The salt marsh at Lower Marsh Farm is part of the Tamar-Tavy Estuary, a marine inlet on the Cornish coast into which the Tamar and Tavy rivers flow. It was made a Site of Special Scientific Interest in 1991, due to its international importance for nature conservation, in particular as a wintering site for wildfowl and wading birds, the national importance of the marine biology, and the rare and rich flora and fauna on the site.
The marsh is cut off from the estuary by a sea wall (built in 1810), which was once used to drain Landulph Marsh to provide grazing pasture. The salinity of the land varies from the fresh water entering from the rivers and the sea water entering from the sea wall, providing habitats for a wide variety of flora and fauna. The majority of the marsh contains soft rush, with greater bird’s-foot trefoil and water mint. Other species include marsh thistle, fleabane, wild angelica and broad-leaved willowherb. Grasses include wavy hair grass with sweet vernal grass, and the locally scarce species plicate sweet-grass and hard-grass. The back of the marsh is dominated by yellow flag iris with water mint and floating sweet grass on the surface of the water. Around the sea wall there is mixed scrub and mature trees of hawthorn, elder, blackthorn, holly, oak, cherry, bramble and gorse. The marsh also supports an area of mud flat which is exposed at low tide when the marsh drains through the flood gate in the sea wall into the estuary.
The Salt Marsh is also an important site for waders which leave the estuary at high tide and feed on the marsh. A number of nationally scarce birds have also been recorded, such as water pipit, water rail, green sandpiper, avocet, little egret and kingfishers. Other species recorded include great crested grebe, dunlin, curlew, shelduck, bartailed godwit, golden plover teal, sedge warbler, heron and barn owls. There is also a good population of the short-winged conehead cricket, often found in costal locations with maritime rushes and wet sites, and can often be heard chirping away.
The marsh is also home to the 4 beautiful resident Highland Cows whose grazing conserves the habitat in a natural way by helping to keep the vegetation under control to maintain its open aspect.