The giant hogweed best practice manual. Guidelines for the management and control of an invasive weed in Europe

Summary

Study 1

Giant hogweed Heracleum mantegazzianum (native to eastern temperate Asia) is a tall perennial plant that is now widespread in many temperate areas of Europe. It has significantly increased its geographical range in recent years, being especially abundant along stream and river banks. Because of its tall height and that it often forms dense mono-species stands, giant hogweed often stifles out native plant communities.

A European Commission funded project was undertaken to provide scientifically-based guidelines for management and control in Europe of an invasive plant, giant hogweed Heracleum mantegazzianum, and two closely related species, H.sosnowski and H.persicum. As part of this project, the effectiveness of several manual and mechanical control techniques were reviewed, including ploughing, root cutting, stem cutting, mowing and umbel removal.

Ploughing: On agricultural land, deep ploughing (up to 24 cm) is an effective means of control. It significantly reduces hogweed seed germination as the upper soil (where the majority of the seeds are concentrated) is buried. Studies indicated that the best results were obtained if hogweed plants were cut or treated with herbicide, prior to ploughing.

Root cutting: Root cutting (usually performed with an ordinary spade with a sharpened blade and cut parts pulled out) is best undertaken in early spring and if regrowth appears, should be repeated in summer. Experience suggests that the main tap root should be cut 10-25 cm below soil level. The method is very effective but labour intensive (therefore potentially costly) and thus suitable only for single plants or small stands.

Mowing: Mechanical mowing e.g. using a flail mower, has been shown to be useful for clearing large areas of hogweed. Smaller stands can be strimmed or scythed. After cutting, there is rapid basal regrowth and mowing must be repeated 2-3 times during the growing season to reduce nutrient build up in the roots and to prevent flowering and seed-set. One cutting regime which appeared more efficient than others was to cut only the flowering plants at mid-flowering stage to prevent seed production. Repeating this cutting regime should with the least effort (in comparison with other cutting strategies) eradicate the population in a few years as the seed bank and recruitment dwindles.

Umbel removal: Removal of umbels can be as effective as cutting the whole plant, but often fails to prevent seed production due to vigorous regeneration and therefore necessitates several visits over the flowering season. Umbel removal is most effective when terminal umbels just start to flower. Even then, there is some regeneration and treated stands must be checked to prevent release of seeds produced by regeneration. Cut umbels must be collected and destroyed as if left on the ground seeds may ripen. It is recommended that this method only be considered as a control of hogweed stands where no other control has taken place earlier in the season. Stem-cutting, a similar technique to umbel removal, necessitates the same crucial timing due to the regrowth potential, otherwise the treatment must be repeated later in the season to prevent the seed from maturing.

Conclusions: One root cutting or ploughing treatment are both effective in controlling giant hogweed. Manual/mechanical control requires two to three treatments per year over several growing seasons to be effective, and is often labour intensive. Regardless of control method, the treatment of plants should commence early in the growing season.

Study 2

Giant hogweed Heracleum mantegazzianum (native to eastern temperate Asia) is a tall perennial plant that is now widespread in many temperate areas of Europe. It has significantly increased its geographical range in recent years, being especially abundant along stream and river banks. Because of its tall height and that it often forms dense mono-species stands, giant hogweed often stifles out native plant communities.

A European Commission funded project was undertaken to provide scientifically-based guidelines for management and control in Europe of an invasive plant, giant hogweed Heracleum mantegazzianum, and two closely related species, H.sosnowski and H.persicum. As part of this project, the effectiveness of grazing with domestic livestock was reviewed.

The studies reviewed showed that grazing with domestic livestock is a very effective method of control of large stands of invasive hogweed. Evidence for the effects of grazing came mostly from the use of sheep, but giant hogweed is also very palatable to cattle. These grazers eat most above ground plant parts, thus greatly reducing photosynthesis and depleting energy resources stored in the root. Sheep and cattle prefer young and fresh plants, and the most efficient control is obtained by commencing grazing in the spring when the plants are small. Studies indicate that usually livestock require a familiarization period before they regularly eat hogweed but that they soon develop a preference for it. In areas with dense stands of hogweed, a single cut is recommended to encourage fresh growth and allow establishment of other plant species as livestock are less likely to be detrimentally affected by eating hogweed if the diet is mixed.

Giant hogweed contains toxins (furanocoumarins) whose toxicity is enhanced in the presence of ultraviolet radiation. These cause inflammation of the skin and mucus-secreting membranes e.g. lips and nostrils, when exposed to light. Bare and unpigmented skin is particularly susceptible whereas darker and hairy skin is more resistant. Choosing livestock with dark pigmentation of the bare skin, e.g. blackfaced sheep, can reduce potential inflammation of mucus-secreting membranes. Clinical studies have shown reduced fecundity after oral application of furanocoumarins to livestock but this has not been reported so far for animals grazing on hogweed in the field.

Grazing can be a cheap method of control where large areas can be fenced, and it may be possible to incorporate smaller stands into adjoining areas if these are already grazed. Where possible, grazing should be undertaken in areas surrounding existing hogweed stands where seeds may have dispersed. Evidence indicates that over time (several years), grazing promotes a dense sward of grazing-tolerant species which limits the amount of suitable substrate in which hogweed seeds can germinate and become established.

 

Study 3

Giant hogweed Heracleum mantegazzianum (native to eastern temperate Asia) is a tall perennial plant that is now widespread in many temperate areas of Europe. It has significantly increased its geographical range in recent years, being especially abundant along stream and river banks. Because of its tall height and that it often forms dense mono-species stands, giant hogweed often stifles out native plant communities.

A European Commission funded project was undertaken to provide scientifically-based guidelines for management and control in Europe of an invasive plant, giant hogweed Heracleum mantegazzianum, and two closely related species, H.sosnowski and H.persicum. As part of this project, the efficacy of application of the herbicdes glyphosate and triclopyr for control purposes was reviewed.

Effective herbicides: Results of trials have demonstrated that giant hogweed is susceptible to systemic herbicides such as glyphosate (a broad spectrum, non-selective herbicide effective in killing many plants including grasses, perennials and woody broadleaves) and triclopyr (a selective herbicide detrimental to many broadleaf species but not monocots such as grasses). Glyphosate and triclopyr treatment are both considered effective and cheap methods of control of invasive colonies of giant hogweed. Glyphosate is currently the only herbicide approved (including locations close to water) for control of tall invasive species of hogweed in all European countries.

Timing of application: Evidence suggests that treatment is best undertaken in early in spring when seedlings are germinating, fresh leaves have emerged from the base of the previous years old stems, and when the vegetation is at a height of 20-50 cm. Application at this early growth stage ensures an adequate covering of the herbicide to all leaf surfaces and has the added advantage of allowing access throughout a large hogweed stand before the it becomes tall and dense. A follow-up application may be required before the end of May if additional seedlings have germinated after the first treatment. Whilst an overall springtime spray with glyphosate at the manufacturers recommended dose is an effective treatment for hogweed, other vegetation may be adversely effected. However, native vegetation is sparse under invasive hogweed colonies and application in early spring will avoid harmful effects to later emerging species. When treating smaller stands, individual plants or in more sensitive areas e.g. nature reserves, spraying can be carried out more precisely using a nozzle that constricts the spray, by weed-wiper or a brush.

Treatment combinations: Combinations of different control methods can be more efficient than spraying alone. For example, if there is minor hogweed regrowth after an early season glyphosate application, an additional treatment of mowing or cutting by scythe of the surviving plants can replace a second glyphosate application. In contrast, a stand that has grown tall and dense is not amenable to herbicide treatment due to the protection of smaller plants by mature plants, difficulty of access and the health hazard which the plants represent to operators. Cutting plants to ground level with a follow-up herbicide spot treatment of regrowth is then preferable.

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