Sheep Grazing as a Management Tool to Control Giant Hogweed

Giant hogweed (Heracleum mantegazzianum) is a non-native invasive plant that has become widely distributed in the UK. It is especially abundant along watercourses but is also commonly found in rough pastures and wasteland.

WARNING – Giant hogweed sap is phytotoxic. It causes human skin to become sensitive to UV light, leading to severe burns and blisters on exposure to sunlight. Affected skin can be sensitive for several years.

Overview of Giant Hogweed

Description: Giant hogweed is an umbellifer (member of the cow-parsley family) with long flowering stems, typically 2-3m in height. The umbrella-shaped flowers are large (up to 80cm across) with white or pinkish flowers. It has jagged leaves with serrated edges and the stems have sharp bristles and purple blotches.

Life cycle: Giant hogweed typically matures after 2-3 years, when it then flowers and sets seed – the plant normally dies after flowering. It has long branching taproots which store energy and enable re-growth following grazing or cutting until the energy is depleted. Flowering may be delayed or prevented by repeated grazing.

Seedlings with varied leaf morphology		Sharp bristles and purple blotches on stem of immature plant		Jagged, lobed leaves up to 3m long		Flowering plants can be up to 5m tall with white / pinkish flowers

Impacts: Giant hogweed grows in dense, impenetrable stands, outcompeting native plants and reducing species diversity. It can cause riverbank erosion. Human health risks complicate eradication efforts – care must be taken when carrying out control work. Typical control methods include herbicide application and cutting of flowering heads.

Giant hogweed is listed under Schedule 9 (established non-native species) of the Wildlife and Countryside Act as amended by the Wildlife and Natural Environment (Scotland) Act 2011). It is an offence to plant or otherwise cause to grow it in the wild outwith its native range.

Sheep Grazing Trials

Between 2013 and 2022, sheep grazing as a method to control giant hogweed has been trialled at two sites in North-East Scotland.  The Scottish Invasive Species Initiative has run a trial at Macduff from 2019 to 2022. This work has been informed by earlier trials at Auldtown which began in 2013.

Macduff Grazing Trials (2019 - 2022):

• The trial site is a 1km (7 hectare) stretch of woodland along the River Deveron.
• Scottish Greyface and Blackface sheep have mostly been used. Wethers (non-breeding males) were used as this was the land managers’ preference for ease of management of the flock.
• In 2019, 25 sheep were put on the site from April to November. This led to overgrazing and numbers were reduced in later years. In 2022, only 11 sheep were put on the site from May to September. Full details on grazing pressure adjustments are detailed in the table below.

Key findings:

• Sheep have been highly effective controlling giant hogweed, tackling the majority of seedlings, immature plants and large mature plants.
• Initial overgrazing increased the presence of unpalatable species (nettles, bracken and thistles), poaching of the ground, loss of herb species and grazing of shrubs and tree seedlings.
• Low grazing intensity by sheep should be used initially. Grazing pressure can be adjusted - informed by observations of giant hogweed abundance and changes in vegetation and bare ground. 
• The sheep may require a familiarization period before grazing giant hogweed. However, they soon develop a preference for it which is retained across grazing years.

Results from these and previous trials can be found here and inform this management guidance

June 2019 (Year 1): Dense stands of giant hogweed visible at monitoring point		October 2019 (Year 1): Giant hogweed growth much reduced but site overgrazed		June 2020 (Year 2): Seedlings have grown but no large plants are present

Management Considerations

Poaching of the ground in a giant hogweed monitoring plot		A carpet of hogweed seedlings and no mature plants – a sign of progress		Extensive area covered by hogweed seedlings at the grazing trial site

Monitoring Progress

It is important to monitor for overgrazing and progress of giant hogweed control. Visit the site regularly to look for signs of overgrazing and adjust sheep numbers if needs be. To assess progress with giant hogweed control, consider the site on an annual and long term basis - Is giant hogweed abundance decreasing overall? Are there fewer mature (flowering) plants present? These are promising signs.

Giant Hogweed Management

In well-established infestation sites giant hogweed control is a long-term strategy as the seedbank can survive for up to ten years. The sheep may not remove all plants (particularly those in difficult terrain or boggy ground) and it is essential to annually cut and remove flowering heads. This must be done before the plants set seed (in August) to prevent replenishment of the seed bank.  We suggest flower head removal is best undertaken in July – when heads are visible and before seeds have set. Flowering heads should be cut safely using protective equipment and long-handled saws, taking care to avoid contact with the sap. Flowering heads can be removed or composted on site. 

Flower heads must be cut before seeding in July/August		Stems can be cut carefully with a long-handled saw		A single plant produces 20 - 50,000 seeds which can be viable for up to 10 years

If the flowering head is starting to develop seeds, extra care must be taken to ensure seeds aren’t spread. A cotton bag or sack should be tied over the flowering head prior to cutting the stem. These bags containing the flower/seed head can then be burnt. However, it is best to avoid this and ensure flowering stems are cut before seeds develop.

*Contractor costs ~£200 per day. We estimate that control by contractors at the Macduff trial site would initially have cost ~£2000 – £2500 per year. After initial control costs would have reduced to ~£1200 – £1600 annually.

Key Tips for Success

• Low grazing intensity over several years is likely to be most effective to control giant hogweed.
• Start with a low grazing intensity and assess the impact of grazing annually.
• Avoid winter and early year grazing to reduce the risk of overgrazing.
• Some giant hogweed plants may escape grazing and survive to flower - it is essential that flowering plants are cut before they seed each year. Do this in July.
• Control of giant hogweed by sheep grazing is a long-term strategy. Seeds can last for many years - persistence is key!

Sheep will graze on both seedlings and mature giant hogweed plants. When grazing larger plants, they can knock over tall stems to graze the leaves. They will graze at night and during the day.

Acknowledgements

This study has been undertaken as part of the Scottish Invasive Species Initiative with the Deveron, Bogie and Isla Rivers Charitable Trust.  Monitoring was undertaken by Dr Annie Robinson (University of Aberdeen).  The study was formulated with essential input from Professor Rene van der Wal (then Professor of Ecology at the University of Aberdeen) and Dr Annie Robinson.

We are particularly grateful to Dan Gordon of Kirkside Farm and Roger Polson of Knock Farm, landowners of the Macduff and Auldtown trial sites, for their support, patience, hard work and commitment to this work. These trials could not have taken place and this guidance could not have been prepared without their cooperation and agreement.   

The Scottish Invasive Species Initiative is a 5-year partnership project led by NatureScot and funded by the National Lottery Heritage Fund and NatureScot.
 

Further Information

Further information on the Macduff Sheep grazing trial is available here
See also our Seedling Emergence Study investigating the timing of giant hogweed seedling emergence in Scotland

Contact: [email protected]
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