Whereas vegetation of organic and mineral soils have been dealt with in the preceding sections, this section is about more unusual substratums and soils, such as inorganic materials, like bricks or rocks that are found in many shapes—walls, broken rubble, lava, minestone and slate—and very shallow soils situated or created with the former as its base. With these rocky materials, all sorts of habitats and substratums may be created. They will initially look very artificial and barely natural, but they may lead to very special, unusual and surprising results. The most interesting characteristic of these materials is their suitability for creating special reliefs and gradients, impossible to achieve with normal soils. In the wild flora, many species prove to adapt to these conditions quite well. The results one may obtain are not only unusual, their variety of designs and shapes may also be very attractive, both in an aesthetic and an ecological respect.
The situations dealt with in these paragraphs concern the sunny artificial ones; conditions that are stony or rocky by nature are left out. The most familiar rocky habitat or substrate for plants are walls—mortared or dry—and the artificial arrangements of rocks called rock gardens. Less well-known is the fact that demolition products, such as old or used bricks, roof tiles, shingles, paving stones, curb stones, tombstones and the such, have great potential for being used in rocky substrates. During the last few decades, many fine examples of these have been created in the Netherlands, both on a small and a large scale. One of the materials’ special advantages is that they are perfectly suited for working in relief, especially in height or three-dimensionally. This makes it possible to create a lot of growing and living space for plants on a small surface. In addition, these constructions are less susceptible to damage caused by trampling or by animals such as cats and dogs. Wherever little space is available and much ‘human pressure’ is present, rocky substrates offer many opportunities.
Rocky materials of very small sizes offer further opportunities. Lava, crushed rock, brick and rock split in 0-40 mm (0-1.5 inch) fractions are excellently suited for creating typical habitats. The finer fractions (0-5 mm) provide water retention and a root penetrable substratum. In addition, material rich in lime results in the most diverse vegetation.
Thin soil layers on top of stones, bricks or paved surfaces (e. g. 5-10 cm) and layers of sand or sandy clay are called shallow soils. A special type of shallow soils are roofs covered with gravel and roof covers specially constructed for growing plants on them, known as green or Sedum roofs, the latter after the genus usually grown on them.
Rocky habitats are situations in which large differences in temperature are present within short periods of time and, consequently, large differences in humidity. Drought and low soil fertility are its common traits. In the more extreme conditions, only plants that are very well adapted to these can survive.
The application of rocky materials starts with a preconceived plan or design, worked out in detail or in broad outline. In the latter case, one may work largely intuitively Using the materials in a construction requires knowledge of its qualities. The design, the material and its utilisation determine the final visual result to a much larger degree when compared to the usual soils. The chances for plant growth will increase with the quantity of soil applied or mixed in. As the quantity of nutrients and moisture are increased, more species will be able to grow on the substratum. These quantities and the composition of the added soil allow one to create different habitats and opportunities for plants. One may thus provide living space for high-impact species, such as Salvia verticillata, S. pratensis, Malva moschata, M. alcea, Coronilla varia, Verbascum spp., Galium verum, Ononis spinosa, Silene nutans, Silene cucubalus, Campanula rapunculus, C. rapunculoides, Centaurea scabiosa, Genista germanica, G. anglica, etc. A final important factor is the exposure to sunlight. Northern and southern expositions each have their own possibilities, caused by the presence or absence of shade, sunlight, heat, light and moisture. Where it is necessary to use a certain quantity of soil for moisture and nutrient provision, soils such as clay, sandy clay or loam are preferable. Peaty soils are reduced by oxidation and are therefore unsuitable. For shallow soils, sandy types are useful as well.