Giant Hogweed ‘Most Unwanted’ in contrast to native variety
HOGWEEDS ON roadside verges are making their presence felt as they grow in stature and flower. One of our most abundant wayside wild plants, Hogweed plants are supported on stout stems that are both grooved and hollow. Their large, creamy-white flowers are more or less flat-topped and are borne on several stalks that all radiate from a central point like the ribs of an upside-down umbrella.
Hogweed is native to Ireland and was the only species we had until a closely related species, the Giant Hogweed, was introduced here from south-west Asia. While our native Hogweed normally stands 1-1.5m tall the alien Giant Hogweed is unmistakable as it towers to the impressive height of 3-4m or more. It is such a tall and statuesque plant that estate owners introduced it here in the late 1880s as an ornamental plant and conversation piece. That was fine but there were two downsides to its introduction that are now so serious that the alien is rated ‘Most Unwanted' and is the subject of control and eradication schemes.
First, it is invasive. Its preferred habitat is damp wasteland and the banks of rivers. It grows so well in our climate and seeds so freely that it has infested several river banks suppressing the indigenous local flora.
Second, the plant is a public health hazard in that its sap is hazardous to people. Contact with the giant plant causes severe itching and nasty skin blisters. Exposure to direct sunlight exaggerates the undesirable effects. The phototoxic sap is found in hairs on the stems and leaves. The sap makes human skin photosensitive. Severe irritation and blistering may not occur for some days after contact with the plant and can recur for several years.
While the alien Giant Hogweed is a public health hazard, our local and native Hogweed is entirely harmless. The two can be told apart in two ways: our native Hogweed never grows more than 2m tall and has flat-topped flowers; adult Giant Hogweed plants are always more than 2m tall, have purple blotches on their stems and have flower heads that are slightly hemispherical, that is, curved like an upturned saucer.
Giant Hogweed is found scattered throughout Ireland in damp places especially along streams and riverbanks. Where it grows it is usually common and spreading. Using the herbicide Glyphosate staff of Inland Fisheries Ireland have successfully carried out eradication programmes and have devised a protocol for getting rid of the 'Most Unwanted' alien.