The Making of a Peaty Tarn

Upland tarns in northern Britain — those dark, peaty pools cradled by crags and moraine — are not just wild swimming spots or lunch stops on a fell walk. Around their boggy margins, in the wet hollows where water lingers, a miniature landscape thrives. Sphagnum mosses, cotton grass and sundew reveal how water and soil shape these fragile habitats. Peaty tarns form where drainage is poor, often in glacial hollows or on a flat col between fells. Rainfall is high, evaporation low, and the ground stays waterlogged. In these anaerobic conditions, dead plant material does not fully rot; it accumulates as peat, sometimes a metre or more deep. The water is typically acidic, low in nutrients and stained brown with humic acids. That chemistry dictates which plants can survive. Only specialists tolerate the low pH and constant saturation. The result is a habitat that looks simple — a boggy ring around dark water — but is finely tuned to subtle changes in water level and soil.

Sphagnum: The Architect of the Bog

Sphagnum mosses are the engineers of these habitats. You will see them as spongy, colourful carpets — greens, reds, ochres — often forming raised hummocks or floating rafts at the tarn edge. Each plant holds water in large, empty cells, so a sphagnum lawn can store many times its own weight in water. That keeps the bog saturated even in dry spells and slowly acidifies its surroundings, favouring itself over other plants. Different species occupy different micro-habitats: Sphagnum cuspidatum trails in the wettest hollows, while Sphagnum papillosum builds firmer hummocks. When you walk, avoid stepping on these lawns. A boot print can compress decades of growth and create a drainage channel that dries the bog. If you are wild swimming, enter the water on firm gravel or rock, not through the sphagnum fringe.

Cotton Grass and the Look of the Fell

From late spring into summer, cotton grass transforms upland bogs. Its white, fluffy seed heads — actually bristles — catch the light and seem to float above the peat. Two species are common: hare's-tail cottongrass, with a single dense tuft, and common cottongrass, with several nodding heads. Both thrive where the water table sits near the surface. They are not grasses but sedges, and their roots help bind the peat. For fell walkers, cotton grass is a useful indicator: if you see it, the ground is likely to be wetter than it looks. For wild swimmers, it marks the transition from firm fell to boggy margin — a good place to keep your boots on. Tread on tussocks of cotton grass or heather if you must cross wet ground, rather than on the moss itself.

Sundew: The Little Carnivore

Perhaps the most surprising plant around a peaty tarn is sundew. Round-leaved sundew grows in bare, wet peat, often on sphagnum or between cotton grass tussocks. Its leaves are covered in red tentacles tipped with sticky droplets. Insects mistake the droplets for nectar and become stuck; the leaf slowly curls around them. Why eat insects? Because peaty soil is so poor in nutrients, especially nitrogen. Sundew gains what the soil cannot provide. It is small — often no bigger than a 50p coin — so you need to look carefully. Kneel down, but keep your knees off the moss. A single footfall can crush a colony. These plants are slow-growing and do not recover quickly from disturbance.

Walking and Swimming with Care

Upland tarns are fragile. A few simple habits make a big difference.

• Stay on established paths where possible. Sphagnum and sundew recover slowly from trampling.
• If you must cross boggy ground, step on tussocks of cotton grass or heather, not on the moss itself.
• Wild swimming is best in deeper water away from the boggy margin. Enter and exit on firm ground, such as gravel or rock, to avoid disturbing plants.
• Keep dogs on a lead around nesting birds and rare plants. This is especially important between March and July.
• Take all litter home, including fruit peel, which can harm grazing animals and change soil chemistry.

Cold water is a serious matter. Enter slowly, know your exit, and never swim alone in remote tarns. The plants are not the only fragile thing up here.

Reading the Bog on Your Walk

Next time you walk the fells, treat the bog around a tarn as a living text. Sphagnum tells you where water seeps and lingers. Cotton grass shows the height of the water table. Sundew reveals just how nutrient-poor the peat is. These plants together create a habitat that stores carbon, filters water and supports insects, birds and amphibians. They have been growing here since the last ice age, inch by inch. A little care from walkers and swimmers helps them continue. So pause, look, and leave only footprints — preferably on the path.