At first, the small purple flowers are hard to spot in the weak May sunshine. Slowly, drifts of delicate mountain pansies, along with white rosettes of alpine pennycress, begin to emerge across an area little bigger than a football pitch on the banks of the River Allen in Northumberland. This is a pocket of calaminarian grassland, an increasingly rare habitat where specialist plants called metallophytes have adapted to live in soils deeply contaminated by heavy metals—the legacy of more than 1,000 years of lead mining.
A Human-Made Habitat
“This is absolutely a case of nature responding to pollution caused by humans,” says Geoff Dobbins, estates manager for the Northumberland Wildlife Trust, who is passionate about saving these grasslands. But as they become cloaked in more aggressive plants such as gorse and broom, and the zinc and lead brought by mine-wash become buried beneath a blanket of humus, a growing debate has emerged: should these human-made meadows be protected or allowed to gently fade away?
The grasslands are named after Viola calaminaria, a rare yellow violet found in metal-rich soils of northern Europe, itself named after calamine, the old term for zinc ore. About 30% of Europe’s calaminarian grasslands are in the UK, but they are scarce, covering just 450 hectares (1,100 acres), with pockets in northern England, mid-Wales, and the Highlands of Scotland.
How Metallophytes Thrive
These grasslands originally evolved in small patches around rocky upland outcrops, where veins of lead, cadmium, and zinc were exposed by the elements. As mining began, a biocrust of lichens and mosses developed that could tolerate toxic wastewater, according to Dr. Ruth Starr-Keddle, a botanist at the North Pennines National Landscape. Then plants like spring sandwort—once known as leadwort—and alpine pennycress took hold, along with tough species like sea thrift, bladder campion, and kidney vetch, often found by the coast or in limestone soils.
Despite their delicate appearance, these specialist plants can live in soils 30 times more toxic than most other species can tolerate. As they grow, metallophytes act as “hyper-accumulators,” cleansing the soils through phytoremediation. They absorb metals through their roots and convert them into complex organic compounds, which are locked away below the surface when the plants die. “They don’t get rid of the contaminants—they’re accumulated into the plant,” says Starr-Keddle. Absorbing metals also acts as a defence mechanism, making the plants unpalatable to herbivores and insects, while the toxicity prevents fungal diseases.
Historical Context: Lead Mining Legacy
The barren, rocky uplands of the northern Pennines were first mined by the Romans, but the industry peaked in the mid-18th century. Today, the landscape is dotted with abandoned workings and spoil heaps. Miners used a process known as “hushing” to uncover ore veins: artificial dams were built high above the seams, and when opened, sent torrents of water cascading across the landscape, washing away topsoil and rocks. The ore was then carried by packhorses to smelting mills, such as Plankey smelt mill at Briarwood Banks, near where the mountain pansies now bloom.
The remains of the mill poke out from under brambles and hazel trees, near a dressing floor where rocks and ore were sorted. Water separated heavier ores from rocks, and fragments of heavy metals were washed into rivers. Over centuries, these deposits, together with mine-wash from upstream workings, gathered on shingle banks and silt beds along rivers like the South Tyne and the Allen, providing a perfect base for calaminarian plants to take root.
Conservation Dilemma
The Environment Agency estimates that abandoned metal mines still pollute about 900 miles (1,500 km) of rivers in England, despite most closing over 100 years ago. Pollution reaches rivers from point sources—contaminated water flowing from drainage tunnels—and diffuse sources, when rain washes metals from waste heaps. Starr-Keddle notes a balancing act between improving water quality and preserving this rare habitat. “If you took samples from most rivers in the North Pennines, most have contamination from lead mining,” she says. “We’re just trying to reduce it to safer levels.” By improving watercourses, “we are going to naturally impact the development of calaminarian plants.”
In the North Pennines alone, more than 60% of upland calaminarian grasslands have been lost since the 1970s. These are shifting, “successional” habitats; as they move from bare soil to denser grassland, grasses outcompete the flowers, and they disappear.
Innovative Solutions
Across northern England, projects are harnessing the unique properties of metallophytes to help clean up rivers while keeping the grasslands blooming. At Nenthead in Cumbria, vast spoil heaps protected as a scheduled monument by Historic England are being scraped—topsoil stripped back to re-expose metal-rich subsoils, encouraging calaminarian species. In County Durham, the government’s Water and Abandoned Metal Mines (WAMM) programme is working with organisations like the North Pennines National Landscape and the Rivers Trust. At three former mining sites near the River Tees, soil has been stabilised with fast-growing grasses and coir matting to reduce runoff. Over 1,500 plugs grown from seeds of seven key calaminarian species, including non-metallophytes like wild thyme and mouse-ear hawkweed, have been planted on the edge of spoil heaps. The plants, grown in a special mix of compost, stones, and metal-rich soil, act as a natural barrier to stop metals leaching from the site, and their seeds are expected to spread across the whole site.
At Briarwood Banks, Dobbins uses a simpler solution to keep the pansies flowering: a strimmer. Twice a year, he cuts back grasses, shrubs, and perennials that lurk around the margins. But he is realistic that as water quality improves, these riverine sites built on a legacy of pollution will eventually be lost. “There’s good and bad in that outcome,” he says.
