Why Plants Sometimes Struggle Near a Geothermal Loop Field

A residential geothermal, or ground-source, heat pump does not generate power the way a large geothermal power plant does. Instead, it uses a buried network of pipes, called a loop field, to move heat into or out of the ground. This is how the system heats and cools a house. Homeowners sometimes notice that grass and other plants growing near these buried loops struggle or die. Usually, the cause is a combination of two separate factors, not just one. The first cause is simply physical. Installing a horizontal loop field requires digging trenches four to twelve feet deep across a wide area. This digging can cut through roots and pack down the surrounding soil. This alone can damage or kill nearby plants, no matter how the system later runs. The second cause is thermal, and it develops more slowly. Over a long heating season, the system pulls heat out of the soil around the pipes faster than the surrounding earth can replace it through natural conduction. This leaves the soil measurably colder for weeks or months at a time. The opposite happens in cooling mode during summer, when the system adds heat to the soil instead. Shallow-rooted plants, such as grasses and small perennials, tolerate this fluctuation fairly well. Trees and other deep-rooted plants are much more vulnerable, both to the temperature swings and to the risk of roots growing into the pipes. Because of this, landscaping guides for geothermal systems generally recommend avoiding trees directly on or near the loop field.