The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner. For example, where pests, diseases or weeds pose a serious threat to the wider environment, to important heritage specimens, to habitat, or to native wildlife.
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.
Due to 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 are competent to deal with this weed.
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.
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 like 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.
Where there are many plants, try applying a tough weedkiller containing glyphosate (e.g. Roundup Tree Stump & Rootkiller, SBM Job done Tough Weedkiller (soluble sachet only), SBM Job done Tough Tree Stump Killer (soluble sachet only), Doff Weedout Extra Tough Weedkiller or Westland Resolva Pro 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. 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 (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.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 1a and 4)
Chemicals: using spot and broad-scale weedkillers
Chemicals: using a sprayer
Chemicals: using safely and effectively
Weeds: non-chemical control