In the immediate vicinity of the large crevice bed there is a bed with tufa rocks, some of them weighing more than a ton. This tufa stone is a very porous limestone material, created by a continuous process of mineral deposition from mineral-rich waters in places where they come in contact with the air. As calcium-saturated water comes to the surface, carbon dioxide is released and calcium is deposited as calcium carbonate – i.e., limestone.
Bacteria, cyanobacteria, moss, leaves, and twigs are encapsulated in the calcium carbonate, making the resulting limestone extremely porous and very suitable for growing alpine plants.
Formation of tufa stone is a very slow process. Fossils found in these stones indicate that they are around 60 million years old and they originate from a quarry near Munich. The acquirement of these stones was sponsored by The Friends of the Garden.
These stones are placed in a ‘basin’ of sand created by an underlying impermeable geomembrane, which facilitates movement of moisture upwards in the porous limestone, thereby providing water for the alpines planted in the stone.
The techniques used in planting of these alpines is unusual, in that holes are bored in suitable places in the stone into which the small plants are inserted. The holes are made using a percussion drill with a 15 mm bit – preferably boring obliquely downwards and as deep as the drill can reach. The material bored out is saved and, afterwards, carefully packed around the roots of the plants placed into the planting-holes with a thin wooden stick. This calcareous material can be mixed with a small amount of growing-medium and sand before finally inserting a small piece of clay to hold the plant in place until it becomes established.
It is clear from our observations, that the many plants placed in cracks and crevices in the sand and tufa growing media have been very quick to establish, while those carefully planted in bore-holes require rather more time to recover and form new rosettes.