Giant hogweed (Heracleum mantegazzianum) can colonise waste land and river banks (riparian zones) and can produce a dense colony growing up to 5m high as well as producing 30 to 50,000 viable seeds per year.It is a member of the parsley or carrot family, Apiaceae. It is a large herbaceous plant and can be either biennial or perennial. It can easily disperse seeds downstream and spread the growth quickly along the length of a watercourse. Like many non-native invasive plants it has built in defense mechanisms, if the plant is cut down before it produces seed, it will survive into a third or subsequent season, attempting to flower each year.
Plants are generally capable of flowering and setting seed only once in a lifetime (termed monocarpic). The seeds produced are easily dispersed by wind, water, animal or human influence. The seeds can start to sprout in early spring and can continue to do so throughout the growing season. Seeds are known to be viable for several years. The tall stem produced is rigid and hollow, up to 10cm in diameter with purple blotches on the surface. The leaves are deeply lobed and sharply pointed with soft hairs appearing on the under surface. Flowers are white and borne in clusters on a large umbrella shaped head.
Giant hogweed is native to Asia but is now invasive in North America, continental Europe and Britain
This species represents a public health hazard. The toxins in giant hogweeds sap react with sunlight/UV rays. When the sap produced comes in direct contact with the skin can cause the skin to blister. Hand cutting should never be undertaken unless the operator is wearing full protective clothing to prevent skin contamination by the sap.
Infestations need to be controlled by digging out the whole plant as cutting through the stem must be done below ground level to ensure damage to the rootstock and to prevent regrowth from the base. This is known as 'tap rooting'.
This species has a high competitive advantage over native plants. It can out compete for space and resources by shading out native and desirable plants. There is also a loss in invertebrate diversity as the plants these animals depend on become scarcer. Other impacts include a significant increase in riverbank erosion In summer time rights of way and access points for water users such as fishermen may become impassable due to high infestation rates of giant hogweed.
For information on Giant hogweed, see:
- Identification Guide for Giant hogweed (Biodiversity Ireland)
- The Giant Hogweed Best Practice Manual- Guidelines for the management and control of an invasive weed in Europe
For a list of Invasive species Indentification guides, see invasives.ie
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