Late one frosty morning, I paused my walk to admire ice crystals that had grown from a small branch lying on the ground. Delicate and lacy to the extreme, the ice had a silky and well-kempt appearance. The formation was gorgeous.

This was my first glimpse of hair ice, a phenomenon that originates in a surprising way.

If you live in a temperate climate that experiences hard frosts, you might be familiar needle ice. Even though it forms on frosty nights, this type of ice isn’t frost because it doesn’t condense out of the atmosphere. According Dr. James Carter of Illinois State University, it forms instead from in water in soil through ice segregation, a process when “above freezing and below freezing temperatures are juxtaposed. At the Earth’s surface this is most common in fall at night as the air cools to below freezing while the land surface stays relatively warm.” As ice forms on the soil surface, liquid water is pulled up from below through capillary action and freezes to the existing ice. This forces the ice to grow away from the freezing surface. The process stops when the temperature becomes cold enough to freeze everything up, the temperature rises above the freezing point of water and everything melts, or the soil surface becomes too dry.

Hair ice however, forms under even more specific, and perhaps unusual, circumstances. Like needle ice in soil, hair ice needs air temperatures just below freezing and a water saturated substrate. Unlike needle ice though, hair ice forms only on wood, specifically the dead and bark-free wood of broadleaf trees. Why only on dead wood?

See more photos of hair ice on iNaturalist

In 2015, researchers from Germany and Switzerland published a very interesting (and highly readable for a scientific paper) study titled, “Evidence for the Biological Shaping of Hair Ice.” Through repeated observations and laboratory experiments, they confirmed that the biological action of a winter-active fungus, Exidiopsis effuse, is required to enable the growth of hair ice.

Looking at the cross section of a small branch, wood rays radiate from the center of a branch like spokes on a bicycle wheel. From these rays, hair ice threads emerge and grow perpendicularly from the wood surface. The thickness of individual hair ice stalks corresponds to the diameter of the wood ray channels. Perhaps for the first time in my life, I could visualize the true scale of these cellular channels.

But this doesn’t explain how the ice maintains its shape. Threads of hair ice are extremely thin, sometimes .02 millimeters in diameter or smaller. Yet, they can grow to be 20 centimeters long (that’s 1,000 times longer than it’s thickness!) and maintain their shape for days. Normally, ice this fine couldn’t retain its shape for so long. It would recrystallize into larger crystals quickly at temperatures near freezing.

While the chemical process that preserves its fine and delicate structure is not fully understood, it seems that the ice, according to the 2015 study’s authors, is “doped” into maintaining its shape by fungi. Samples of melted hair ice contain lignin, tannins, and other compounds. Lignin cannot be digested by animals, only by fungus and some bacteria. It’s presence in the water, therefore indicates fungal activity. (We can thank fungi that forested habitats aren’t buried in dead trees.) The lignin and tannins might act as a crystallization surface for the ice and the fungi might help to initially shape the ice as it forms at the surface of the wood rays.

When researchers applied fungicide or hot water (90-95˚C) the hair ice wood for several minutes, hair ice formation was suppressed for many days. Instead of hair ice, an simple ice crust formed on the wood. This indicates that hair ice formation is somehow catalyzed by fungal activity and that high temperatures inhibit the activity of Exidiopsis effusa.

Since I first observed it, air temperatures have been too warm in my neck of the woods for hair ice to reappear. Given its ephemeral nature and remarkable delicacy, I’ll be sure to search for it once the temperature drops again. If I find it, I’ll surely be astonished by ice that was—in a sense—doped by a magic mushroom.