Of all the things moss does, building peat is the one with the biggest consequences. It is worth understanding, because it is why "peat-free" is printed on compost bags now.
How sphagnum makes peat
Sphagnum, the bog mosses, grow in waterlogged, acidic, oxygen-poor ground. The old growth beneath the living surface does not fully rot, because there is too little oxygen and the conditions are too acidic for the usual decomposers to work. Instead it accumulates, layer on layer, over thousands of years, as peat. A deep peat bog is, in effect, millennia of moss stacked up and half-preserved, growing only a millimetre or so a year at the top while holding centuries of carbon below.
The moss that makes its own bog
Sphagnum does not simply tolerate these conditions; it creates them. Like all mosses it has no roots and takes up water across its whole surface, and its leaves carry large dead cells that act as reservoirs, so a living carpet holds many times its own weight in water and keeps the ground saturated. It also acidifies its surroundings, which suppresses the bacteria and fungi that would otherwise break down dead plant matter. Between them, the waterlogging and the acidity switch off decay. The moss engineers a wet, sour, airless world that suits itself and almost nothing that would rot it, and peat is the slow result. There is more on the plant in sphagnum moss and its many uses.
Why bogs matter
Because the carbon in all that moss never finished decomposing, peatlands lock away enormous amounts of it, more per hectare than forest. They also hold and slowly release vast quantities of water, steadying rivers and reducing floods, and they shelter specialised plants and animals found nowhere else, from sundews to wading birds. Intact, a bog is quietly doing several important jobs at once, and doing them for free.
The cost of digging it
Cutting peat for fuel or compost drains the bog, lets air into ground that has been airless for millennia, and so restarts the decay that was halted. The stored carbon oxidises and escapes to the atmosphere. A resource that took thousands of years to form is gone in a season or two of growing, and it does not come back on any human timescale. That is the case against peat compost in a sentence, and it is why bog drainage is now treated as a climate problem rather than a merely local one.
Peat-free for growers
Modern peat-free composts use coir, wood fibre, bark, composted green waste and similar materials. They behave a little differently from peat, tending to dry on the top while staying wet below, so they reward checking the moisture lower down with a finger rather than watering to a fixed schedule. They also drain faster and hold nutrients differently, which is worth allowing for when you feed. For most sowing and growing they now work well, and the good ones keep improving. Choosing them leaves the bogs where they are.
Letting bogs recover
Damaged peatland is not always lost. Where drains are blocked and the ground is rewetted, sphagnum can return and begin laying down peat again, though it works at the moss's own unhurried pace rather than ours. Buying peat-free, and supporting the restoration of drained bogs, are two sides of the same choice: keep the carbon in the ground and let the moss carry on doing what it has done for thousands of years. See moss through history for the older human uses of the same plant.