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!
Dale
