Giant hogweed (Heracleum mantegazzianum), is a tall, cow parsley-like plant with thick bristly stems that are often purple-blotched. It can reach a height of 3.5m (11.5ft) when the flat-topped flowers appear, and has a spread of about 1m (3.5ft). It is usually biennial, forming a rosette of jagged, lobed leaves in the first year, before sending up a flower spike in the second. After flowering the plant usually dies, but not before spreading its seeds.
Although it makes an impressive plant, it is invasive and potentially harmful - chemicals in the sap can cause ‘photodermatitis’ or ‘photosensitivity’; the skin becomes very sensitive to sunlight and may suffer blistering, pigmentation and long-lasting scars.
Always wear gloves, and cover your arms and legs when working on or near it. Contaminated clothing and tools are potentially hazardous too. Wash any skin that comes in contact with the plant immediately.
Cultural control
Ideally, pull up young plants by hand when the soil is moist. Do this in May when it has reached a reasonable height, but before it has produced its flowering spike. For larger plants it might be necessary to loosen them with a fork first.
Never let hogweed seed, but allow the flower spike to form and remove it before the flowers fade. At this stage, the plant is less likely to survive trimming than earlier in the year. Again, protect yourself from any sap, especially your face, when cutting stems. Although a powered trimmer can ease the work, beware flying sap and fragments. Face protection is essential if you have to use a trimmer.
Chemical control
Where many plants have to be dealt with, try applying a weedkiller containing glyphosate such as Roundup or Tumbleweed. 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.
Applying sodium chlorate crystals to the hollow cut stems after cutting back is said to be effective. But be careful as sodium chlorate is highly soluble and any spillage can affect nearby plants.
Leigh Hunt