Giant Hogweed

Giant Hogweed is a notoriously dangerous plant which though uncommon in the UK is something you are likely to see if you spend a lot of time walking beside rivers and streams.

Giant Hogweed is a near relative of cow parsley that originated in Southern Russia and Georgia. It can grow to be beyond 3m (10ft) tall.

Although this beautiful plant can be lovely in some conditions, most gardeners will want to get rid of it because it is invasive and the sap can cause severe skin burns.

It’s widely disseminated in the wild, posing a major threat to those who aren’t aware of its dangers.

This blog post will provide an in-depth guide on what Giant Hogweed are, how they grow, and how to eradicate them from your garden.

Quick Facts

Common names: Common Hemlock, Cartwheel Flower, Hogsbane, Giant Cow Parsley, Giant Cow Parsnip
Scientific name: Heracleum mantegazzianum
Family: Apiaceae
Origin: non-native
Flowering season: June to August
Habitat: riverbanks, streams, wetlands, farmland, woodland, gardens, common land and waste ground

Giant Hogweed Identification

Giant hogweed out-competes other vegetation due to its size and forms pure stands that expand from year to year if not controlled.

Giant hogweed is an immensely tall umbellifer (member of the carrot family) that displays large, white, umbrella-like clusters of flowers. Its hollow stem is ridged and purple-spotted, and its leaves are large and divided.

Because of its preferred location along riverbanks, gigantic hogweed has the ideal opportunity to spread as it sends its seeds into the water to be transported by the current.

Each plant may generate between 30,000 and 50,000 seeds per year, and colonies can swiftly take over a habitat, displacing native species.

The main differences between Giant Hogweed and Cow Parsley / Cow Parsnip are:

  • Giant Hogweed Height 15 -20 feet: Cow Parsley Height 5 – 8 feet
  • Giant Hogweed Stem Width 1 – 3 inches: Cow Parsley Stem Width 1 – 2 inches
  • Giant Hogweed Leaf Width <5 feet: Cow Parsley Leaf Width 2 – 2.5 feet

What does Giant Hogweed look like?

Characteristics that make up the Giant Hogweed leaf, stem, root and flower are detailed below.

In the first year, plants can develop leaves over 1 m wide. The dense rosette of leaves suppresses other vegetation. The plant dies down in winter then regrows in March-April from the large fleshy taproot.

In the second, third or fourth year it sends up huge flat-topped flower heads on a hollow stem up to 4 m tall. Plants may need to develop taproots of a minimum size before flowering can occur.

Giant Hogweed leaves
Giant Hogweed leaves

Leaves: these are huge, up to 1.5m wide and 3m long and is deeply divided into smaller leaflets. It looks a bit like a rhubarb leaf, with irregular and very sharp or jagged edges – which has given rise to one of its other common names – wild rhubarb.

The underside of the leaf is hairy.

Giant hogweed stem with shoot and tiny hairs
Giant hogweed stem with the shoot and tiny hairs

Stems:  The stem and leaf stalks contain a sap that is released by handling, cutting or just brushing against the plant.

Stems are green with purple blotches and stiff, white hairs. Stems are hollow with ridges and have a thick circle of hairs at the base of each leaf stalk.

The stem can grow up to 20 feet tall with leaves roughly 6 inches wide, but it should be noted that Giant Hogweed is not just found in gardens; it also grows along riversides, roadsides, forests, and other areas where there is dense shade.  

One characteristic of this plant that distinguishes it from other weeds is red spots on the stem.

Roots: over time the taproots develop several metres underground and grow to be thick roots that dominate the eco-environment beneath the surface.

Giant hogweed develops an extensive taproot during its first season of growth. When fully developed, this extensive taproot along with secondary roots provides support for the enormous plant.

Despite being labelled a biennial, giant hogweed appears at times to give rise to new plants from the branched taproot it develops; however, it does not reproduce vegetatively.

The umbrella head of the Giant hogweed
The umbrella head of the Giant hogweed

Flowers: 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).

It can take 4 years for giant hogweed to flower after which the whole plant normally dies. The flowers are insect-pollinated and self-compatible. A single flower head may have over 5,000 seeds and a plant may produce 50,000 to 80,000 seeds.

Seeds: Seeds are shed from late August to mid-October.

Fresh seed may have an after-ripening requirement. Most seeds germinate in the year after shedding and following a period of natural stratification over the winter. Seed mixed into the surface soil and stirred periodically emerged from January to June. The main peak of emergence was in April.

Other studies suggest that a few seeds germinate in autumn and in mild spells in winter and early spring, with most germination from January to March. In the wild, seeds germinate well in surface organic matter and detritus if adequate moisture is present. A sufficient depth of soil is then required to allow the taproot to develop.

Seeds can remain viable in the soil for 15 years.

The flat seeds are dispersed by the wind, water and by man. Normally, most seeds fall around the parent plant.

Seeds from plants growing on riversides are carried downstream and deposited further along the riverbank. Infestations often begin as a single plant that sets a seed and soon forms a small colony.

The Problem

Giant Hogweed produces a phototoxic sap which can cause serious skin inflammations, burns and blisters.

Hogweed can be a very dangerous plant that can harm not only humans but animals as well. It spreads quite quick across the land. It is also an offence to allow the plant to spread into the wild so needs to be controlled.

Giant Hogweed Seasonal Changes

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.

Giant Hogweed in Spring

Giant Hogweed is a perennial plant with large white flowers that grow in the Spring. The leaves are smooth and shiny, about four to six feet long hairy stems emerge from these plants which can grow up to 20-feet tall.

In springtime, you might notice this tall-growing perennial coming up as an invasive species under trees or at forest edges.

It has broad leaves of between 2 1/2 to 5 feet long as well as conspicuous pale yellowish pink flowers clustered thickly on top of 3 foot stems higher than your head.

Giant Hogweed in Summer

Giant hogweed is a weed with large leaves and white flowers that grow in early summer. When it’s flowering, the plant can be identified by its distinctive flower clusters which will gradually turn into green seed heads for later dispersal.

In late summer it bears little white flowers on long stems which turn into black spade-shaped seeds that will release next year when touched by rain or wind blowing them off their parent plants’ stalks onto the soil below for new growth.

Giant Hogweed in Autumn

In autumn Giant Hogweeds leaves turn from green to red or brown as they fall.

During autumn the large white flowers may be apparent, however, these do not last for long as they become covered in thousands of seeds.

Giant Hogweed in Winter

Giant Hogweed dies back in the winter and has pink flowers which are poisonous to humans and animals alike during warmer seasons.

The Giant Hogweed flower head in winter is enclosed within a stiff, leaf-like bract and looks like an upside-down ice cream cone. The stem (thickest part) can be up to 15 cm wide.

Giant hogweed dominates stream and river banks
Giant hogweed dominates stream and river banks

How to get rid of Giant Hogweed

Seedlings and young plants can be pulled by hand; larger plants can be dug out or chopped down to ground level, but full protective clothing must be worn to avoid contact with t, but he sap. 8-12 cm below ground level, the taproot should be cut.

Plants that are cut at or above ground level have a more vigorous regeneration.

In the spring, plants should be pruned every two weeks. Cutting plants that are taller than 1.5 metres is not recommended.

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.

Method One – Herbicide Treatment

Before buying or using a weed killer, make sure it’s right for the job by reading the label carefully.

Contact weed killers like pelargonic acid, for example, have a low persistence and will kill the top growth.

Systemic weed killers based on glyphosate, on the other hand, are usually the best option because they also kill roots.

Because residual weed killers remain in the soil for several weeks, extra caution must be exercised while using them.

Giant hogweed grows best in damp, fertile locations near streams. Weedkillers must never be allowed to enter waterways under any circumstances. Before spraying near rivers, streams, or ponds, seek advice from the Environment Agency.

Glyphosate weed killers

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.

Used with care, glyphosate is safe to use around the base of non-suckering woody plants, as long as the bark is woody, brown and mature. Glyphosate is not active through the soil and there is, therefore, no risk garden plants will absorb it through their roots.

Triclopyr weed killers

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

Method Two – Organic treatment

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.

Take extreme care with Giant hogweed as it can affect your health
Take extreme care with Giant hogweed as it can affect your health

Take extra care with this weed

The danger with Giant Hogweed is not poisoning, but in the way that its sap reacts with your skin. If you get the sap on you then it will react with the melanin in your skin and removes any protection that patch has from UV light.

If the hairs or sap come into contact with your eyes they can cause blindness!

The sap of giant hogweed can cause burns. It contains furocoumarin, which makes skin extremely sensitive to sunlight (phytophotodermatitis).

If the sap gets onto your skin, then you are exposed to the sun, your skin can blister badly and blistering can recur over months and even years. This is known as phytotoxicity.

The best way to avoid injury is to familiarise yourself with the plant. Avoid brushing through patches of giant hogweed and exposing yourself to plants that have been cut which might cause you to get sap on your skin.

With Giant Hogweed you don’t have to break the plant and rub the sap on you for the effect to take place either. The stems have fine needle-like hairs that will cause irritation simply by touching them. So the rule is just to stay away from the plant if you see it.

Always wear gloves, cover your arms and legs, and ideally wear a face mask when working on or near it.

Beware as strimmers send sap and fragments flying so face protection is essential. Cut plant debris, contaminated clothing and tools are potentially hazardous too.

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 any skin that comes in contact with the plant immediately with lots of cold water and avoid sunlight for several days. Ensure that contractors working on your land are aware of the risks and are competent to deal with this weed.

Disposal 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.

Management of Giant Hogweed

Although there is no statutory obligation for landowners to eliminate giant hogweed, local authorities will often take action to remove infestations in public areas.

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.

The control must start with preventing seed dispersal, but a seed that has already emerged from the soil seed bank will continue to sprout for another 3-4 years.

To help prevent recolonization by the giant hogweed, bare portions of soil left following weed eradication should be sowed or planted with native plants.

Cutting it back will manage the population but if it is not killed it will just force the plants’ energy into rooting deeper. A targeted herbicide application is the only way to really control it for good.

In Conclusion

If you come across this weed on your property be careful. Play safe and call in the experts.

This weed is far too dangerous to tackle on your own and needs to be treated with the utmost respect.

Most weeds are visually intrusive, whereas the Giant Hogweed is not only this but dangerous as well.

Take care and definitely remove this weed from your property as soon as you can.


How does Hogweed spread?

Through its seeds.