Sphagnum, cranberries and cotton grass: reading the bog from the planks

Plank 1

A bog that only drinks rain

Tyrelis and Kamanos are raised bogs, which means they are fed almost entirely by rain and snow rather than by streams or groundwater. Over thousands of years the peat has built up into a very shallow dome, higher in the middle than at the sides, and the living surface on top has lost touch with the mineral soil far below. The water up there is acidic and very poor in nutrients, and only a small cast of plants can cope with it. That is why the view looks so uniform from a distance, and why each of those few plants is worth learning.

Walking out from the forest you can feel the change even before you name it. At the fringe there is a wetter, greener belt where water running off the bog meets water from the surrounding land, with birch, sedges and the odd alder. Then the trees shrink to knee-high and chest-high pines, the ground turns red and ochre, and the air smells of resin and wet moss. The dome itself is far too gentle to feel as a slope, but the stunted pines tell you plainly that you have crossed into somewhere else.

Plank 2

Sphagnum: the moss that builds the ground

Almost everything under the boardwalk is sphagnum, alive on top and dead underneath. The mosses grow upward a few centimetres a year while their lower parts slowly stop decaying and turn into peat, which builds up by roughly a millimetre a year. Sphagnum holds many times its own weight of water and makes the water around it more acidic, and between those two tricks it shuts out most other plants. It feels like a cold sponge, as anyone who has knelt on the planks for a close look and put a hand down for balance will know.

The colours are what hold our attention. On the hummocks the sphagnum is often deep red or wine-coloured, especially in autumn and after a sunny spell; in the wet hollows it is green or yellowish and grows loose and floppy. We do not attempt species names beyond that. There are several kinds on any bog, and telling them apart properly takes a hand lens and a willingness to take samples that we do not think is ours to exercise. Red for dry and high, green for wet and low, is enough to read the ground.

That pattern of hummocks and hollows is the key to the whole surface. The hummocks are drier on top, and that is where you find heather, bog rosemary, crowberry and the cranberry runners. The hollows hold water for most of the year and belong to sedges, sundews and the looser mosses. The bog pools, the dark open eyes of water that the boardwalk sometimes skirts, sit at the far end of the same scale, and they are much deeper than they look.

Plank 3

Cotton grass: the bog under snow in May

The first flowering plant most visitors notice is hare's-tail cotton grass, and they notice it twice. In April its flowers are small, yellowish and easy to walk past, tucked into dense tussocks that look like clumps of old hair. Then at the end of May the seed heads open into white tufts, thousands of them across the open bog, and for about two weeks Tyrelis looks as if a light snow has fallen that the sun cannot melt. That is the fortnight we invite friends who have never been.

The tufts are seeds, each fixed to fine white bristles that catch the wind, and by the middle of June most have blown away or turned grey. Around the same time bog rosemary opens its small pink bells on the hummocks, and the Labrador tea along the drier stretches comes into white flower with a heavy, resinous scent. That scent is pleasant for a minute and gives some people a headache on a warm, still day. All three plants are poisonous to some degree, and none of them are for picking.

• Late April: cotton grass in flower, small and yellowish in the tussocks
• Late May to mid-June: cotton grass seed heads white across the open bog
• June: bog rosemary and Labrador tea in flower, cranberry flowers on the runners
• July: sundews flowering in the wet hollows, the bog at its hottest
• September: cranberries red, the sphagnum at its deepest colour
• October and November: berries softer after frost, first ice on the pools

Plank 4

Cranberries, and why we pick so few

Cranberries on a raised bog do not grow on bushes. They grow on fine red runners that creep across the sphagnum, with tiny leaves and, in June, pink flowers whose petals fold back like the head of a very small crane, which is where the English name is said to come from. The berries form over the summer, turn from speckled white to red in September, and are at their best after the first frosts, when they soften a little and lose some of their bite. Berries left on the runners through the winter are sweeter still in spring.

We pick very little, and never where the boardwalk is the only way to reach the berries. Leaning out or stepping off to reach a good patch is exactly how a bog surface gets trampled into mud, and that damage takes decades to heal. In strict reserves such as Kamanos nothing may be picked at all. Elsewhere there are rules about when picking may start, so that unripe berries are not stripped, and we check the current ones every autumn rather than relying on what someone remembers from last year. When we do pick, it is by hand on the fringe, a small bag each, never with a berry comb that tears up the runners.