“Ice flowers?” Never heard of them. That is, until last Tuesday, when the buzz at the Arboretum was all about the ice flowers on Isodon henryi (593-2010*A; 鄂西香茶菜), a Chinese perennial herbaceous plant in the mint family. Needless to say, I was there first thing the next morning! This is what I saw.
Word on the street is that you need air temperatures below freezing and soil temperatures above freezing. I have googled around and checked out the very limited scientific literature on the topic, and I don’t think anyone has proposed an explanation that I buy. So naturally, I will propose one (which may be entirely wrong and has not been properly vetted by card-carrying plant physiologists). Normally, water moves through plants under negative pressure (tension)—it is literally being pulled from the top. But, on certain nights, when humidity is relatively high and the soil is moist, water in the soil can essentially force its way into a plant’s roots (osmosis) and push (positive pressure) right to the top of the plant.
So, now my speculation: in our Isodon henryi plants, positive root pressure was generated last Monday night and the water was forced up into the leafless stems of these plants, where it began to freeze, expand, and create a slit in the stem (you can see it in the top image). As the liquid water kept pushing up into the stem, it oozed out of the stem and froze solid in the cold air as a narrow ribbon that continued to be extruded for many hours. The result was a set of dozens of ice flowers on the stems of these plants—looking a bit like ribbon candy—but made of ice. Totally amazing and most ephemeral.