Abstract
The impact of plants growing on buildings remains controversial, especially for vulnerable historic walls and ruins requiring on-going conservation. English ivy (Hedera helix L.) can cause considerable damage where it is able to grow into deteriorating masonry, yet in some circumstances it may be protective. Here we focus on the potential of ivy to buffer damaging thermal cycles and frost events that can contribute to the deterioration of masonry materials. On limestone masonry test walls in central Southern England (Wytham near Oxford, UK), ivy foliage had a significant influence on stone-surface freezing regimes. Over two successive winters (2012/13 and 2013/14) the frequency of freezing events under ivy was reduced on average by 26%, their duration by 34% and their severity by 32%. A subsequent laboratory simulation showed that stone mass loss, surface softening, and textural development were all significantly reduced under an ‘ivy covered’ thermal regime. Cautious extrapolation indicates that ivy can reduce frost-driven granular-scale decay of limestone by the order of 30 g m−2 yr−1, depending on the local freezing regime. Whilst the capacity of ivy to cause damage should not be underplayed, vertical greenery can aid heritage conservation efforts by mitigating specific environmental threats.
Highlights
The potential for ivy and other higher plants growing on masonry structures to cause damage in some circumstances is unquestionable[1,2,3,4,5,6]
Accepting its potential to cause damage, there is evidence to suggest that in some cases a cover of ivy can be beneficial for historic walls, and that costly removal is unnecessary or entirely inappropriate
The ‘thermal blanket’ effect of ivy may be important for vulnerable stone, by buffering freezing events that otherwise occur on exposed walls[24,26,31]
Summary
The potential for ivy and other higher plants growing on masonry structures to cause damage in some circumstances is unquestionable[1,2,3,4,5,6]. Factors having even a slight influence on wall microclimates (such as vegetation cover) can have significant consequences for frost-driven deterioration Complicating this is the fact that—in addition to frequency of occurrence—the severity, rate and duration of freezing have bearing on the potential for damage[42,45,46,47]. Existing studies provide a strong evidence base for a protective mechanism with respect to frosts, yet these effects are likely complex and a more detailed assessment of ivy’s influence on the nature of frost events is missing It is not known how the efficacy of frost buffering varies for events of differing severity, or whether the effect is consistent between wall aspects. It remains untested whether thermal buffering (including frosts) leads to reductions in rates of physical deterioration of masonry materials
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