Giant hogweed

Meet the plant

Giant hogweed (Heracleum mantegazzianum) is an umbellifer (member of the cow-parsley family) with flowering stems typically 2-3m in height bearing white flower heads up to 80cm across. The lower leaves are often 1m more in size and distinctively spiky.

It grows by lowland rivers, in rough pastures and on wasteland and is widespread throughout Great Britain.

It was introduced into gardens as a curiosity around 1820 and was often planted by rivers and ponds.

Giant hogweed reproduces entirely by seeds and is monocarpic i.e. it reproduces only once in its lifetime, normally after 2-3 years. Plants are able to self-fertilise and each can produce 20-50,000 seeds – some of which survive more than 10 years in the soil.

Download the giant hogweed ID guide

Warning!

The sap of the plant is phototoxic causing serious skin burns under sunlight – so if you come across it DO NOT TOUCH! Keep dogs and pets away too as can also get burns.

Giant hogweed sap contains photosensitizing furanocoumarins – which, when it sap comes into contact with human skin and exposed to UV radiation (sunlight) can cause severe burns. This danger to human health complicates eradication efforts.

Impacts

Individual giant hogweed plants grow to a great size and collectively form dense, impenetrable stands – particularly along riverbanks but also alongside roads and railway lines. It can quickly become the dominant species, out-competing native flowers and vegetation and reducing species diversity.

The dangers posed to humans (and pets) by the phototoxic sap impact recreational activities. Walking, angling or doing water sports near giant hogweed is extremely hazardous due to the risk of skin contact and as such it can block paths and prevent access for recreation.

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Giant hogweed management

We take a catchment-wide approach to giant hogweed control, first identifying upstream sources on the main stems of rivers and tributaries and starting work here. As areas are cleared our control systematically moves downstream – with work on tributaries and lower catchment areas following management upstream.

This strategic approach reduces the risk of re-infection of controlled areas and allows us to work towards eradication across the entire catchment. By 2024 we had some 1043km of river length being managed for giant hogweed in our project area.

Control work is carried out by project and fishery trust staff and local volunteers – including individuals, community groups, angling associations, land managers, ghillies and conservation groups. We can provide training in pesticide application for local volunteers to enable them to assist in this work and create a network of people with the skills to undertake giant hogweed control as part of our overall programme.

Giant hogweed control

Herbicide application

The most effective control method for giant hogweed is to treat the emerging leaves with herbicide (Glyphosate) in spring to early summer. We generally treat between March and July and often visit each treatment site twice, to apply a follow up treatment to any late emerging or missed plants. Preventing seeding is a priority as allowing this would replenish the seedbank in the soil and undermine control work.

We apply herbicide using backpack sprayers using a spot spraying technique. This work is undertaken by qualified staff, land managers and volunteers.
Cutting flowering heads

As the year progresses giant hogweed starts to flower. We aim to treat our control sites before this stage but this is not always possible – and flowering can begin as early as May and continue into June and July. To prevent seeding we remove flower buds and flowering heads.

Flower heads are cut using a long-handled pole saw and herbicide sprayed into the cut stem and remaining basal leaves. This work is hazardous as cutting the plant stems releases the phototoxic sap. Protective clothing must be worn and appropriate equipment used to ensure those carrying out this work do not come into contact with the sap.
Grazing

From 2019 to 2022 the Scottish Invasive Species Initiative ran a sheep grazing trial at our trial site near Macduff, in the Deveron catchment. This trial assessed the use of sheep grazing as a method to control giant hogweed.

Annual monitoring was carried out to assess the impacts of sheep grazing on giant hogweed and native vegetation. The trial was a success and in 2024 we released our Sheep Grazing Management Guide with lessons learnt and advice for land managers and conservationists interested in using this method.

Visit our Sheep Grazing Trial page for further information and annual reports.
Digging

Giant hogweed can be dug and the tap roots cut, but this is only suitable during the first stage of the growing season on young plants. Roots must be cut at least 15cm below ground level.

This can be extremely hazardous as it puts people in close contact with the plant and increases the risk of contact with the phototoxic sap.

This method is not used within our control programme.

Seedling emergence trial

Although we know that, in general, the seedlings of all plants emerge in the spring, there has been little information or data setting out the timing of giant hogweed seedling emergence in Scotland. This information would help to ensure that control work is delivered at the optimal time of year to make management most effective and impactful.

The Scottish Invasive Species Initiative, volunteers and partners, ran a seedling emergence trial from March to October 2021. The full study can be found here. Results from this study help to inform seasonal management plans for giant hogweed control, including our sheep grazing trial near Macduff in the River Deveron catchment.

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