Although mental images of quaint cottages and historic collegiate campuses are often filled with ivy-covered buildings, the nasty truth is that allowing ivy – and many other types of vegetation – to grow on your historic building can be extremely detrimental to its survival. Usually the method by which the plant attaches itself to the building is mechanically harmful to historic building materials. In addition, vegetation holds moisture against buildings and causes chemical deterioration of building materials. Even the growth of such small plants as mosses and lichens on historic buildings has a similar detrimental effect. Finally, substantial growth of vegetation can hide problems in the underlying wall, such as cracking or loss of mortar caused by settlement.
Chemical Deterioration Caused by Vegetation
All living things, including lower order plants such as algae, fungi, and lichens, and large plants like vines, require air to survive – indicating respiration. Respiration of many plants produces acids. These acids – through a process known as chelation – dissolve minerals in stone or extract specific elements out of the minerals comprising stone, resulting in deterioration of the stone’s surface. Although perhaps not as drastic as the damage caused by acid rain, the effect is similar.
Mechanical Deterioration Caused by Vegetation
All plants that grow on historic buildings attach themselves to the structures in some way. In many cases, the chemical action described above softens the underlying wall material and provides a pathway for the plant to insert a portion of itself into the stone as an anchor. As the plants grow, the anchors get larger as well. With higher order plants, like ivy, the anchors may actually break off small pieces of masonry or cause a total disintegration of small areas of mortar. Secondly, the anchors provide a pathway for water to run into the building material. Water saturation will cause deterioration and rot in wooden materials. In cold climates, water in masonry subject to freeze/thaw cycles generally causes spalling. If a plant grows sufficiently large, the main vine can develop into a trunk, which is strong enough to heave stone, unsettle a foundation, or even collapse part of a structure.
Water Retention Caused by Vegetation
In addition to the mechanical and chemical destruction caused by the growth of vegetation on historic structures, the plants keep water trapped against the structure. Because they need it to survive, plants efficiently collect precipitation on their leaves and elsewhere in their structures. This water will remain against the exterior wall, encouraging rot and insect infestation, as long as the plant remains in place. Particularly dense plants will even prevent sunlight from reaching a moist wall, which limits the beneficial process of natural drying after a rainfall or snow shower.
Before you remove vegetation from your historic building, it is important to consider whether the vegetation is considered character defining. For example, the ivy-covered buildings that are prominent at so many older college campuses may well be considered an important part of the campuses’ historical development. Similarly, plants growing on a structure that were placed by a well-known landscape architect or famous gentleman farmer may be significant to the story of a historic site. If this is the case, then it is advisable to create a system of support for the plants that it is independent of the structure, e.g., a trellis that supports the weight of the plant and also creates an airspace between it and the wall.
If the plants are not a character-defining feature of the historic property, it is usually preferable to remove them. The least damaging way to remove vines and other large plants is to cut them off at the roots, allow the plants to die and wither, and then remove any easily detachable pieces of plant material from the building. Removing live plants from the structure can cause additional damage because it may also tear off surrounding building materials that have been weakened. To kill the roots, a “cut-and-paint” technique – where the plant is cut and the cut surface is then painted with a herbicide – is usually recommended. Moss and lower order plants can be effectively removed with an appropriate biocide and a soft bristle brush, being sure to wet the surface prior to cleaning and to work from the bottom up to minimize streaking. Historic buildings should never be powerwashed to remove biological growth. As with any preservation work, the best course of action is to begin with the gentlest means of removal possible and advance to more aggressive techniques in small steps.
Once the vegetation is removed, the wall surface should be checked for needed repairs. Often masonry will require repointing, and wooden elements may need repair or consolidation. If the damage is severe, replacement may be warranted. The best way to protect your historic building is to prevent the problem from the start by keeping thriving vines where they belong – in your garden.
For additional information:
Butcher-Younghams, Sherry. Historic House Museums: A Practical Handbook for Their Care, Preservation, and Management. New York: Oxford University Press, 1993.
National Trust for Historic Preservation Forum-L Listserve.
New York Landmarks Conservancy. Historic Building Facades: The Manual for Maintenance and Rehabilitation. New York: John Wiley & Sons, Inc., 1997.
Weaver, Martin E. Conserving Buildings: A Manual of Techniques and Materials. New York: Preservation Press, 1997.
Young, Robert A. Historic Preservation Technology. New York: John Wiley & Sons, 2008.