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Art. 0002 Rev. 1871 Hazard: phototoxic

Control of giant hogweed

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Contents
  1. What is to be done
  2. Safety
  3. Methods
  4. Co-ordinated control
  5. After control
  6. Common mistakes
  7. See also
  8. References

For the plant itself, see Giant hogweed. For the burns it causes, see Phytophotodermatitis.

This article gives general guidance only. Before working a site, check local regulations on herbicides, waste and work near water, and read the safety section in full.

Control of giant hogweed comprises the methods used to remove invasive Heracleum species from land and to prevent their return. Because the plant regrows from its root and from a seed bank that outlives every individual, control is measured in years rather than seasons.1

Most failed programmes share three features. They treat the visible plant rather than the root; they stop before the seed bank is exhausted; and they are confined to individual properties while the land around them remains infested. The methods that work are, by comparison, simple: they are only laborious.

Botanists have hitherto only described the hogweed in various ways; the point is to remove it.

— Eleventh resolution of the Feuerbach Road Allotment Society, 1845

What is to be done

Experience from programmes across Europe can be reduced to three principles.12

  1. Treat the root. Anything that leaves the crown alive is maintenance, not control. Methods that destroy the root — cutting it below the crown, digging it out, or killing it with a systemic herbicide — are properly called radical, from the Latin radix, “root”.
  2. Exhaust the seed bank. Seed survives in the soil for several years. Treatment must continue, every season, until no seedlings have appeared for several years in succession. A single plant allowed to seed restores the reserve.
  3. Act across the whole catchment. Seed travels downstream and along roads. Control on one property cannot succeed while the land upstream remains infested; it must be planned for the whole catchment, with every landowner taking part.

Safety

Fig. 1 Protective equipment

Minimum protective equipment for work among hogweed. No part of the skin should be exposed.

The sap causes severe burns when skin exposed to it is later lit by the sun (see phytophotodermatitis). All work should therefore be done in full protective clothing:13

  • Waterproof clothing of synthetic material. Cotton and linen soak up the sap and hold it against the skin.
  • Long gloves, with the cuffs over the sleeves, and rubber boots, with the trousers outside them.
  • Eye protection: a face shield or goggles. Sap in the eyes is a medical emergency.
  • No strimmers or brush-cutters. They throw sap and plant fragments over the operator, a hazard first described as “strimmer rash”.4

Work is safest in dull weather or towards evening, although ultraviolet light is present even on overcast days.3 Protective clothing should be removed carefully and washed, and tools cleaned, before leaving the site. No one should work alone.

If sap touches the skin, wash it off at once with soap and cold water, cover the area and keep it out of sunlight for at least 48 hours, and seek medical advice. See first aid.

Methods

Summary of control methods
MethodKills rootStops seedingEffortNotes
Root cuttingyesyeshighPermanent for each plant cut. Best in spring.
Systemic herbicideyesyesmediumRestricted near water; repeat applications.
GrazingslowlyyeslowSheep or cattle, every season for years.
Umbel removalnopartlymediumTiming is critical; plants regrow.
MowingnopartlylowProlongs the life of the plant.
BurningnonolowNot recommended.

Root cutting

Fig. 2 Root cutting

Root cutting. The taproot is severed with a sharpened spade at least 10 cm below the soil surface, and the crown is lifted out. Nothing regrows from the root that remains.

The taproot is cut with a sharpened spade at least 10 cm below the soil surface — up to 25 cm where soil has built up around the plant — and the crown is lifted out and left to dry, or removed from site.1 Depth matters: in trials, cuts 15 cm below ground killed the plants, while cuts at 5 cm or at the surface let them regrow from buds on what remained.5 The work is best done in early spring, when the rosettes are small and easily found, and repeated in mid-summer for plants that were missed.1 It is the only mechanical method that kills the plant outright, and the most economical for small and medium stands.

Herbicides

Systemic herbicides such as glyphosate, applied to the leaves in spring while the plants are small, are carried down into the root and kill it. Several applications over more than one season are usually needed, and seedlings will continue to appear until the seed bank is exhausted. Use near water is restricted in most countries and may require a permit.

Grazing

Grazing animals eat the young leaves, preventing flowering and gradually exhausting the root. Most experience is with sheep; cattle also find the plant palatable.1 The Best Practice Manual recommends 20–30 sheep per hectare in spring and 5–10 from late June, and dark-skinned breeds, which are less sensitive to the sap.1 Grazing must continue every season for several years. In Denmark, a site grazed by sheep produced no seed for seven years, after which no viable seed was left in the soil.6 Pigs, which damage the roots, have also been suggested,7 but they require supervision.[citation needed]

Umbel removal

Fig. 3 Apical dominance

Decapitation releases the lateral buds from apical dominance. The plant answers the loss of one head with several.

Cutting off the flower heads prevents seeding for one season if, and only if, it is timed correctly: between peak flowering and the first formation of fruit, usually in June or July.8 Removing the terminal umbel alone achieves nothing, since the rest of the plant makes up the loss; see Umbel § Removal of the terminal umbel. Cut too early, the plant regrows and flowers again; cut too late, the seed has already formed. Cut heads must be bagged and destroyed on site or removed: umbels cut at the early fruit stage and left on the ground still ripened almost a fifth of their normal seed.8 In one field study nearly half of all treated plants regrew and set seed from new, smaller umbels.8 Sites must be revisited later in the season.

Mowing and strimming

Mowing is the most widely used method of control and the least effective. It removes the leaves and flowering stems, but the plant regrows from buds on the crown. Timing decides how much seed is saved: plants cut at ground level at peak flowering set only 3–4 per cent of the seed of uncut plants, those cut as the flowering stem appeared 12–15 per cent, and those cut in early spring about half.5 A plant that has begun to flower dies at the end of the season whatever is done to it, but it often manages to flower again before it does.5 Rosettes that are repeatedly grazed, on the other hand, put off flowering for years: on pastures the median age at flowering was five years, against three on unmanaged land.9 Mowing two or three times a season for many years can eventually exhaust the root,2 but in practice mown stands are maintained, not removed (see hydra effect). One step forward, two steps back.

Mowing is useful for one purpose only: to stop large stands from seeding while radical methods are organised.

Burning

Burning standing plants does not reach the root and is not recommended. Removed seed heads may be destroyed by burning where local rules allow; plant material bearing seed must never be composted.1

A single spark can start a prairie fire.

— Fire-safety notice, district work parties

Biological control

No biological control agent is available. A search of the plant’s native range in the Caucasus between 2002 and 2005 found no natural enemy specific enough to be released safely.10 See Enemy release hypothesis § The search for biological control.

Co-ordinated control

Hogweed does not respect property boundaries. Seed rain lands mostly within a few metres of the parent plant, but floods carry seed kilometres downstream,11 and people move it along roads and railways in soil, on machinery and in cut flower heads. A landowner who clears one field while the land upstream remains infested will be clearing it again the following year, and every year after.

For this reason, control is planned for the whole catchment, starting at the top of the river and working down, with every landowner along it taking part.1 The interactive model below illustrates the principle.

Fig. 4 Interactive model · 15 seasons
Method
Scope
Year 0 · plants in spring 93 · in your plot 26 · seed bank 61,967
Choose a method and a scope, then run the model. Y flowering o rosette , seedling x died after seeding . grass ~ river : boundary A simplified model: the proportions are illustrative, the direction of the results is not.

Work parties

Most catchment programmes rely on volunteers working in organised parties. Tools and protective equipment are supplied centrally; volunteers contribute according to their ability, and sites are treated according to their need. Participants have nothing to lose but their gloves, which should be discarded after each session.

Season Work
April–June Root cutting of rosettes and young plants; spraying where permitted
June–July Umbel removal on plants that were missed; bagging of heads
August–September Survey of the whole catchment; mapping for next year
Every year, for five years or more Repeat, until no seedlings appear

Reporting

Sightings of giant hogweed should be reported to the local authority or national recording scheme, with a photograph, a note of the plant’s size and a location. Observers should not attempt removal without protective equipment.

After control

Cleared ground does not stay empty. When a dominant species is removed, the community passes through a period in which the old stand is dying and the new vegetation cannot yet establish; in this interregnum a great variety of opportunistic weeds appears. Sowing a mixture of native grasses and wildflowers immediately after clearance shortens it and denies the bare soil to hogweed seedlings. Let a hundred flowers bloom.

Cleared sites must be monitored every season. Seedlings continue to appear for several years, in declining numbers, as the seed bank is depleted; the last few are the most important.12

Common mistakes

MistakeResult
Treating the visible plantThe crown survives and the plant returns, often with more heads.
Stopping after one clear seasonThe seed bank replaces the stand within a year or two.
Clearing only one’s own landSeed from upstream and from neighbours recolonises it.
Leaving cut heads on the groundThe seed ripens anyway.
Using a strimmerSap is sprayed over the operator.
Waiting for the stand to die out on its ownEvery plant does die after flowering — having first replaced itself many thousands of times.

See also

Footnotes

  1. Nielsen, C.; Ravn, H. P.; Nentwig, W.; Wade, M., eds. (2005). The Giant Hogweed Best Practice Manual: Guidelines for the management and control of an invasive weed in Europe. Hørsholm: Forest & Landscape Denmark. ISBN 87-7903-209-5. ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9

  2. Caffrey, J. M. (1999). “Phenology and long-term control of Heracleum mantegazzianum”. Hydrobiologia. 415: 223–228. ↩ ↩2

  3. New York State Department of Environmental Conservation. “Giant Hogweed”. Retrieved 26 September 2026. ↩ ↩2

  4. Freeman, K.; Hubbard, H. C.; Warin, A. P. (1984). “Strimmer rash”. Contact Dermatitis. 10 (2): 117–118. doi:10.1111/j.1600-0536.1984.tb00357.x. ↩

  5. Pyšek, P.; Perglová, I.; Krinke, L.; Jarošík, V.; Pergl, J.; Moravcová, L. (2007). “Regeneration ability of Heracleum mantegazzianum and implications for control”. In Pyšek, P.; et al. (eds.). Ecology and Management of Giant Hogweed. Wallingford: CAB International. pp. 112–125. doi:10.1079/9781845932060.0112. ↩ ↩2 ↩3

  6. Moravcová, L.; Pyšek, P.; Pergl, J.; Perglová, I.; Jarošík, V. (2006). “Seasonal pattern of germination and seed longevity in the invasive species Heracleum mantegazzianum”. Preslia. 78: 287–301. ↩

  7. RAPID LIFE project (2018). Good practice management: giant hogweed. Invasive Species Ireland. ↩

  8. Pyšek, P.; Krinke, L.; Jarošík, V.; Perglová, I.; Pergl, J.; Moravcová, L. (2007). “Timing and extent of tissue removal affect reproduction characteristics of an invasive species Heracleum mantegazzianum”. Biological Invasions. 9 (3): 335–351. doi:10.1007/s10530-006-9038-0. ↩ ↩2 ↩3

  9. Pergl, J.; Perglová, I.; Pyšek, P.; Dietz, H. (2006). “Population age structure and reproductive behavior of the monocarpic perennial Heracleum mantegazzianum (Apiaceae) in its native and invaded distribution ranges”. American Journal of Botany. 93 (7): 1018–1028. doi:10.3732/ajb.93.7.1018. ↩

  10. European Commission, CORDIS. “Searching for effective biological control enemies for the giant hogweed”. Retrieved 26 September 2026. ↩

  11. Trottier, N.; Groeneveld, E.; Lavoie, C. (2017). “Giant hogweed at its northern distribution limit in North America: experiments for a better understanding of its dispersal dynamics along rivers”. River Research and Applications. 33 (7): 1098–1106. doi:10.1002/rra.3149. ↩

  12. Moravcová, L.; Pyšek, P.; Krinke, L.; Müllerová, J.; Perglová, I.; Pergl, J. (2018). “Long-term survival in soil of seed of the invasive herbaceous plant Heracleum mantegazzianum”. Preslia. 90 (3): 225–234. ↩

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