Individual study: A review of the effectiveness of herbicide application for control of giant hogweed Heracleum mantegazzianum in Europe
Nielsen C., Ravn H.P., Nentwig W. & Wade M. (2005) The giant hogweed best practice manual. Guidelines for the management and control of an invasive weed in Europe. Forest & Landscape Denmark, Hoersholm.
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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