Meet the plant
Giant hogweed (Heracleum mantegazzianum) is an umbellifer (member of the cow-parsley family), and its flowering stems are typically 2-3 m in height bearing 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 deliberately planted by rivers and ponds.
Giant hogweed reproduces entirely by seeds, it is monocarpic –i.e. it reproduces only once in its lifetime. Plants are able to self-fertilise and each plant can produce 20-50,000 seeds, some of which can survive more than 3 years in the soil.
The sap of the plant is phototoxic causing serious skin burns under sunlight so if you come across it DO NOT TOUCH!
Keep dogs away too as they've known to get burnt as well.
Download the Giant hogweed ID guide
Read more about Giant hogweed
Seedling Emergence Trial
There is little information and data detailing the timing of giant hogweed seedling emergence in Scotland. This information would be helpful in ensuring that control work is delivered at the optimal time of year to make management most effective and impactful.
To fill this gap in knowledge the Scottish Invasive Species Initiative, with the help of volunteers and partners, ran a seedling emergence trial from March to October 2021. The full study can be found here. Results from this study will help to inform seasonal management plans for giant hogweed control, including our sheep grazing trial near Macduff in the River Deveron catchment.
Giant hogweed grows to a huge size and forms dense, impenetrable stands, particularly along riverbanks but also alongside roads and railway lines. This dominance out-competes native flowers and reduces species diversity.
The plant produces a phototoxic sap, which in the presence of sunlight will cause serious skin burns. This makes walking anywhere that giant hogweed is growing extremely hazardous and as such it blocks paths and prevents access for recreation.
Giant hogweed management
We take a catchment approach to our giant hogweed control and work on the source patches of hogweed first - i.e. those the furthest upstream. As areas are cleared our work moves gradually down river, including clearing the tributaries. Taking this strategic approach means that there is no risk of re-infection of the areas that have been controlled.
At the start of our project we identified the priority areas on which to focus our work and we currently have 753km of river length being managed for giant hogweed.
This control work is carried out by our SISI project and fishery trust staff and local volunteers - including individuals, community groups, angling associations, land managers, ghillies and conservation groups. We have been able to provide training in pesticide application for local people to enable them to assist with this work and create a network of local volunteers who have the knowledge and skills to carry on with giant hogweed control after our project finishes.
Giant hogweed control
The most effective treatment method for giant hogweed is to treat the emerging leaves with herbicide (Glyphosate) in the 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.
We apply herbicide using backpack sprayers using a spot spraying technique.
Cutting flowering heads
As the year progresses we see giant hogweed starting to flower so to ensure we prevent any seeding occurring we remove flower buds and flowering heads as well as treating with herbicide.
Flower heads are cut off using a long-handled pole saw and herbicide is sprayed to the cut stem and basal leaves.
If the flowering head is starting to develop seeds then extra care must be taken to ensure seeds aren't spread. A large cotton bag or sack should be placed over the flower head and tied prior to the stem being cut. These bags containing the flower/seed head can then be burnt.
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.
We have our woolly warriors hard at work over on the River Deveron where we are running a trial to assess the use of sheep grazing as a control method for giant hogweed.
Since 2019 our fearless flock has been munching their way through the giant hogweed on our trial site. We are monitoring their grazing and it's impacts on the hogweed and surrounding vegetation. At the end of the trial we will produce a guidance document of recommendations to enable other conservationists and land managers to use this method.
Visit the Sheep Trial Case Study page for further information about the grazing trial, results to date
and annual trial reports.