Northern Ireland's Priority Species

Ceratophyllum submersum – soft hornwort

Ceratophyllum submersum

Ceratophyllum submersum L.
Family: Ceratophyllaceae

Formerly known as a ‘honewort’, Ceratophyllum submersum is a plant that grows completely submerged just below the surface of shallow, still or slow-moving waters. It has no true roots and even flowers underwater. It is a member of a group of two species which have no close relatives in the plant kingdom and are candidates for consideration as ‘living fossils’. It has a widespread global distribution, but remains rare in Ireland.

In brief

  • Grows submerged in shallow water, often in coastal or hard water situations

  • Where it occurs it may be locally plentiful from May to September

  • In Northern Ireland, only found in a handful of lakes in the Lecale, County Down

  • Very rare in the rest of Ireland, so these sites are the Irish stronghold for the species

  • Used as an aquarium plant when it may be known as tropical or spineless hornwort.

Species description
A fully aquatic plant growing submerged and without true roots. Mid-to dark green whorls of narrow, repeatedly forked leaves occur at intervals along the main axis of the stem, giving the plant an attractive feathery appearance, though the leaves themselves are quite stiff and brittle (despite its English name). Inconspicuous green flowers occur in the leaf axils.

Life cycle
In bright conditions this plant can grow quickly to form abundant masses. It promotes its own growth by the release of chemicals that can suppress growth of other water plants, including algae, which would otherwise cloud water and intercept sunlight. Male and female plants are separate. Pollen-bearing anthers from the male plants detach in the summer and float slightly under the water surface where they may contact the stigmas of the female plants to form fertile seeds which may be dispersed by wildfowl; however, vegetative reproduction is important as flowers do not reliably form mature seeds. The stems are brittle and readily regenerate from snapped-off fragments. In autumn the vegetative masses die back, but not before specialist buds have formed which detach and sink to the bottom ready to start the following year’s growth.

Similar species
The genus Ceratophyllum is sufficiently unique in the plant kingdom to justify its own family, the Cetatophyllacaea, in which most people recognise two species, soft hornwort (Ceratophyllum submersum) and rigid hornwort (Ceratophyllum demersum) although there is variability that has prompted some researchers to propose division into more species.

Rigid hornwort (Ceratophyllum demersum) grows in similar situations and is superficially very similar to soft hornwort. In Northern Ireland it occurs mainly in County Down, so the geographical ranges of the two species concur. It is more frequent that soft hornwort both in Britain and Ireland. Rigid hornwort is distinguished by the degree of branching of the leaf whorls, usually only forked once or twice rather than three to four times in soft hornwort, and in having two spines on the fruit base that are absent from soft hornwort. The comparative rigidity of the leaves is not a reliable character.

How to see this species
Naturally this species inhabits still water ditches, ponds and small lakes. It tolerates very slightly brackish water and to date its Irish stations are not far from the coast. It is of very restricted occurrence in Northern Ireland so to see it in its wild habitat would require a trip to the Lecale lakes where Wellington boots would be required. The easier place to find this species is in fresh water aquaria where it is favoured because of its attractive appearance and high oxygen production and where it may also be known as Tropical or Spineless Hornwort.

Current status
It is widespread globally, being found throughout most of Europe, Asia, Africa and North and South America. It occurs frequently in parts of south-east and central England, but the first Irish record (apart from a couple of ‘false alarms’ in the nineteenth century) was in 1989 when it was discovered in Loughkeelan, a small lake in the Lecale area of County Down, which was included in the Environment and Heritage Service’s Survey of Northern Irish lakes. It was subsequently found in two other nearby small lakes as a part of the same survey. In 1991 it also turned up in the Wexford Slobs and, more recently in a coastal lagoon also in Wexford. Both of these sites are much used by wildfowl, which may have provided the plant with its means of transport.

Why is this species a priority in Northern Ireland?

  • Rare – confined to a small population of one or two sites in Northern Ireland with Northern Ireland being a stronghold (S) consisting of >50% Irish population.

Because aquatic habitats are often neglected by botanists and because it is easy to confuse this species with rigid hornwort, it is possibly more widespread in the Republic of Ireland, but in Northern Ireland the records were part of a fairly comprehensive province-wide survey of small lakes.

Threats/Causes of decline
Unlike many other rare aquatic plant species it is fairly tolerant of high nutrient levels, however excessive nutrient input remains a threat as does infilling of small lakes and ponds.

Conservation of this species

Current action

  • Loughkeelan has been designated ASSI and is an interesting calcium-rich lake and fen site as well as hosting an important population of this species which is likely to be secure.

Proposed objectives/actions

  • Maintain the number of viable populations of the species.

What you can do
If you are an aquarium owner, do not dispose of surplus plants or animals into waterbodies such as ponds, streams, lakes, rivers etc. where they can disrupt the ecology and displace native species or genotypes.

If plants which appear to be this species are found at new sites, whether they are considered as aquarium escapes or natural populations, the find should be reported to the Botany Department, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5251. Record the exact details of the location in order that the plant may be refound.

Further information


Smith, S.J. and Wolfe-Murphy, S.A. (1991). Ceratophyllum submersum L. Soft Hornwort, a species new to Ireland. Irish Naturalists’ Journal 23: 374-376.

Text written by:
Shaun Wolfe-Murphy

iNaturalist: Species account : iNaturalist World Species Observations database