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

There is also a native hogweed, Heracleum sphondylium, which will be a familiar plant to gardeners and those who like to go walking in the UK. It can grow to six foot or so when in flower but is nevertheless a much smaller plant than giant hogweed. It can cause rashes and other skin complaints but reactions tend not be as severe as with the larger species. Heracleum sphondylium can support lots of pollinators, including beetles, flies, hoverflies, bees and wasps. 

The giant hogweeds were introduced into Britain and Europe from the Caucasus Mountains in the nineteenth century. The earliest documented reference to their introduction into Britain that has been traced 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.

Their occurrence is monitored and it appears that they are reported to be continuing to spread and can be found in every part of the British Isles. Details can be found on the Non-native Species Secretariat (search 'Heracleum').

This page looks at options for gardeners when giant hogweed is a problem.

Giant hogweed (Heracleum mantegazzianum) is tall with thick bristly stems that are often purple-blotched. It is in the same family as cow parsley but cow parsley has much finer, fern-like foliage and generally flowers from April to June, whereas giant hogweed tends to flower later in June and July. 

The flowers are white and held in umbels, (flat-topped clusters, like those of carrots or cow parsley), with all the flowers in the umbel facing upwards. The flower heads can be as large as 60cm (2ft) across. It can reach a height of 3.5m (11.5ft) or more and has a spread of about 1-2m (3.5-7ft).

Giant hogweed is usually biennial, forming a rosette of jagged, lobed leaves in the first year before sending up a flower spike 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 RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner. For example, where pests, diseases or weeds pose a serious threat to the wider environment, to important heritage specimens, to habitat, or to native wildlife.

Although there is no statutory obligation for landowners to eliminate giant hogweed, local authorities will often take action to remove infestations in public areas. Plants that are undesirable, out-compete desired plants, or simply invade half the garden are classed as weeds and require control. Weeds from abroad with strongly invasive tendencies are termed ‘invasive aliens’ and pose a severe threat to wild or other uncultivated environments, such as railway embankments.

Due to the severity of the threat, legislation has been applied to invasive aliens, including giant hogweed. The Wildlife and Countryside Act 1981 (as amended) lists it on Schedule 9, Section 14 meaning 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 where occupiers of giant hogweed infested ground can be required to remove the weed or face penalties.

Local Authorities have powers under certain circumstances to require giant hogweed to be removed.

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.

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 are competent to deal with this weed.

Cultural control

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 by RHS research and preventing them from setting seed will not reduce giant hogweed populations quickly.

Protect yourself from any skin contact with the sap, especially your face, when cutting stems, and carry out control measures in overcast weather avoiding sunny periods. Wash off any sap as soon as possible with plenty of cold water.

Larger scale areas are probably best left to the professionals, who should wear full protective clothing, especially if they are using a strimmer. Strimmers send sap and fragments flying so face protection is essential.

Weedkiller control

The RHS does not support the use of weedkillers and recommends that alternative control methods are used. However, when invasive plants are a threat to biodiversity and gardeners struggle to control them with cultural methods, targeted use of regulated weedkillers may be an option. Garden centres and large retailers selling weedkillers have trained staff who can advise on suitable products for your needs.

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.

Weeds: non-chemical control