When openings have been left in walls, these are filled with soil, offering immediate growing opportunities for plants. On built walls without openings, the start for plants is slowest. The mortar has to be weathered to a certain extent before it is ready for plants, a process that may take many years. Once the environment is amenable to it, fern spores can be blown in or seeds can be smeared on the surface using some clay or old manure. On dry walls or in flat situations, sowing or planting is obviously much easier. When a number of desired individuals have been established they will usually propagate and distribute themselves, for example species such as Linaria cymbalaria, Corydalis lutea and C. ochroleuca, Cheiranthus cheirii, Hieracium amplexicaule, Phyllitis scolopendrium, Asplenium ruta-muraria andA. trichomanes.
Maintenance
Vegetations on rocky substratums are largely maintained by weeding out unwanted species and cutting off wilted and dead parts. Blown in leaves are removed. With increasing nutrient and moisture quantities in rocky substratums, plant growth will become more abundant and, consequently, weeding will become more time-consuming. As a rule, maintenance will vary between very little in dry poor walls and relatively intensive in flatter, richer and moister situations and shallow soils. For the latter, one may choose either the mowing or the weeding maintenance method, corresponding to spontaneous or guided naturalism. In the former, grass species will be part of the desired plant community, resulting in a (free-flowering) low-fertility meadow vegetation. Mowing once a year and carrying off the hay will suffice. If the weeding method is preferred, entailing more direction and guidance, a great variety of vegetations is possible. One may freely choose from a variety of compositions and combinations, ranging from vegetations with a more ecological layout and impact to artificial vegetations aimed at an aesthetic impact.
The timely weeding of trees and shrubs appearing spontaneously is of special importance in keeping built and dry walls in good condition. Trees such as Acer pseudoplatanus, Fraxinus excelsior and Ulmus spp., and shrubs such as Sambucus nigra, Crataegus monogyna, Viburnum lantana, Rubus fruticosus and Hedera helix may cause severe damage to the construction. These unwanted species are weeded out as young as possible; larger individuals are hard to remove without damage. In doing so, one should obviously use one’s own judgement: rare or special species may be spared in places, only trimming them back when they grow too large, for example Ficus carica, Buddleja davidii, Rubus uvacrispa and the such. On weak slopes and flat situations, weeding in rocky substratums may cause painful fingertips. A useful method for preventing this is to apply a layer of sand (a few centimetres (an inch) or so in thickness) immediately after planting and supplementing it as required. An important aspect of built and dry walls is the correct spatial and visual balance between subtratum and vegetation, since it determines the charm and beauty of such environments. Walls that are largely or completely overgrown are less adventurous and are aesthetically less appealing. By weeding out or cutting back parts of the vegetation—i. e. of the desired species!—one may maintain this balance.
