of horticulture, Andrea DeLong-Amaya. She says Verbesina virginica does well in dry to moist shade, where plants in the wild form handsome colonies that help counteract the individual leaning that can result if light comes primarily from one direction; stakes or a nearby fence are also useful. She advises cutting plants short through the spring, then letting them grow the rest of the year. Another shade-loving plant like Turk’s cap (Malvaviscus arboreus) can be placed in front of the frostweed to conceal brown leaves or any spaces that result from removing them.
Steven Schwartzman is a writer and nature photographer whose work can be read and viewed on his Portraits of Wildflowers blog.