Photo by JosiahJohnston licensed under CC BY 3.0
Those who are hardy enough to venture out on cold mornings during late fall may be lucky enough to encounter ice flowers. Though these beautiful ribbons of ice are not actual flowers, they are produced by some plant species. For at least one species (Verbesina virginica), this phenomenon occurs frequently enough to earn it the name frostweed. The production of these "ice flowers" has everything to do with plant physiology.
Ice flowers result from a perfect storm of sorts. They only occur on herbaceous tissues that are still actively transporting fluids. During the first hard freezes of fall, while the ground is still warmer than the air, the roots are still drawing up water into the stem. As ambient temperatures dip, the water within the stem starts to freeze. As it expands it causes the stem to rupture.
As the water and sap within spill out into the cold air, they too begin to freeze. The result is the formation of a ribbon of ice that seems to flow from the wound. As the ice continues to grow, more liquid is pulled from the stem and the ribbons grow more ornate. Contrary to popular opinion, these formations are not the result of frost deposition but rather a process called ice segregation.
Along with V. virginica, dittany (Cunila origanoides) and stinkweed (Pluchea camphorate) are also known to regularly produce ice flowers. Why it happens more frequently with these species and not others remains a mystery. It could be that they simply remain active longer into the fall than other plants species. Regardless of the reason, ice flowers are an interesting natural phenomenon to witness. Sometimes it pays to be an early riser on those first few cold days of late fall.
Further Reading:
http://my.ilstu.edu/~jrcarter/ice/flowers/