Key to the health of the environment at Tarn Hows is the quality of water flowing into it.
The National Trust works with farm tenants to ensure the good water quality, this means keeping it free of pollution and naturally low in nutrients (excess nutrients can lead to algal blooms in the tarn). The farmland in the tarn's water catchment is managed in a low-intensity way using minimal amounts of fertiliser and not too many livestock.
Grazing with hardy breeds like belted galloway cattle and herdwick sheep helps to maintain a rich variety of native plants.
 
During winter, the cows are introduced into the woodlands where the touch-me-not balsam grows. They have helped bring about a dramatic rise in the amounts of balsam and consequently a large increase in the number of rare netted carpet moths.
Site of Special Scientific Interest
The Tarn Hows area is protected as a Site of Special Scientific Interest because of it's important aquatic flora, its wetland mires and ancient woodlands:
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