Mosses belong to the bryophytes, a group of small, non-vascular land plants that also includes the liverworts and hornworts. Understanding a few basics about how they are built explains almost everything about where they grow and how to look after them.
No vessels, no roots
Unlike the familiar garden plants, a moss has no internal plumbing. There are no vessels to pump water and dissolved nutrients from a root system up to the leaves. Instead each leaf, often only a single cell thick, takes up moisture directly from rain, mist and dew across its whole surface. What looks like a root is a mass of fine threads called rhizoids, and their job is to anchor the plant, not to feed it.
This is why moss is happiest on surfaces that a conventional plant would find impossible: the shaded face of a boulder, a north-facing wall, the mortar between paving stones, the bark of an old tree. It is not drawing anything from those surfaces. It only needs something to hold onto, plus shade and the occasional wetting.
The drying-out trick
The cleverest thing a moss does is survive drought by switching itself off. When the air dries, the plant dries with it, shrivelling to a crisp brown mat that looks dead. It is not. Within minutes of the next shower it rehydrates, greens up and resumes photosynthesis as though nothing happened. Botanists call this poikilohydry, the ability to let internal water content track the surroundings rather than defend a fixed level.
For the gardener the lesson is simple. Brown moss is usually thirsty, not dead. A misting will tell you which.
Two plants in one life cycle
A moss lives in two stages that look quite different. The soft green cushion or carpet you recognise is the gametophyte, the long-lived stage. After fertilisation, which needs a film of water for the sperm to swim through, the gametophyte raises slender stalks tipped with capsules. These stalks and capsules are the sporophyte, the second stage, and they exist mainly to make and scatter spores.
When a capsule ripens it dries, opens, and releases dust-fine spores to the wind. A spore that lands somewhere damp and shaded germinates into a fine green thread, the protonema, which in turn buds into a new cushion. The cycle begins again. Moss also spreads the lazy way, simply by fragments: a broken-off piece that lands somewhere suitable regrows into a whole new plant, which is the trick gardeners exploit when they propagate it.
The two growth forms
Look at a few mosses and they sort into two camps, a split worth knowing because it decides how a moss behaves. Some grow upright in tufts and tight cushions, staying compact and slow-spreading; these are the acrocarpous mosses, the bun-like domes on a wall top. Others creep and branch sideways into flat, spreading carpets and wefts; these are the pleurocarpous mosses, the feathery mats over logs and soil. Cushion-formers give shape and texture, carpet-formers give fast continuous cover, and knowing which you are looking at is about the most useful thing you can know about them. It has its own guide in acrocarpous and pleurocarpous mosses.
Where mosses grow
Because a moss feeds from the air and only needs something to cling to, it grows where rooted plants cannot: bare rock, tree bark, brick and mortar, roof tiles, the compacted ground under trees, the splash zone by streams. What it cannot abide is the combination of strong sun and dry wind, which strips its moisture faster than it can take it up, so you find it on the cool, shaded, damp side of things. That same independence is why moss is one of the first life to colonise bare ground after disturbance, slowly building the conditions other plants then move into.
An ancient and successful design
Mosses are among the oldest land plants, with a lineage going back something like 450 million years, long before flowers, before dinosaurs, close to the time plants first left the water at all. That they are still here, in their thousands of species, on every continent including Antarctica, says how well the simple, rootless, dry-out-and-wait design works. A moss is not a primitive failure that never invented roots; it is a different and very durable answer to the problem of living on land.
How to tell a moss from a lookalike
Three plants are often mistaken for moss. Liverworts are flat and ribbon-like or have rounded lobes, and lie closer to the surface. Lichens are not plants at all but a partnership of fungus and alga, usually crusty, leafy or shrubby and often grey or yellow. Clubmosses and spikemosses look mossy but are larger, with tougher stems, and belong to an entirely different, vascular group. A true moss has the soft, leafy-stemmed, cushion-or-carpet form, and very often those fine stalked capsules standing above it.