Ivy on Walls

A few years ago English Heritage funded research into the role that English Ivy (Hedera Helix) plays with regard to historic monuments. The idea was to find out, over 3 years, the positive and the negative impact that ivy can have on historical walls and buildings, as well as how to manage the ivy.

I volunteer at a Living Churchyard where there is plenty of ivy. I'm quite a fan of it, but realised that I knew next to nothing about it. So I decided to read the seminar report PDF produced for English Heritage so I could have a better understanding of whether my fan-status of ivy was justified.

The research focussed on:

1. Temperature and humidity conditions at the wall face. Conditions behind a layer of ivy compared to uncovered walls. Anything which moderates the freeze/thaw cycle or the wet/dry cycle may be potentially protective.
2. The mechanism of attachment. How the aerial rootlets stick to walls and if this causes damage. Understanding how these rootlets adhere helps to determine the severity of damage caused.
3. What causes the ivy to send ‘proper’ roots into walls. These roots, unlike aerial roots, are not normally produced by climbing stems and are damaging (displacing masonry, causing cracking, and possible destabilisation).
4. Particulate filtering. Whether ivy leaves prevent dust and pollution particles from reaching the wall surface. If ivy does, then this could potentially reduce deposition of damaging pollutants on to vulnerable wall surfaces.

The results gathered from the experiments in the field and the lab showed the following:

1. Temperature and humidity conditions at the wall face. Hourly data showed a general mediating effect of ivy canopies on both temperature and relative humidity over the five sites used in the project. iButton Hygrochron recording devices were used to monitor temperature and relative humidity.
• The ivy reduced the extremes of temperature and relative humidity – the most clear-cut differences found for temperature. On the five sites used for the project – exposed surfaces were 36% higher and 15% lower than ivy covered surfaces.
• The ivy affected the diurnal range in temperature and humidity throughout the year. The average showed that exposed surfaces had a mean daily temperature range 3.6 times greater AND a humidity range 2.7 times greater than those of ivy covered walls.
• Important factors influencing results: shading by trees or other walls, aspect of the wall and thickness of the ivy canopy.
2. The mechanism of attachment. This is an ongoing part of the project. The report mentions that no damage has been recorded from the rootlets as yet on the test wall – but does not go into detail about the mechanism of attachment. Page 33 of the report notes that careful removal of ivy from limestone revealed ‘fresh and clean surfaces’.
3. What causes the ivy to send ‘proper’ roots into walls. This is another ongoing part of the project. The report mentions that the ivy on their test wall has not produced any roots as yet. Page 35 of the report notes that where ‘proper’ roots were seen by the investigators it was because the ivy was cut off at the base in an attempt to kill it off. Roots were seen only where there were holes in the wall.
4. Particulate filtering. The project examined three sites in Oxford to assess how the ivy interacted with airborne dust and pollutants. A scanning electron microscope was used to investigate the deposition of particulate along the roadways of these three sites. The findings showed that ivy trapped particulate matter and acted as a ‘particle sink’ – especially in areas with high volumes of traffic. The results show higher amounts of particles (per mm2) on the outer ivy canopy than on the inner parts of the canopy. The separate paper detailing the results of particle filtering suggests that ivy and other higher plants could have potential in conservation as a protective layer that mitigates particulate deposition on historic stone surfaces in metropolitan areas.

The research team took a broad view of the major processes of deterioration that affect masonry walls:
Physical: Freeze-Thaw, Wetting and Drying, Heating and Cooling, Salt weathering.
Chemical: Runoff- or rising damp-induced chemical weathering, Pollution-induced chemical weathering.
Biological: Roots, Lichen weathering, Microbial weathering.

Which helped them interpret the results from the study and discuss the roles that ivy plays:
Positive roles of ivy

• Passive roles of ivy: may prevent excessive heating and cooling- moderating freeze/thaw; may regulate humidity and stop rainfall hitting the wall – reducing chemical weathering; may absorb pollutants and salts – reducing weathering

Negative roles of ivy

• Passive roles of ivy: may keep walls damp through reducing evaporation – potentially enhancing chemical weathering

• Active roles of ivy: roots penetrate vulnerable walls and cause physical breakdown; aerial rootlets may chemically deteriorate vulnerable minerals.

Ivy Removal
Importantly, the study also looked at three methods of ivy removal. Firstly, cutting the ivy at the base to let it die before removal. Secondly, poisoning the ivy in an attempt to kill it before removal. Thirdly, removing the ivy carefully with no other treatments. They found that:

• Cutting the ivy off at the base and leaving it to die is detrimental to the wall as the plant may grow sporadically – potentially causing damage to the wall.

• Poisoning the ivy can pull of mortar and additional work may still be required to fully remove the plant from the wall. Poison may also damage other plants close to the ivy plant.

• Gently removing the ivy with no other treatments to be the best method.

I find it exciting that this type of research is receiving funding as it can provide new knowledge for the wider community as ivy is so common and isn't selective towards historic stone buildings to climb up. This research can help us consider the state of the structure ivy is growing up and if ivy will benefit the wall (if it's in good condition), or perhaps where ivy needs monitoring or managing, especially if the wall is in poor condition.

As this post is getting a bit long, I'll write about my thoughts on ivy in a future post! Until then, it'd be great to hear your thoughts about ivy in the comments :)

Resources
Sternberg T., Viles H., Cathersides A. & Edwards M. (2010). Dust particulate absorption by ivy (Hedera helix L) on historic walls in urban environments, Science of The Total Environment, 409 (1) 162-168. DOI: 10.1016/j.scitotenv.2010.09.022
http://www.geog.ox.ac.uk/research/landscape/rubble/ivy/