Soft Capping in Scotland: The context and potential of using plants to protect masonry
8. A NOTE ON green walls
ground moisture, tendrils of climbing plants will seek out moisture by growing into the masonry. Even if a substantial amount of the plant subsequently dies and is removed, such rooted-in tendrils may be impossible to remove without considerable disturbance of the masonry. Systemic poisons must be comprehensively applied in such circumstances.
The steepest pitch at which soft cappings were found to have been successfully applied was around 60 degrees (CS23). This is closer to being a vertical surface than a horizontal one. There is no reason, in principle, to assume that vertical masonry surfaces could not benefit from the same thermal buffering and moisture inhibiting effects that vegetation commonly gives to horizontal ones. An assessment of the potential for such ‘green walling’ shows that exactly the same issues pertain as apply to soft capping, though plant species vary, different technical approaches are required and aesthetic considerations become more important.
As with wallhead vegetation, it is prudent to undertake a careful assessment of the benefits and disadvantages of all individual mural plants, to inform a targeted programme of intervention and long-term maintenance, which sensibly might include some retention of vegetation. In only one case study (CS 28) was there evidence of deliberate retention of plants growing on the faces of walls. These were small plants, which were recognised as being an important part of the character of the place.
The removal of mural vegetation from monuments is a standard procedure of long standing, based on the premise that climbing plants, especially ivy, cause damage by rooting into masonry, while also obscuring the monument.
The romantic associations of mural vegetation have long been recorded, especially in England. These have been echoed in the designed green walls used in a number of recent high profile new buildings. These have been a logical development of the increasing use of green roofs in new buildings, especially in urban areas, to reduce heat gain, improve air quality and enhance biodiversity.
However, there has been some recognition of the potential protective benefits of plants on walls: Some kinds of wall climbing plants do not damage masonry directly but must none the less be kept away from the eaves and gutters to avoid blockages. These include Ampelopsis veitchii, a form of Boston ivy with small ovate or trifoliated leaves, which is often incorrectly referred to as a Virginia creeper; Hydrangea petiolaris, a climbing hydrangea; and Hedera canariensis, Canary Island ivy. The last is evergreen and can be grown over unburnt brickwork that is decaying due to frost as a form of protection.
Feilden, 2004
English Heritage have recognised that the removal of ivy from some monuments has accelerated masonry decay, by increasing the climatic exposure of the wall faces (pers. comm. A. Cathersidea). Examples include Wigmore Castle and Fountains Abbey. A similar consequence has been noted in Sweden. There is no doubt that climbing plants can cause damage to masonry walls in certain circumstances. They can root into decayed mortar and stone. Their growth can also add a significant load to masonry structures. However, there are many examples where attempts at complete ivy removal have been unsuccessful, triggering exactly the decay mechanism that was being targeted. If the lower stems are cut, removing plants’ source of
Fig. 8.2: St. Olof’s Church, Visby, Sweden. The ruin is utterly cloaked in ivy, an enigmatic form in the botanical gardens.
96