Geothermal energy is heat from within the Earth that has been present since the original formation of the planet. It is contained in the rocks and fluids beneath the earth's crust, distributed through layers of magma several thousand miles thick.
You can store geothermal energy by providing points of contact to the nearby soil and your greenhouse. This can be done by partially or completely burying the greenhouse, or excavating a hillside in order to embed the greenhouse within.