Pour water over a moss that has baked brown in a drought and the change is almost startling: the shoots swell, the green floods back, and within minutes a plant that looked finished is quietly at work again. Hardly any other land plant can manage this. How moss pulls it off explains a great deal about where it thrives and why some patches shrug off a heatwave that kills their neighbours.
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A plant with no waterworks
A flowering plant is plumbed. Roots draw water up and vessels distribute it, while a waxy skin and adjustable pores keep it from escaping, so the living cells stay plump whatever the weather does. Moss has almost none of that. It gathers moisture over its entire surface, straight from rain, dew and humid air, and it can barely hold on to what it takes. A moss shoot is therefore about as wet or as dry as the air around it, a condition the references call poikilohydry. On an exposed wall a cushion can pass from sodden to brittle over one sunny, breezy afternoon, and the plant has no way to slow that loss. So instead of resisting the drying, moss has evolved to survive it.
Folding the cell away
What happens inside a drying cell is where the real feat lies. In an ordinary plant, water loss wrecks the cell from within: membranes crack and proteins lose their shape, while the stalled machinery of photosynthesis leaks reactive fragments that scar whatever they touch. A desiccation-tolerant moss heads most of this off as it dries. According to the botanical literature, it floods its cells with sugars, sucrose chief among them, and these stand in for the departing water, propping up membranes and proteins in something near their working form. Picture setting a fragile object in a block of resin rather than leaving it loose in a box. Alongside the sugars the cell makes protective proteins and mounts a chemical guard against those damaging by-products. The moss does not so much endure the drought as pack itself down tidily and wait.
The speed of coming back
Recovery is quick, which is part of what makes it look like a conjuring trick. Add water and the dried cell takes it up in seconds; respiration picks up within minutes, and photosynthesis follows over the next few hours as the machinery is checked over and restarted. Repairing the ordinary wear that drying always leaves behind runs on for a day or two afterwards, which is why a hard-baked cushion can seem sluggish at first and only reach full colour and vigour after a couple of good soakings. The greening you notice in the opening minute is genuine, but the plant is still finishing its repairs behind that first flush.
How long it can wait, and which mosses are hardiest
The waiting can be astonishingly long. A moss will sit dry and dormant for weeks or months and still revive, and specimens pressed dry in herbaria have been coaxed back to life after years shut in a drawer. Tolerance varies widely between species, though, and it maps neatly onto where each one lives. The champions are the mosses of sun-baked rock, walls and roofs, among them the screw-mosses (Syntrichia and Tortula) and the pincushion grimmias, for which drought is a routine part of ordinary life. Mosses of damp woodland floors and bogs are far less rugged; they dodge drying rather than tolerate it, and a long drought can genuinely kill them. Working out which sort you have tells you how much thirst it will forgive.
What it means for growing moss
Two practical lessons fall out of the biology. Established moss browning in summer is nearly always dormant rather than dead, so the response it wants is patience and water, never the trowel; the companion piece on reviving dried-out moss walks through the how. The second lesson concerns young moss. Fresh transplants and slurry have not yet laid down the sugar reserves and tuned-up defences that let a mature cushion dry safely, so a drought kills them far more easily and they need steady moisture until they settle in. Matching the moss to the spot matters too: a bog species asked to hold a hot, dry wall will perish where a screw-moss would hardly notice. Keep those points in mind and a browning drought becomes a passing nuisance rather than a loss, much as the watering guide sets out.