Frostweed

Frost flower on Frostweed, Verbesina virginica

A shade loving biennial, Verbesina virginica can be found in portions of the southern United States. It has dark green leaves with white autumn flowers and can grow up to 1.8 m (6 ft) tall. It is a favorite nectar plant of many bees and butterflies, including migrating monarchs and has past use as tobacco by some Native American tribes. There are many common names for this plant including white crownbeard and Indian tobacco, but it’s mostly known as frostweed.

A shade loving plant that does well around oak trees.

As it is, Verbesina virginica is not the only plant known as frostweed. Others share this common name including Helianthemum canadense and H. bicknellii for the reason that when temperatures get cold enough, the stems exude water which forms beautiful ice crystal formations. These formations have many names: frost flowers, ice ribbons, frost castles, crystallofolia, and rabbit butter; but the process itself is known as ice segregation.

It is not the full stem that splits, but rather the epidermis of the stem.

Ice segregation is when cold water moves through a medium towards the presence of ice, freezes at the interface, and adds to the ice structure. For a frost flower to form, the air temperature must be below freezing, but the moisture in the stem must remain liquid. The ground must also be warm enough for the roots to be active in moist soil. Water in the stem is drawn upward, expands as it starts to freeze, and results in a vertical split on the stem, specifically in the epidermis. As more water is drawn from the ground, the thin ice layer exudes further to form beautiful and interesting shapes.

Each frost flower is unique and different for each plant and season.

While frost flowers are the most common instance of ice segregation, it is not limited to the stems of plants. Similar phenomena occur in loose soils forming ice needles, also known as kammeis or pipkrake, as well as dead tree branches forming hair ice, also known as haareis, silk frost, or cotton candy frost, and also on rocks forming pebble ice.

Different location from above with frostweed growth and frost flowers.

In addition to Verbesina virginica, Helianthemum canadense and H. bicknellii, the phenomenon of frost flowers has also been reported for Cunila origanoides, Pluchea odorata, P. foetida, and P. camphorata, as well as on the lower stems of some species of Lamiaceae, Verbenaceae, and Apocynaceae. The key to all species confirmed to form frost flowers is that they are herbaceous and perennial, and thus they maintain a root system that overwinters.

Splitting stems of frostweed surrounded by new and old Turk's Cap growth.

The adaptive role of frost flowers is still speculative, especially considering that it occurs in many different species. Plants that adapt to live in a freezing climate have three options for survival: die and leave seeds, remain active, or become dormant. While annual herbaceous species most often follow the first option, long-lived woody species often follow one of the other two. For the plants that exhibit frost flowers, they don’t seem to fit either category nicely. For now, the best guess, provided by Dr. Bob Harms who coined the term crystallofolia, the phenomenon is an adaptive self-pruning. It is possible that frost flowers allow for a head start on spring growth, but as it is, it is simply a best guess for now.


The original post on frost flowers can be found on Google+ here with different photos than the ones shown here, but with the same text that has been provided above.