All materials have their own unique ability to store, absorb, and conduct heat. Dense, highly conductive materials are the best options for a geothermal greenhouse. This can include concrete, brick, tile, and even water. A well insulated greenhouse will be better at storing and holding onto heat.
Dark colored barrels of water are popular choices for 'thermal batteries', additions you can make to your greenhouse that store extra heat. In the daytime, the barrels absorb ambient warmth, and release it in the coolor nighttime. These barrels are cheap, and can serve many purposes for water storage within a greenhouse.
