Mighty Hogweed is Avenged.....

Turn and Run

There has been a lot of talk about the dangers of the Giant Hogweed plant taking over riverbanks and waste ground. The band Genesis warned of the impending invasion back on their Nursery Crime LP in the 1970’s, but I for one was too pre occupied with cleaning my chopper bike and fraying my flared jeans to listen. Now they are here with vengeance and as Gabriel stated “They are invincible. They seem immune to all our herbicidal battering”. Which isn’t entirely true, so what‘s all the fuss about?

What is it?

Giant hogweed is usually biennial, forming a rosette of jagged, lobed leaves in the first year before sending up a flower spike up to 4m high in the second year and then setting seed. True biennials only live for two years, dying after flowering, but giant hogweed does not always behave as a true biennial and in fact some are perennial, coming up year after year. The plant tends to grow in really inaccessible places.

Background

The giant hogweeds were introduced into Britain and Europe from the Caucasus Mountains in the nineteenth century. The earliest documented reference to their introduction was 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, so were widely planted in ornamental gardens.  Unfortunately they quickly escaped from cultivation and came to Ireland in the 1880’s, and are now widely naturalised as invasive species throughout much of Britain and Europe.

Impressive

Although an impressive sight when fully grown, giant hogweed is invasive and potentially harmful. Chemicals in the sap can cause photodermatitis or photosensitivity, where the skin becomes very sensitive to sunlight and may suffer blistering, pigmentation and long-lasting scars.

The large leaves of the plant create sufficient shade to suppress indigenous herbaceous understorey plants along banksides.

OPW

Between 1998 and 2001 the Office of Public Works commissioned the Central Fisheries Board (now IFI) to conduct a four-year control/eradication programme in the Mulkear River County Limerick catchment area where Giant Hogweed was widespread and restricting angler access.  Colonies often presented continuous linear stands that extended up to 2km along the river bank.  The herbicide Glyphosate was applied to plants in a catchment-wide approach in March, May, July and September of each year. The application was similar to the Japanese Knotweed treatment where the stems were injected.

The control programme almost completely eradicated Giant Hogweed from the treated sites by the end of the four year study.  Native plants such as Butterbur (Petasiteshybridus) were able to re-establish in the area.

The facts: what does it look like?

• Giant hogweed (Heracleum mantegazzianum) is a close relative to cow parsley.
• It has thick, bristly, reddish-purple stems and can reach over 3m (10ft) in height.
• The flowers are white in a flat-topped clusters that can be as large as 60cm (2ft) across.
• It commonly grows on riverbanks and wasteland.
• Its leaves, stems, roots, flowers and seeds contain toxic components which can be transferred by contact and make exposed skin extremely sensitive to sunlight.

What to do if you come into contact

After coming into contact with the plant, the burns can last for several months and the skin can remain sensitive to light for many years.

If the sap of the giant hogweed comes into contact with your skin, it can cause severe, painful burns and make your skin sensitive to strong sunlight.

If you touch a giant hogweed, cover the affected area, and wash it with soap and water.

The blisters heal very slowly and can develop into phytophotodermatitis, a type of skin rash which flares up in sunlight. If you feel unwell after contact with giant hogweed, speak to your doctor.

Herbicidal Battering

Although there is no statutory obligation for landowners to eliminate giant hogweed, local authorities will often take action to remove infestations in public areas and it’s best to leave it to the professionals if there are a lot of plants.

First, consider whether this can be done using non-chemical means such as digging out or suppressing with mulch. Where these methods are not feasible, chemical controls may need to be used. Seek advice from the Environment Agency before undertaking spraying near rivers, streams and ponds.

When controlling giant hogweed always wear gloves, cover your arms and legs, and ideally wear a face mask when working on or near it. Cut plant debris, contaminated clothing and tools are potentially hazardous too. Wash any skin that comes in contact with the plant immediately. Ensure that contractors working on your land are aware of the risks and competent to deal with this weed. 

Strike by Night - Naturally

Consider if non-chemical controls are an option;

On a garden scale, appropriate measures include pulling up young plants by hand when the soil is moist. Do this in May when the giant hogweed has reached a reasonable height, but before it has produced its flowering spike. For larger plants it might be necessary to loosen the roots with a fork first.

Never let hogweed set seed, but allow the flower spike to form and then remove it before the flowers fade. At this stage, the plant is less likely to survive trimming than earlier in the year. Remember that perennial forms have been identified and preventing them from setting seed will not reduce giant hogweed populations quickly.

Giant hogweed prefers moist fertile areas often near waterways. It is essential that weedkiller never under any circumstances enters waterways. 

The seeds are viable for up to seven years so the natural method of eliminating the plant on a small scale would be to chop the plant down as they flower

Disposing of giant hogweed

Giant hogweed is a controlled waste (similar to Japanese knotweed) so, if it is taken off site, can only be disposed of in licensed landfill sites with the required documentation. To avoid this, dispose of any plant material (dug up or cut down) by composting or burning.

The smaller, native hogweed, Heracleum sphondylium, is not classed as controlled waste but should still be disposed of with care to avoid human contact.

“Mighty Hogweed is avenged. Human bodies soon will know our anger.” Why didn’t everyone listen to Genesis in the 1970’s? All of this could have been avoided.