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How to remove Giant hogweed
By Jennifer Holmes

How to remove Giant hogweed

Giant Hogweed (heracleum mantegazzianum)

In this article we look at the many reasons Giant hogweed must be removed. Though it is impressive in stature, and some may consider it a beautiful plant species, Giant hogweed is a danger to man, wildlife and biodiversity. First we will explain how it can be identified, why, as a non-native species it thrives in the UK, and lastly – some effective methods of control or removal.


How did it get here?

The Giant hogweeds were introduced into Britain and Europe from the Caucasus Mountains in the nineteenth century. The earliest documented reference to the introduction of Giant hogweed into Britain is from the Royal Botanic Gardens Kew Seed List of 1817 where Giant hogweed, under the name of Heracleum giganteum was listed among seeds supplied to Kew by the Russian Gorenki Botanic Gardens. They were soon introduced into the horticultural trade and, being aesthetically impressive plants, were widely planted in ornamental gardens throughout Britain. Unfortunately, they quickly escaped from cultivation with the first naturalised (‘wild’) population recorded in Cambridgeshire in 1828 and are now widely naturalised as invasive species throughout much of Britain and Europe.


Legislation

Giant Hogweed is listed under Schedule 9 to the Wildlife and Countryside Act 1981 with respect to England and Wales. As such, it is an offence to plant or otherwise allow this species to grow in the wild. Seeds are dispersed over short distances by wind but considerably longer distances by rivers and streams. They can also be transported in soil adhering to shoes, machinery and clothing, which is why biosecurity measures are paramount for anyone coming into contact with this plant, either socially or when working to control it.



Species characteristics

Giant hogweed is a biennial or perennial herbaceous plant that can grow up to 5 metres in height. Flowering does not occur until the second or third year, or longer if conditions are unfavourable, but when it does, the white flowers look like very large cow parsley, with a pale, swollen rootstock. It produces 20-50,000 viable seeds a year, which are penny sized and paper thin and germinate quickly. Giant hogweed can be identified by a number of key characteristics:

  • Hollow stems
  • Hairs on the stems
  • Stems grow to 5-10 cm diameter
  • Stems have purple speckling on
  • Large spiky leaves up to 1 metre across (as seen in the picture below)
  • Flower umbrells can be 80 cm across and grow from a central stem
  • Flower heads first appear as a closed pod which unfurls to reveal clusters of small cream-white flowers
  • Seeds are small, paper like texture with dark stripes

Habitat

Giant Hogweed is a pioneer species that spreads easily by human disturbance of habitats. The climate in Britain is ideal, as it prefers temperate, moist conditions, with a cold winter to initiate germination. The seeds have been known to germinate in Britain up to 600 metres in altitude. This species prefers high nutrient (particularly nitrogen) and moisture levels but can tolerate a range of pH values and soil conditions. It will grow in semi-shade but does not tolerate heavy shading.

Giant hogweed is often associated with riparian habitats, where waterways can provide corridors along which it can spread. As well as river margins, it can also be found on roads and railways, derelict land and rubbish tips.


A risk to health

Giant hogweed is highly invasive, but more concerning is its ability to inflict serious injury and even blindness on people or animals that come into contact with the plant.

  • All parts of the Giant hogweed plant, including the hairs on the stems and leaves (which can penetrate light fabric) contain a toxic sap, which can cause severe skin blistering.

Humans, pets and wildlife can all be affected by contact with Giant hogweed, which as it prefers to grow along river and canal banks is an obvious danger to walkers with children or dogs that may brush past these impressive looking tall plants.

Contact with the cut material in sunlight produces a reaction in almost everyone. The sap of Giant hogweed contains a toxic chemical which sensitises the skin and leads to severe blistering when exposed to sunlight. The sap reacts with the skin and makes the skin sensitive to ultra-violet light, though no pain or irritation is felt at the time of contact. Any subsequent exposure to sunlight can cause the skin to burn and will result in large, watery blisters that may not become evident until 15 to 48 hours following contact, by which time the damage has been done. Blistering and dense post-inflammatory hyper-pigmentation is visible after 3-5 days and may persist for at least 6 years. Contact with eyes can lead to temporary, or in some cases permanent, blindness.

The burns can last for several months and even once they have died down the skin can remain sensitive to light for many years after. The degree of symptoms will vary between individuals, but children are known to be particularly sensitive. It’s important to note that any cut material remains active for several hours after cutting.  The highest concentration of toxins is in the leaves, the lowest in the stems, petioles, and the roots.


a risk to the environment

Growing in dense stands Giant hogweed can suppress all plants other than trees or shrubs. It absorbs up to 80% of sunlight, suppressing and out-competing native plants and inhibiting species development, and ultimately reducing plant and invertebrate diversity. Dense stands can also restrict access to amenity areas and riverbanks and reduce sight lines on roads. Furthermore, dense colonies die back in the winter leaving river and canal banks bare of vegetation which increases the risk of erosion and flooding, as well as re-colonisation from seeds washed downstream. The effects of losing the native riparian vegetation and the resulting bank erosion are serious, including physical impacts on the geomorphology of the river, increased sedimentation and changes in temperature all of which leads to decreased invertebrate and fish populations.

In addition to this, it is thought that furanocoumarins (toxic chemicals produced by the plant as a defence against insect herbivory) may be a cause of loss of insect biodiversity in dense stands of this species. Insects play a vital role in waterway ecosystems, which is why Giant hogweed must be controlled. These toxic chemicals (as well as being a public health hazard) are also known to be antifungal, which may lead to suppression of soil fungi, which is essential for soil fertility.

So, to sum up, Giant hogweed is harmful to humans, animals, insects, soil quality and waterway ecosystems. It is not to be ignored!


How to get rid of Giant hogweed

Before we dive into specific methods of control, it’s important to note that as contact with sap can occur by brushing against any broken plant parts, handling plant material, or even by touching tools or equipment that was used for Giant hogweed control, full PPE is required to be worn by all operatives. Combinations of control methods can be more efficient than a single method, but here’s a summary of the options:

Herbicide

Spraying the plant with herbicide (Glyphosate) is an effective method of control, it is recommended that after treatment sites are monitored annually and re-treated as necessary to take account of newly emerged seedlings. It’s important to ensure full coverage of the leaves both top side and underneath to improve the efficacy and ensure there is sufficient die back after treatment. Herbicide can also be applied via stem injection. This is a suitable method if single plants are in evidence or are surrounded by areas of grassland or other shrubs that are required to remain unharmed and where a foliar application (spraying) may prove difficult.

Excavation

Eradication of a Giant hogweed infestation can be achieved via careful excavation of the above ground and below ground elements of the plant. To be effective all parts of the plant must be removed including surrounding soils to ensure no fragmentation is left behind that would result in regrowth.

Manual

Manual methods include cutting down the stems and/or digging the plant up. It is possible to manually dig up small areas of Giant hogweed. This method is best used on young plants, when the leaves are not too large, and the soil is moist (for easier digging). Checking for regrowth must be done during the summer months. Cutting the stems down before flowering is an on-going method of control, as the roots remain underground they will produce regrowth that will need further cutting.

Flower head removal

Trials carried out in the Czech Republic found removing all Giant Hogweed flower heads at peak flowering time reduced seed production. Cutting the heads off the plant removes its seed-producing parts. This should be done early in the year when the plants are small and have not already set seed, but not too early that cutting the plant stimulates a more vigorous rate of growth. Whilst this method does not guarantee regrowth it is often used as a ‘stop-gap’ or interim method of control to limit the seed heads producing seeds before a more permanent method of removal can be carried out.


And finally

Because this invasive species is so dangerous to be handled, if you need our help to remove Giant hogweed please get in touch. We can arrange a survey, talk to you about the options and provide a quote. All of our technicians are fully qualified and experienced in dealing with Giant hogweed – this is one plant that’s best left to the professionals!

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