Sium latifolium L., Great Water-parsnip
Account Summary
Native, varying from locally common to scarce. Eurosiberian temperate.
1806; Scott, Prof R.; Co Fermanagh.
May to October
Growth form and preferred habitats
S. latifolium is a hairless, robust, bright green, hollow stemmed perennial, 8-150(-200) cm tall, growing erect from a short, horizontal rhizome, 5-10 mm thick. It occupies shallow nutrient-rich, often still alkaline water, over clay or mud substrates (Tutin 1980). The hollow stem is strongly ribbed and the aerial leaves, usually 6-8 on the stem, can grow to 30 cm. The stems smell rather strongly of paraffin or petrol when bruised. The aerial leaves have stalks which are also hollow and which clasp the main stem. They are usually simply pinnate, often with only 5-7 pairs of finely-toothed leaflets.
S. latifolium seed germinates underwater, produces 1-2 large, typical aquatic, finely divided, 2-3 pinnate, submerged leaves, ie the species is heterophyllous, producing two quite different leaf types. It also dies down in the autumn and overwinters underwater. Thus, there is absolutely no doubt that S. latifolium is a member of the aquatic flora of B & I, rather than an emergent terrestrial species (Cook 1998). Some Flora writers go halfway and describe it as semi-aquatic (eg Jonsell & Karlsson 2010), but the current author (RSF) believes the species is a good example of an aquatic plant, pure and simple.
The typical habitat is very wet, species-rich, tall-herb fen, developing as a floating mat of vegetation on the margins of large lakes and slow-flowing rivers (J.O. Mountford, in: Stewart et al. 1994). In England, it grows along the edges of lowland dykes, ditches and drains in fens and alluvial levels where the water is shallow and calcareous or base-rich (Garrard & Streeter 1983). In lowland Fermanagh, it grows in slow streams and muddy ditches. It prefers still or slow-moving, shallow water that is not acidic, is nitrogen-rich, and where the soil is formed from sedge peat, or has been deposited by rivers (ie alluvial).
Greater Water-parsnip can compete with and tolerate the shade of other tall emergent herbs in species-poor reed-swamp, including reeds (Phragmites) and bulrushes (Typha), but it becomes shaded out if wet woodland (ie swampy fen-carr) with species such as Alder (Alnus spp.), Ash (Fraxinus excelsior) and Willow (Salix spp.), encroaches upon the fen or ditch habitat (J.O. Mountford, in: Stewart et al. 1994).
The only other umbellifer of similar size to S. latifolium in this type of habitat is Cicuta virosa (Cowbane), which has much narrower, more finely cut leaf segments, so the two plants cannot readily be confused, even when not in flower. Another comparison is that Cicuta virosa is extremely poisonous and is avoided by cattle, whereas Sium latifolium is readily grazed and stands of it are regularly subjected to trampling.
Since it is intolerant of grazing and frequent cutting, S. latifolium is often most common in ditches adjacent to un-reclaimed fen or arable land, provided the water is kept open by occasional use of a weed bucket or scythe (J.O. Mountford, in: Stewart et al. 1994).
Toxcity and herbivory
WARNING – parts of the S. latifolium plant are poisonous (particularly the roots) and direct physical contact with it should be avoided at all times. Having said this, there are no recent reports of poisoning by this species (Cooper & Johnson 1998).
Cattle and other stock seem to be immune from the plant's poison, but farmers discourage dairy cows from browsing it, since it gives their milk an unpleasant taste. S. latifolium can survive moderate browsing and trampling by cattle. However, drainage and excessive mechanical cleaning of waterways can kill it off.
Flowering reproduction
S. latifolium flowers in July and August. The inflorescence is a compound umbel of c 16-40 smooth rays, 6-10 cm across, each umbellule bearing c 16 small, 4 mm diameter white flowers. The terminal umbel consists of hermaphrodite (bisexual) flowers and the lateral umbels are almost entirely of male flowers. The outer flower petals are not radiating. Bracts number 2-6 and are often large and leaf-like. The flowers are pollinated by a range of insects including beetles, bees and flies (Fitter 1987). In the absence of pollinators the flowers probably self-pollinate (https://pfaf.org/user/Plant.aspx?LatinName=Sium+latifolium, accessed 18 November 2021).
The fruit is ovoid, 3-4 mm and laterally flattened. The mericarps have distinctive thick ridges and short recurved styles (Tutin 1980; Blamey & Grey-Wilson 1989). The ridges on the fruits contain large, air-filled cavities that allow the mericarps to float for up to ten days, greatly enhancing the dispersal ability of the species (Egholm 1951). Otherwise, the seed is transient, surviving for less than one year (Thompson et al. 1997). Soon after flowering the aerial part of the plant dies down and disappears.
S. latifolium can live totally submerged for several years without flowering. In some years, it may produce abundant seed, but opportunities for seedling establishment are few in the tall fen communities that it typically frequents. In ditches, particular individuals can survive in tall reed for over ten years, but fresh recruits to the population are only seen following major work to cut down the tall vegetation, or following the use of a weed bucket to open up the community and create opportunities for re-colonisation (J.O. Mountford, in: Stewart et al. 1994).
A history of rapid decline
Previously, this large umbellifer was widespread and typical of very wet, shallow, marshy waterside vegetation in B & I, but it suffered a catastrophic decline during the last 200 years and has become scarce over much of its range. Changes in land use, particularly urban encroachment and the intensification of agriculture, resulted in the rapid loss of ponds and wetlands, leading to habitat fragmentation and isolation, and deterioration of both habitat and water quality.
Fermanagh occurrence

This previously quite widespread marsh umbellifer of soggy lakeshores and ditches has declined significantly and become scarce in both B & I so that it is now listed as a 'Priority Species of conservation concern' in both UK and N Ireland Biodiversity Action Plans (J.O. Mountford, in: Stewart et al. 1994; An Irish Flora 1996). However, it remains extremely frequent and abundant around Upper Lough Erne, which must now be its current stronghold in these islands. The Fermanagh Flora Database has records of Great Water-parsnip in a total of 66 tetrads (12.5%), only one of which does not contain a post-1975 record.
The very common and abundant occurrence of S. latifolium around Upper Lough Erne reflects the ideal nature of the habitat for this tall and distinctive perennial, which although it appears physically robust, must in fact be very sensitive and demanding in terms of its physiological ecology. As the tetrad map clearly indicates, around Lower Lough Erne to the NW and along the River Finn in the SE of the VC, the plant is very much more local in its occurrence.
British and Irish occurrence
Elsewhere in B & I, the downward spiral of this species presence is very marked and obvious. In Britain, Stewart et al. (1994, p. 389) plotted records from 149 hectads with pre-1970 dates where S. latifolium was present and only 66 hectads with subsequent finds. The New Atlas map shows a total of 297 hectads with British records, but 175 of them (almost 59%) are pre-1970 in date. In England, it is still found in fair numbers south and east of a line drawn between the River Humber and the Bristol Channel, but appears confined to Lincolnshire, the East Anglian Fens, the Norfolk Broads, the coastal levels of Kent and Sussex and the Somerset Levels. It appears most common in the coastal levels and flood plains of major rivers in Britain (J.O. Mountford, in: Stewart et al. 1994). Previously it was widespread along the Thames valley (Garrard & Streeter 1983).
In Ireland as a whole, there has also been a marked decline: from the 14 vice-counties listed by Scannell & Synnott (1987), mainly strung along the River Shannon up through the midland counties from Limerick (H8) to Fermanagh (H33), the latest information in the New Atlas indicates that S. latifolium survives in just ten VCs (Preston et al. 2002). The New Atlas representation in Ireland totals 34 hectads, eight of which are pre-1970 (ie 23.5%), a rather more comfortable figure in terms of species survival. However, no one should be complacent when the total number of squares is as low as this and there is any evidence of a decline. This message is confirmed in the seventh revised edition of An Irish Flora (Webb et al. 1996), where the authors regard S. latifolium as 'rare', whereas previous editions of the work described it as 'rather rare'.
The Irish records are basically in two areas: on Lough Erne and along the course of the River Shannon in the RoI.
Current conservation status in B & I: Classified as Nationally Scarce and endangered in the UK, S. latifolium is actually too common around Upper Lough Erne to be listed for specific protection in Northern Ireland under Schedule 8 of the Wildlife and Countryside Act and, similarly, it is not protected in the Republic of Ireland. It is listed as a NI Priority Species since more than 20% of the UK population is in Northern Ireland.
The Great Water-parsnip is listed in the UK Biodiversity Action Plans (UK BAP) and is included in English Nature's Species Recovery Programme involving translocation studies in several areas that previously supported populations of the species. In England, The Environment Agency, who are the lead partners in work to recover this species, have produced plans to improve river and floodplain management that will benefit other plants and animals that are features of these important habitats.
Seed from the Millennium Seed Bank, managed by the Royal Botanic Gardens, Kew at Wakehurst Place, is currently being used to try to restore populations of S. latifolium at suitable sites within its former range. Conservation projects involving translocations of S. latifolium have occurred independently in at least seven counties of England, re-introducing the species in regions where it has been lost or declined; however, the success of these translocations has so far been mixed.
European and world occurrence
S. latifolium is widespread across most of Europe, although it is very rare near the Mediterranean region and is absent from Portugal. Nevertheless, Hultén & Fries (1986, Map 1396) show it recorded from the very south of both Spain and Italy, although it is absent from all the Mediterranean islands. It is common to rare and mainly southern in Fennoscandia (common in both Denmark & S Sweden), but it has markedly declined in Finland (Jonsell & Karlsson 2010). The indigenous range of S. latifolium extends across temperate Asia as far east as W Siberia. It is recorded as an introduction in south-eastern Australia and Chile, but is otherwise unknown in both Japan and N America (Hultén & Fries 1986).
Names
The genus name 'Sium' is an old or classical Greek name for an unknown water plant, possibly a name given to at least two plants, thought to be Sium angustifolium (= Berula erecta) and Veronica anagallis-aquatica (Blue Water-speedwell) (Gilbert-Carter 1964). The Latin specific epithet 'latifolium' means 'broad leaved'.
The English common name 'Water Parsnip' appears to have been shared with S. angustifolium (= Berula erecta, Lesser Water-parsnip), a stoloniferous species. The name is inappropriate for both these species, since neither possesses anything remotely similar to the fleshy rootstock of the true vegetable Parsnip (Pastinaca sativa).
Threats and causes of decline: The tall-herb fen vegetation that supports S. latifolium grows in a sensitive aquatic habitat which has recently suffered major decline across much of Britain and Ireland due to development, drainage and pollution, including nutrient run-off from agricultural land. S. latifolium is sensitive to disturbance and cannot survive regular cutting or ditch clearance. It can only tolerate very occasional dredging or reshaping of its ditches, or moderate levels of grazing and trampling by stock. The neglect of ditches, allowing scrub and young woodland to invade and become established, also kills off the plant. Even in conservation protected sites, decline has been observed, and new plants appear only rarely (J.O. Mountford, in: Stewart et al. 1994).