On a late October day, a fall thunderstorm gathers its skirt of clouds and rumbles off to the east. The wind drops as the clear evening sky deepens to indigo blue. The unmistakable bite of winter does not bode well for the few remaining tomatoes in the garden, and a "hard freeze" warning posted on the 6 o’clock news confirms their fate.
The next morning, most Missourians will bemoan the loss of those tomatoes and the last few roses of summer, but those who know where to look will discover a whole new garden of incredibly delicate beauty. Whether deep in an Ozark forest or in the weedy corner of a vacant metropolitan parking lot, a perfect natural brew of biochemistry, physics and meteorology has again produced the miracle of the frost flower.
Although the name is colorfully descriptive, a frost flower is really neither "frost" nor "flower," but layers of ice squeezed from the stem of a plant. True frost is formed similarly to dew on the surface of objects, such as rose petals, window panes and pump handles. When the air or an object is below freezing, dew converts directly to ice upon touching a surface, forming crystalline rime or hoar frost.
Large frost crystals sometimes build one on top of the other to form "ice flowers," another natural phenomenon often found on rocks near Ozark springs and around city manhole covers. If the condensing moisture converts to water before freezing, amorphous frost or glazing occurs. This can be the protective coat of ice on fruit trees or the dangerous ice build-up on power lines and airplane wings.
Frost flowers, on the other hand, form when moisture already inside the stem of a plant freezes. One of the special properties of water is that it expands when it freezes. Like a bursting water pipe, the tender skin of the plant stem splits under the pressure of the expanding ice.
Because of the cell structure of the plant’s stem, fissures occur in tiny vertical rows. Capillary action in the plant’s veins pumps the moisture out through these minuscule cracks like translucent ribbon candy.
As crack after crack yields a layer of ice, the total effect resembles the many layers of a flower petal. Air bubbles trapped in the ice makes it appear frothy white.
If the air is cold and still when a frost flower forms, the ice layers push straight out into large silvery feathers. When