Looking After Monuments and Sites
Vegetation on walls
Most old walls have some vegetation growing on them unless it has been recently removed. Much of this vegetation will be harmless and may be left in place unless it obscures important historical features.
Vegetation that produces woody growth, especially trees and shrubs, will invariably be harmful and should be removed at an early stage.
Ivy on walls
Ivy (Hedera helix) is commonly found growing over walls. A covering of ivy may not necessarily be a bad thing as it can protect surfaces from weathering processes. However, ivy can sometimes root into a structure and this is likely to cause damage.
Historic England has researched the benefits and problems caused by ivy and this can be found in our Ivy on Walls research report.
Physical damage by ivy growth
The potential for ivy to cause damage to historic walls is primarily controlled by the condition and physical characteristics of the underlying structure.
Ivy cannot actively ‘bore’ its way into walls but it can cause serious problems where it grows into existing defects such as holes, cracks and crevices. Where defects do exist ivy stems can grow into them, and may push stones apart or dislodge them.
The potential for damage can be minimised through appropriate management, including regular pruning to prevent growth onto roofs and over guttering, and removing excessive arboreal growth.
The greatest damage often occurs when ivy ‘roots-in’ to walls. This is not common, but can be stimulated when shoots come into contact with darkness, moisture and weathered material (protosoil) within already deteriorating walls.
The common practice of killing ivy by cutting it at its base can also stimulate rooting-in and this is no longer recommended.
Surface attachment: juvenile ivy stems attach and climb up walls via aerial rootlets. Attachment is remarkably strong but is entirely superficial – aerial rootlets do not penetrate into the materials they are attached to and they do not extract moisture or nutrients (the same is true for ivy growing on tree trunks). Forceful removal of stems can cause physical damage to the substrate underneath, and it may leave marks that may be an aesthetic issue for some assets.
Microclimate buffering and weathering
Ivy is very effective at reducing extremes of temperature and relative humidity, and the frequency and range of variations that can otherwise contribute to deterioration over time. Ivy also reduces the frequency, severity and duration of frost events that cause damage to vulnerable masonry materials.
Surface soiling and pollution filtering
Ivy foliage is an effective trap of fine airborne particulates. It reduces the amount of pollution reaching the surface of walls that contributes to soiling and chemical degradation. Ivy cover can limit greening or blackening of stone (by algae growing within the stone matrix) through its shading effects.
The influence of ivy on the moisture content of walls is complicated and there is no simple answer as to whether it increases or decreases damp. Its relative importance varies between different construction materials and locations, and between different wall heights and aspects.
A thick cover of ivy certainly shields walls from rain, but it may reduce evaporation of ground-level moisture where there is an existing damp problem.
Long-term monitoring of test walls shows that a covering of ivy typically stabilizes surface humidity to levels well within the typical range of moisture variations found on exposed wall surfaces.
There is no evidence that ivy influences deeper-seated moisture in walls.
Soft capping to protect ruined masonry
The use of soft capping (grass and soft herbaceous plants) on the top of ruined walls rather than hard capping of stone, mortars and cement can have many long term conservation benefits. Historic England’s report looks at the research and trials carried out.
The report provides advice about soft capping and issues to consider. The report concludes:
- Soft capping is a much less disruptive intervention than hard capping
- Soft capping requires much less invasive maintenance and monitoring
- The use of soft capping might secure more historic fabric as it is relatively easy to install, and should also contribute to enhanced conservation and biodiversity at heritage sites
- Soft capping has proved to be a highly cost effective solution to many conservation problems
- The evidence clearly shows it is the natural way to protect walls
- Soft capping can lead to a major change in the appearance of sites and this needs to be considered before installation
As part of a Hadrian’s Wall World Heritage Site project, a manual was developed on conserving archaeological earthworks.
The ‘Managing Earthworks Manual’ provides advice on low cost interventions to prevent damage and erosion. There are four parts:
- Recreational issues
- Livestock issues
- Vehicular erosion
- Burrowing animals
- Scrub, tree and bracken management; and land drainage
- Appendices on:
- Repairing Erosion Scars
- A Guide to Stocking Levels for Lowland Grassland
- List of Suppliers
Grants are available through Countryside Stewardship Scheme.
Trees and archaeology
Whether or not tree cover would be beneficial or detrimental to archaeological evidence will depend upon both the nature of the archaeology, and its surrounding environment. In some cases the trees will be of historic interest in their own right. However, tree planting, tree roots, forestry operations and events such as wind throw can damage archaeology. The issues are reviewed in the Forest Research paper Trees and Forestry on Archaeological sites in the UK: A review document (2004).
Forest Research provides advice on
The Forestry Commission Scotland provides some useful briefing for woodland managers on managing forestry works and likely archaeological surveys needed.
Bracken and archaeological monuments
Not only does rampant bracken growth obscure archaeological sites, it can also damage below-ground deposits because of the fern’s robust and invasive rhizomes.
Bracken, Pteridium aquilinum, is a native fern. It grows fresh fronds each year. These can grow to a height of two to three metres in a single season. After the first frosts the stems quickly die but remain standing, slowly collapsing over the winter. The fallen stems break down slowly, helping to inhibit the growth of other plants and allowing the bracken to dominate.
It mainly spreads by underground rhizomes which can run for long distances and penetrate deeply, especially in sandy soils. Each rhizome contains numerous buds, some of which remain dormant unless damage occurs to growing shoots, thus allowing rapid replacement. Bracken can also spread by spores which are released in autumn.
Bracken is able to rapidly establish itself and out-compete most other native plants. The monoculture it creates is less ecologically interesting than the habitats it invades.
In past times bracken was cut for various uses such as animal bedding and as fuel and the loss of these practices has contributed to the increasing problems with bracken spreading in many areas.
Bracken can be controlled but complete control is very rarely achieved in the first year and allowance must be made for follow up treatments in the second, third and even subsequent years as necessary.
Both chemical and mechanical methods of control can be used for Bracken. Natural England provides further advice on controlling bracken. Up to date information on the regulations around chemical control is provided by the Bracken Control Group.
Historic England has funded and supported research into the best methods for controlling bracken on historic and archaeological monuments. The reports are available within our Research Report Series and the results were presented at a conference organised by Historic England in 2019. Updated advice on the management of bracken on historic and archaeological monuments is being developed.
Natural England offers various options to control bracken under its Countryside Stewardship Scheme.
Publications of interest
Historic England’s Research Report 107/2015 ‘Evaluation of Organic Bracken Control on Archaeological Features at Ingram Farm, Ingram, Northumberland Project Report – Year 4’
Japanese Knotweed, Fallopia japonica, is a tall, non-native herbaceous plant. Introduced from Japan as an ornamental for large gardens it is now considered an invasive weed and is common throughout most of the British Isles. It spreads by the vegetative means of underground rhizomes. The rhizomes can spread for long distances and often penetrate quite deep below the surface and can grow over two metres in a single season. It can cause physical damage to hard surfaces and some structures, and harm to the natural environment.
Under the Wildlife and Countryside Act (1981) it is an offence to plant this species in the wild and care must be taken to ensure that it is not introduced accidentally. This also applies to one other introduced land plant - Giant Hogweed, Heracleum mantegazzianum.
The Environment Agency provides advice on how to identify, prevent spread and dispose of Japanese knotweed including spraying with chemicals, burying, burning or disposing invasive plant waste.
Japanese knotweed can be controlled but complete control is very rarely achieved in the first year and further treatments need to be planned and carried out. The Welsh Government ‘s ‘The Control of Japanese Knotweed (Fallopia japonica) in Construction and Landscape Contracts. Model Specification and Guide to Procurement’ provides further advice on controlling this invasive plant. The RHS provides advice for home owners.
The House of Commons Science and Technology Committee looked at ’Japanese knotweed and the built environment’ in 2019. The report highlights the problems of invasive plants like knotweed and the difficulty of eradicating them. However MPs found there was scant research on the effects of knotweed in the built environment and queried the standard assessment of risks used by mortgage and insurance businesses.
Other harmful invasive and non-native species
Other invasive and non-native species also damage our natural biodiversity and harm the environment. Advice on how to identify, control and dispose of invasive non-native plants that can harm the environment is available on the UK government website.
Public rights of way across archaeological landscapes
The historic landscape is criss-crossed by public rights of way which are defined by the types of use: footpaths, bridleways (for walkers and riders),and byways open to all traffic. Many of these routes are historic features in their own right. By law they are public highways and governed by highways legislation, and managed by the local Highways Authority, usually the County Council. Changes cannot be made without a legal order.
Occasionally increased use of rights of way may result in damage of areas of archaeological interest. Recreational use or ‘greenlaning’ by off-road cyclists, trail bikes, and drivers with 4x4s and cross-terrain vehicles, even horse-riding and hoards of walkers can generate problems. The Managing Earthworks Manual has a section on recreational issues and erosion caused by vehicles.
The national user groups, the Green Lane Association and the Trail Riders Fellowship promote responsible use of rights of ways and publish codes of conduct. If there is a risk of archaeological damage, discuss options like protective surfacing or traffic regulation orders with the local access authority (usually the county council) and archaeological advisers. The Ramblers Association provides useful advice.