Invasive Plants and Archaeology: Bracken, Japanese Knotweed and Others

Bracken

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.

Controlling bracken

In the past bracken was cut for various uses such as animal bedding and as fuel. As bracken is no longer managed for these uses, it has spread in many areas.

Bracken can be controlled but complete control is very rarely achieved in the first year and follow up treatments in the second, third and even subsequent years must be planned.

Both chemical and mechanical methods of control can be used for bracken. Natural England's Technical Information Note 048 'Bracken management and control' (2008) provides further advice. 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 archaeological sites (see references below). Defra/Natural England has followed up this research with a monitoring and evaluation project using remote sensing to assess the success of bracken control in agri-environment schemes and the results will be published in due course. Further research on chemical control of bracken on archaeological sites is needed.

Natural England offers various options to control bracken under its Countryside Stewardship Scheme.

References

Historic England Research Report 107/2015 'Evaluation of Organic Bracken Control on Archaeological Features at Ingram Farm, Ingram, Northumberland Project Report – Year 4'

Historic England Research Report 241/2020 'Monitoring of bracken control methods and their impact on the historic environment'

Historic Environment Scotland's TAN 17 'Bracken and Archaeology'

Historic England's poster on the uses of bracken, past and present, from evidence and the archaeobotanical record

Japanese knotweed and other harmful invasive and non-native species

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, 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 and disposing of 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 invasive non-native species also threaten our natural biodiversity and 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.