Giant hogweed

Giant hogweed (Heracleum mantegazzianum) is a close relative of cow parsley primarily from Southern Russia. It can reach over 3m in height. Even though this impressive plant can be appealing in some circumstances, many gardeners will want to destroy it, as it is potentially intrusive and the sap can cause very bad skin burns.

What is giant hogweed?

Although a striking sight when it is fully grown, Giant Hogweed is intrusive and potentially harmful. There are chemicals in the sap that can cause photodermatitis or photosensitivity, this is where the skin becomes extremely sensitive to sunlight and may cause blistering, pigmentation problems and long-lasting scars.

The Giant Hogweeds are usually referred to by one name, Heracleum mantegazzianum. Research shows that, while this is one of the species, there are as many as four other giant hogweeds in Britain some of which are biennial and others perennial. When tested all of these had very high levels of furanocoumarins (these are the chemicals that cause burning by making the skin sensitive to sunlight) and so all of these pose a risk to health.

The Giant Hogweeds were initially introduced into the UK and Europe from the Caucasus Mountains in the nineteenth century. They were soon initiated into the horticultural trade and were widely planted in gardens across Britain.

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Regrettably, they swiftly escaped from cultivation with the first naturalised population recorded in Cambridgeshire in 1828, and are now widely naturalised throughout much of Britain and Europe. They are monitored and it seems that they are reported to be continuing to spread and can be found in literally every part of the UK.

Appearance

Giant hogweed (Heracleum mantegazzianum), is a tall, cow parsley looking plant with quite thick stems that are often purple-blotched.

The flowers are white and held in flat-topped clusters, like those of carrots or cow parsley, with all the flowers in the facing upwards. The flower heads can be as large as 60cm across and they can reach a height of 3.5m.

Giant hogweed is usually biennial, forming a rosette of lobed leaves in the first year before growing a flower spike in the second year and then setting seed.

Control

Even though there is no legal obligation for landowners to get rid of giant hogweed, local authorities will sometimes take some action to remove infestations in public areas. Plants that are unwelcome, out-compete desirable plants and are classed as weeds that require some kind of control. Weeds from abroad with strongly intrusive dispositions are referred to as ‘invasive aliens’ and they pose a very real threat to wild such as railway embankments.

Because of the intensity of the hazard, legislation has been applied to weeds such as this. The Wildlife and Countryside Act 1981 lists it on Schedule 9, Section 14 which means it is an offence to cause Giant Hogweed to grow in the wild in England and Wales (similar legislation applies in Scotland and Northern Ireland). Also, it can be the subject of Anti-Social Behaviour Orders.

Local Authorities have powers under specific situations to require giant hogweed to be removed.

First, consider whether this can be done using a non-chemical approach such as digging-out or keeping back with mulch. Where these approaches are not practical, chemical control may need to be employed.

When you are controlling Giant Hogweed you should always wear decent gloves, cover your legs and arms, and preferably wear a face mask when working on the plant. Wash skin that has touched the plant right away. Also, make sure that any contractors who are working on your land are fully aware of the risks the plant poses and that they are capable of dealing with this weed.

Non-weedkiller control

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 first.

Never let hogweed get to the point where it sets seed, allow the flower spike to grow, then cut it off it before the flowers just start to fade. At this point, the weed is less likely to survive cutting than earlier in the year. Note that perennial versions have been identified and stopping them from setting seed will not stop giant hogweed populations quickly.

Protect your skin from contact with the sap, particularly your face, when cutting the stems, and carry out the measures in overcast weather and avoid sunny periods. Then wash off any remaining sap as quickly as you can with plenty of cold water.

For larger areas, it is probably best left to the professionals, who should wear full protective clothing, especially if they are using a strimmer. A strimmer can shoot up sap and fragments flying so face protection is crucial.

Weedkiller control

Choose a weedkiller that is suitable for the purpose. Those of low persistence such as contact weedkillers will kill the top growth. However, systemic weedkillers based on glyphosate are usually the best choice as these kill roots also.

Giant Hogweed likes damp fertile areas often close to waterways. It is very important that weedkiller never enters waterways. You should get advice from the Environment Agency before you are intending to spray near streams, rivers or ponds.

Glyphosate

Where there are a lot of weeds you should try to apply a strong weed killer that contains glyphosate. Preferably you should spray the young leaves in May. Plants should be treated again in August or September. Otherwise, cut back the flowering plants and then spray any young leaves that re-grow in August and September. Older plants are probably going to need more than a single treatment to destroy them completely. Please note that glyphosate damages any plants it touches, so you should cover up ornamental plants with polythene before spraying the weed.

Triclopyr (selective systemic weedkiller)

Applying spray that contains triclopyr to the hollow cut stems after cutting back again may be efficient. Triclopyr is a weed killer that doesn’t harm long grass.

Disposing of Giant Hogweed

Giant Hogweed is classed as controlled waste (comparable to Japanese knotweed) so, if it is transported off-site it can only be disposed of in a licensed landfill site with the required documentation. To prevent this, dispose of any organic material by composting or burning it.

The smaller, native variant of Hogweed, Heracleum Sphondylium, itself is not classed as controlled waste but it should still be removed of with caution to avoid human contact.

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