Out walking last week, I came across a group of plants taller than me, with large white umbrella-shaped flower heads held on long, hairy stems, and broad green leaves with toothed edges. My first thought was that these impressive plants, standing at eye level, were giant hogweed (Heracleum mantegazzianum), a toxic and dangerous invasive species. However, a closer look revealed that I had misjudged our native and beneficial common hogweed (Heracleum sphondylium). As common hogweed is native and important within the ecosystem, and giant hogweed is non-native, invasive, and harmful, it is worth knowing how to tell them apart.

What are umbellifers?

Common hogweed and giant hogweed, although very different in their impacts, share some key features. Both belong to the Apiaceae family, also known as the umbellifers. This group also includes angelica, hemlock, parsley, carrot, and fennel. The name comes from the Latin word umbella, meaning parasol or sunshade, which reflects the distinctive arrangement of their flowers. These are typically grouped in umbrella-shaped clusters at the ends of stems, often made up of many smaller clusters. At this time of year, umbellifers are a common sight across West Cork, brightening grasslands, hedgerows, and verges, while providing vital pollen and nectar for pollinators. Among the most familiar species are common hogweed, hemlock water-dropwort, cow parsley, pignut, and, from next month, wild angelica. While some are easy to recognise, others look very similar, and a few are highly toxic.

Giant hogweed

Each year, I notice small numbers of native common hogweed in public areas that have been cut or knocked down in West Cork, likely due to confusion with giant hogweed. This is understandable, but unfortunate. Giant hogweed was introduced to Ireland from Asia as an ornamental plant and remains relatively uncommon in West Cork, at least for now. Common hogweed can grow tall, up to two metres, but giant hogweed is much more striking, reaching heights of up to five metres. Giant hogweed also has far larger leaves, up to three metres across. While common hogweed leaves are toothed, they appear rounded in comparison to the deeply jagged edges of giant hogweed. The stems are another key feature. Giant hogweed stems are exceptionally thick, five to ten centimetres in diameter, and marked with red or purple blotches. Common hogweed stems may show red or purple streaks, but never the distinctive blotches. Location can also help, as giant hogweed is most often found along riverbanks, whereas common hogweed grows in a wide range of habitats. If you do encounter giant hogweed, record the sighting at invasives.ie and do not touch the plant. The sap is phototoxic and can cause severe dermatitis, leading to blistering when exposed to sunlight, with effects that may persist for years. The fine hairs on the stems can also cause irritation on contact. Beyond the risks to people, giant hogweed poses a serious threat to native habitats. Each plant can produce up to 50,000 seeds annually, allowing it to spread rapidly and crowd out other species. They often dominate riverbanks, and when they die back in winter, the exposed soil becomes vulnerable to
erosion.

Dead man’s finger

Another umbellifer, hemlock water-dropwort, is considered one of Ireland’s most poisonous plants and is extremely common in West Cork, particularly along rivers, streams, and ditches. It grows up to 150 centimetres tall, with clusters of white or pale green flowers forming dense heads, green, slightly shiny, hairless leaves, and green, grooved stems sometimes tinged with purple. The plant’s nickname, ‘dead man’s finger’, refers to the finger-shaped roots, which resemble parsnips but are highly toxic and can prove fatal within hours. All parts of the plant are poisonous if eaten. The term sardonic grin is thought by some to originate from ancient Sardinia, where people are believed to have ingested this plant as part of death rituals. The toxins caused involuntary facial spasms, known as risus sardonicus, followed by death. Despite the plant’s toxicity to humans and livestock, some species have adapted to use this plant. The caterpillars of the moth Depressaria daucella feeds on it, gaining protection from grazing animals, which tend to instinctively avoid the plant if other food is available.

Benefits

Although caution is essential if foraging for or handling certain umbellifers, most species, with the exception of giant hogweed, are valuable components of our ecosystems. Plants such as wild carrot, with delicate white and pink flowers, support a wide range of insects, while their seeds provide food for birds, including finches. If you look closely at the flower head of wild carrot, you will often see a single dark red or purple floret at the centre. According to folklore, this represents a drop of blood from Queen Anne, who pricked her finger while making lace, giving rise to one of the plant’s other names, Queen Anne’s lace. I have often looked at this plant and wondered about this distinctive spot. A more likely explanation is that it helps attract pollinators. Take time to look for these remarkable plants in our countryside. Watch the diversity of insect life that they attract, and if you do come across giant hogweed, avoid contact and report the sighting.

Depressaria daucella, a moth caterpillar on hemlock water-dropwort showing why the plant is a member of the “umbrella” group. (Photos: Ann Haigh)