This morning (Nov. 24) Emily and I drove out to the Bot Garden to look for "frost flowers." We found some during our Nature Rambles in the Garden last year (see this link) and the year before (2013, at this link). They only appear (or we only look for them) on the first autumn day when the overnight temperature is below freezing, which is a clue to their origin. They are not flowers, but some do look flower-like. They are, in fact, ice that appears to have been formed from water that is forced out of the stems of certain plants. As the water extrudes it freezes, forming a delicate ribbon of ice that twists and turns into a variety of interesting shapes. You can only observe these ephemeral structures for a short period of time. When the sun hits them they rapidly melt away.
This morning we found the greatest concentration of frost flowers at the bottom of the Dunson Native Flora garden just outside the fence that separates the Yucca planting from the "weeds." Earlier this summer most of the plants growing in this area were White crownbeard (Verbesina virginiana), also known as "Frostweed." Because there are two other species of Verbesina growing in this area of the garden we thought it would be interesting to see if any of them had produced frost flowers. We walked on the path in the power line right of way toward the river, checking the stems on either side and saw no evidence of frost flowers.This part of the garden has both V. alternifolia and V. occidentalis in abundance, so the absence of frost flowers here means that they may not be suitable for their formation. (Or that we were too late and the sun had melted them all.)
If you would like to see the growth and demise of a frost flower view this time-lapse video.
How frost flowers form
The water in the soil enters the root system by a process called osmosis. Osmosis is the term describing the movement of water across a cell membrane from a more dilute solution to a more concentrated solution. In this case the more concentrated solution is the sap in the root system and the less concentrated solution is the water in the soil. This movement of water into the root causes the sap fluid in the plant conductive tissues to rise. If the stem above has been injured or has a weakened skin the water will ooze out at the damaged spots. When the temperature is low enough this oozing water will freeze, beginning a frost flower.
This article by James R. Carter in the American Scientist magazine is the best discussion I have found on how such lovely and delicate features form.
In his article Carter also mentions a Georgia connection to the phenomenon:
". . . physician and naturalist John LeConte of the University of Georgia made many insightful observations about whole and cut-off stems, both of which grew ice. He noted that many plant stems were dead and dry at the time of year when he did his study, although the roots might have been alive, but the ice formation therefore didn’t seem to be connected to the plant’s physiological functions. He observed, “At a distance they present an appearance resembling locks of cotton-wool, varying from four to five inches in diameter, placed around the roots of plants; and when numerous the effect is striking and beautiful.”.
John and his brother Joseph were native Georgians, educated at Harvard and Yale, who became Professors at the University of Georgia, but left after an acrimonious dispute with the then president, Alonzo Church. Church thought that Professors should only teach, but the LeConte brothers wanted to both teach and do research. Both brothers left UGA over the dispute and eventually found their way to University of California where John became President.
If you want to see this phenomenon for yourself wait until the overnight temperatures are below freezing and go to the Bot Garden early (the grounds and natural areas are open at 8AM), before the sun is too high. It would be interesting to see if the same plant can produce a frost flower more than once. You could determine this by tying a marker to a specific plant and observing it on successive days when the conditions are right.
I hope you all have a great Thanksgiving!