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| Blooming from August to November, Frostweed provides late season nectar and a unique surprise at the first frost. |
Accompanying the crunching of fallen leaves and the rattling of seed pods drying in the breeze is the arrival of the first frost. This marks the seasonal change from our relatively warm autumn to the cooler days of a mild central Texas winter. How does frost, this sparkling layer that sometimes covers the fall landscape, form?
When the temperature of the air reaches a point where the water vapor in it can condense out into water, it is called the dew point. The frost point is when the dew point falls below freezing, and rather than producing dew, it creates frost. Consisting of tiny, spike-like crystal structures called spicules that grow out from a solid surface, frost generally forms on surfaces that are colder than the surrounding air. Even the size of the crystals can vary, depending upon the amount of time they took to grow, the relative changes in temperature, and the amount of water vapor available.
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| A frost flower. |
Cold air is denser than warm air, so quite often lower areas become colder on calm nights due to differences in elevation. Known as surface temperature inversion, this phenomenon forms ‘frost pockets’ or areas where frost forms first, due to cold air trapped against the ground. It is here, in these areas, that you can find a rare and wonderful spectacle of nature called ‘frost flowers.’
While many plants can be damaged or killed by freezing temperatures or frost, this varies by the type of plant and tissue exposed to these conditions. In central Texas, there is a common plant called Frostweed (Verbesina virginica), which is found in low-lying areas near streams, creeks, canyon bottoms, and in dappled shade at woodland edges.
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| Bordered Patch |
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| Silvery Checkerspot |
Much of the year, Frostweed goes unnoticed while it grows 5 to 8 feet tall and leafy, the top of each stalk crowned by a cluster of small white flowers. Its stalks are oddly square-like, with fleshy green flanges. Frostweed begins to bloom in the August heat, and continues until first frost, well into the fall. These flowers provide late-season nectar for many insects, including bees, beetles, flies, wasps, and even migrating hummingbirds and Monarch butterflies. It is also a host plant for Silvery Checkerspot and Bordered Patch butterflies.
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| A patch of frost flowers. |
With the first frost, however, the water contained in each Frostweed plant stem expands, causing the stems to crack. Via capillary action, more water is drawn through the cracks, freezes when it hits the cold air, and forms long curls of ice like petals of a flower, ribbons, or other delicate, abstract sculptures. Most often, they consist of longitudinal bands of fine ice threads at right angles to the stem. These delicate flowers of the frost are fleeting in nature, and can only be found in early morning, as the rising temperature quickly melts them away.
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| A prime example of crystallofolia. |
Only a few species of plants exhibit this unique phenomenon, which has been called ‘crystallofolia’ by Bob Harmes at the University of Texas, from the Latin crystallus or ‘ice’ and folium or ‘leaf’. Much is left to be discovered reading the purpose of this process, but further research by Dr. James Carter at Illinois State University has concluded that the ice formation often far exceeds the amount of moisture locally available within the plant’s stem, so it must be augmented by water drawn up from the roots.
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| Delicate curls can form when more plant moisture is available. |
On the surface, fall may seem as if nature is shutting down for the winter, but take the time for a second look and you just might be surprised. The first frost of the season is another intriguing part of the ongoing cycle of life and renewal for our native plants and animals.
