Plants that are undesirable, out-compete desired plants, or simply invade half the garden are classed as weeds and require control.
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.
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 or glyphosate may suffice. Residual weedkillers persist in the soil for several weeks or months and can move deeper or sideways in the soil, leading to possible damage of underlying roots. Particular care must be taken when using residual weedkillers.
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.
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.
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.
Larger scale areas are probably best left to the professionals, who should wear full protective clothing, especially if they are using a strimmer. As strimmers send sap and fragments flying face protection is essential.
Where there are many plants, try applying a tough weedkiller containing glyphosate (e.g. Scotts Roundup Ultra 3000, Scotts Tumbleweed, Bayer Tough Rootkill, Bayer Garden Super Strength or Doff Maxi Strength Glyphosate Weedkiller). 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.
Triclopyr (residual 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.
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