Mystery around origins of Irish Hornwort species
'Wort' is an old Middle English word meaning 'plant' or 'herb' that survives in common names like Ragwort, Woundwort, Hornwort and others.
Hornwort is so called because its leaves are divided and forked into narrow segments like the branches or tines of the antlers or horns of a Red Deer stag.
Hornwort grows underwater. It even flowers and fruits underwater. It has no roots and floats freely just below the water surface. It differs from all other water plants in that its leaves come off the stem in whorls or rings and are forked in very narrow, needle-like segments.
Up to quite recently we had only one species of Hornwort in Ireland: our native Rigid Hornwort a rare and very local plant found in some lakes, pools and canals. Now we have a second species - Soft Hornwort - and it is regarded as an unwelcome invasive alien.
The names are self-explanatory: our native Rigid Hornwort feels rather stiff and inflexible in the hand whereas Soft Hornwort feels less harsh and is flexibly floppy. However, a floppy feel is a pretty subjective description; a more precise distinction between the pair is that in Rigid Hornwort the leaves branch into narrow segments only once or twice whereas in Soft Hornwort they branch three or four times.
On the island of Ireland, Soft Hornwort first turned up in the north-east in four lakes on the Lecale peninsula near Downpatrick in County Down. Then it was found at the Wexford Slobs and in Tacumshin Lake in the south-east. It is considered likely that it is probably thriving and awaiting discovery in other waterbodies around the country, especially those near the sea.
Where it came from and how it got here are questions that may never be answered. Two possibilities have been suggested. First, the waterbodies that it has been recorded from are all well-known wild bird haunts so fragments of brittle plants may have been introduced from Britain, mainland Europe or farther afield on the bodies of migratory water birds.
The second possible source of plants is from garden centres or pet shops. Because of its soft texture, its feathery appearance, its attractive light green colour and its rapid growth, Soft Hornwort is a popular aquarium and garden pond plant. So, fragments of it may have escaped from cultivation, been thrown out or disposed of with unwanted or surplus plants.