Giant hogweed

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.

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 website (search 'Heracleum').

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

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.

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.

Because of 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 competent to deal with this weed.

Non-chemical controls

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.

Chemical controls

Choose a weedkiller that is appropriate for purpose by reading the label carefully before buying or using. Those of low persistence such as contact weedkillers, diquat or pelargonic acid for example, will kill the top growth. However, systemic weedkillers based on glyphosate are usually the best choice as these kill roots also. Residual weedkillers persist in the soil for several weeks so particular care must be taken when using them.

Giant hogweed prefers moist fertile areas often near waterways. It is essential that weedkiller never under any circumstances enters waterways. Seek advice from the Environment Agency before undertaking spraying near rivers, streams and ponds.

Glyphosate
Where there are many plants, try applying a tough weedkiller containing glyphosate (e.g. Scotts Roundup Tree Stump & Rootkiller, Bayer Garden Rootkill, Bayer Garden Super Strength, Doff Maxi Strength Glyphosate Weedkiller or Westland Resolva Xtra Tough Concentrate). Ideally, spray the young foliage in May. Plants should be re-treated in August or September, if necessary. Alternatively, cut back flowering plants and then spray any young foliage that re-grows in August and September. Mature plants are likely to need more than one treatment to kill them. Remember that glyphosate damages any plants it touches, so cover up ornamental plants with polythene or cardboard boxes before spraying.

Triclopyr (selective systemic weedkiller)
Applying Vitax SBK Brushwood Killer (based on triclopyr) to the hollow cut stems after cutting back may be effective. Triclopyr is a residual weedkiller that does not harm long grass.

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.

Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.

Downloads

Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 1a and 4)

Links

Chemicals: using spot and broad-scale weedkillers
Chemicals: using a sprayer
Chemicals: using safely and effectively
Weeds: non-chemical control

Bindweed
Brambles and other woody weeds
Cow parsley
Garden thugs: potential nuisance plants
Ground elder
Himalayan balsam
Japanese knotweed
Ragwort
RHS video: weed control
Wild garlic and crow garlic