Moss takes cold in its stride far better than most garden plants, and a hard frost rarely does an established patch any lasting harm. More often than not, the one thing still quietly growing in a frozen midwinter garden is a cushion of moss, working away at a temperature that has shut everything around it down. Understanding how it manages the cold also explains why winter, for a moss, is closer to a good season than a trial.
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What a freeze does inside the plant
When the temperature drops below zero, ice forms first in the spaces between a moss's cells rather than inside them, where ice crystals would be fatal. As that outside ice grows it pulls liquid water out of the living cells, so the cell steadily dries from within even as the air stays damp. A moss cell that can survive genuine drying can therefore survive freezing by much the same means, leaning on the same protective sugars and proteins and the same hormone cue that let it ride out a summer drought. The two hardinesses run together, which is why the drought specialists of sunbaked walls and rocks tend also to be the ones that laugh off a hard frost. The cellular side of that story is told at length in how moss survives drought; the cold simply borrows the same machinery.
Why the cold season suits moss
A moss keeps its green through winter because it never had leaves to shed in the first place, and it takes its water from whatever is going at the surface rather than pumping it up from frozen ground. Give it a mild, damp hour in December and it will quietly photosynthesise and even grow, in dim light and close to freezing, while the trees stand bare and the grass sulks. That knack of seizing any soft spell turns the dead months into working ones. It is the plainest reason a shaded bank or a north wall looks its best when the rest of the garden has given up, and the seasonal rhythm behind it is laid out in moss care through the seasons.
The shelter of snow
Snow looks like an ordeal and is often a kindness. A covering of it holds the ground beneath at a steady temperature near freezing, far milder and far less variable than the bitter, swinging air above, and it keeps the humidity high so the moss neither bakes nor desiccates. Thin snow even lets a little light filter through, enough that some mosses carry on a trickle of photosynthesis in the muffled world underneath. When the thaw comes, the moss emerges flattened and wet but unharmed, often greener than the patches that lay exposed. Snow, then, is one of the better things that can happen to a moss over winter.
Mosses of the deep cold
Push to the extremes and the toughness becomes extraordinary. Mosses are among the dominant plants of the Antarctic and the high Arctic, building slow banks of peat where almost nothing else will grow, enduring months of dark and cold that would kill any flowering plant outright. The references record a striking case from an Antarctic island, where moss buried and frozen in the peat for something like 1,500 years was coaxed back into fresh growth in the laboratory, having waited out the centuries in suspended animation. No garden frost comes close to testing a moss that hard, worth remembering the next time one looks dead on a cold morning.
So what actually harms it in winter?
Rarely the cold itself. A frost-struck patch goes dark, flat and brittle, and people new to moss often write it off, but it is almost always dormant and will flush green within a day of a thaw. The genuine winter threats lie elsewhere. A sodden mat of fallen leaves pressed over moss through the dark months will smother and rot it, which is why clearing leaves matters more than any worry about frost. Freshly laid moss that has not yet gripped can be lifted clear of the soil by repeated freezing and thawing, so autumn transplants are worth pressing down firmly and checking after a cold spell. And frozen moss turns glassy and snaps underfoot, the trampling question taken up in can you walk on moss, so a frosty path is one to admire rather than tread. Keep the leaves off and firm in any new patches, and frost is the least of a moss's worries.