Frost flowers typically occur in in late autumn when cold weather freezes plant stems but the soil is still warm enough for active roots to push sap into the stem. As the sap freezes, ice is extruded through cracks in the stem, forming thin ribbons or spines. In North Carolina, frost flowers are most often associated with Verbesina species, particularly Verbesina virginica (frostweed), but yesterday I found these examples on Salvia elegans (pineapple sage). They are, perhaps, not as delicate as the frost flowers that form on Verbesina, and from a distance looked more like frost seed pods than flowers, but they seem to have been formed by the same mechanism as the classic frost flowers.
As indicated by the presence of frost flowers, the first freezes of the season occurred on Tuesday and Wednesday nights. A few weeks ago, we were flirting with 90 F (32 C), but this morning the temperature was about 24 F (-4.5 C). The growing season for summer perennials and sub-tropical plants is definitely over. The cannas, bananas, and crinums have all turned black, and in a few weeks I’ll cut them back. The only plants still flowering outside are Crocus cartwrightianus ‘Albus’, Camellia ‘Yuletide’, Cyclamen hederifolium, and Abutilon megapotamicum (which continues to amaze me with its hardiness), but buds are expanding on Edgeworthia chrysantha.