Lythrum salicaria L., Purple-loosestrife
Account Summary
Native, common and widespread. Eurasian temperate, but naturalised in N America and now circumpolar.
1881-2; Barrington, R.M.; Co Fermanagh.
Throughout the year.
Growth form and preferred habitats
The numerous, bright magenta or pink-purple spikes of this 50-200 cm tall, wet ground perennial are a very spectacular feature of the Upper Lough Erne shoreline during the summer from July to September, especially when mixed among Filipendula ulmaria (Meadowsweet) and Senecio aquaticus (Marsh Ragwort) as is often the case. Purple-loosestrife is a common, prominent and sometimes abundant plant of the water-fringing, tall-herb, marsh and fen vegetation, including everything from fen-carr and reed-swamp to margins of acid Sphagnum bogs and under-managed, wet, marshy grasslands that are intermittently or seasonally flooded, and that are neither excessively grazed nor trampled. It is also frequent on floodplains beside muddy, slow-moving drains, streams and rivers and even occasionally along damp roadsides.
L. salicaria produces a strongly developed tap-root system that persists throughout the life of the plant without any adventitious roots developing (Shamsi & Whitehead 1974). The species grows on a wide range of soil textures, but is usually found in mud, silt, clay, or fen peat. While it can tolerate poor mineral nutrition, it most often appears under reasonably fertile, moderately acid to neutral, lime- or base-rich conditions (Sinker et al. 1985; Mal et al. 1992). It tolerates soils in the pH range 4.0 and above (Shami & Whitehead 1974).
Being a tall plant, in some sites it prefers shelter and can tolerate shading to about 50% of full sun from reeds, willows, alder and ash. On the other hand it is most vigorously competitive, abundant and conspicuous in sunny, marshy grassland and in linear, bank-side situations. The established strategy of the species is categorised as a C/CR (Competitor/Competitive Ruderal) by Grime et al. (1988, 2007), reflecting successful germination of its buried seed after habitat disturbance, rapid growth rate, remarkable fecundity and dispersal ability, and vigorous colonising ability under a wide range of soil nutrient conditions. The principal limiting factors are temperature and moisture (it being essentially a wetland species), so it is unsurprising that it performs best and can become dominant in the mild, wet, W of Ireland and in warm, coastal parts of S Europe (Shami & Whitehead 1977).
Experimental evidence on reaction to nutrient levels suggests that L. salicaria may have spread and increased its presence during the last 50 years or more assisted by nutrient enrichment of drainage waters following increased use of agricultural fertilizers (Shamsi & Whitehead 1977).
Growth rate and vegetative reproduction
The plant has a persistent taproot from which it develops a horizontally spreading rootstock or wide-topped crown. From this perennial plate-like base arise 30 or more, crowded, erect, annual stems, some often reaching 150 cm or, occasionally, 200 cm in height. The growth rate of the species is so high that despite its considerable vegetative plant size, L. salicaria seed can germinate in the early summer whenever temperature rises to 20°C, establish, flower and set seed, all within a single growing season (Shami & Whitehead 1977). For this reason it is sometimes misleadingly referred to as a 'facultative annual', although in reality it is both perennial and polycarpic, only the aerial parts dying back in late autumn (Shipley & Parent 1991). The lifespan of individuals is unknown and it is clearly a topic for further study.
The rootstock is the perennating organ and it produces fresh shoots each year from spreading root buds around the crown, a localised form of vegetative reproduction and clonal development, resulting in tightly tufted 'phalanx' plants that can represent an almost completely dominant monoculture on suitable ground (Mal et al. 1992). Wide vegetative spread is unlikely with this form of growth, but the plant can persist in a particular spot for many years using this property (Shamsi & Whitehead 1974). In B & I and across its native territory, these monospecific stands usually do not persist long, but tend to be replaced quickly, active colonisation by other species commencing in the first year after their establishment. L. salicaria also occurs as single individuals in a wide range of other wetland plant community types (Shamsi & Whitehead 1974).
Flowering reproduction
L. salicaria flowering takes place from early July to September or occasionally October. The species is capable of rapid spread in water systems by means of its lightweight seed, produced in vast numbers despite a complex tri-stylous, self-incompatible (but not absolute), out-breeding system that is unique in the flora of B & I (Darwin 1865). The insect pollination requirement involved with these three flower forms is satisfied by bees and hoverflies attracted by nectar secreted at the base of the hypanthium or 'calyx tube' and copious pollen (Hickey & King 1981). Maximum seed production requires the presence of all three flower morphs (Mal et al. 1992).
The ovary is formed from two fused carpels each containing numerous ovules and the fruit is an oblong-ovoid bi-valved capsule, 3-4 mm long, enclosed in the calyx. It splits into two valves along the septum (wall) between the two locules of the ovary (ie septicidally) to release the small, light seeds (Hickey & King 1981). In a study by Charles Darwin, the capsules contained means of 93, 130 and 83.5 seeds in long-, mid- and short-styled flower forms respectively (Darwin 1877). The lowest capsules on the inflorescence are ripe and seed is dispersed whilst the plant is still green and leafy. A normal healthy plant produces about 900 capsules per year (Shami & Whitehead 1974).
Seed is very mobile, dispersal involving wind transport, water flotation and adhesion in mud to birds and other animals, plus on boats, boots and vehicle tyres (Shamsi & Whitehead 1974; Thompson et al. 1987). Seed viability in soil is also ecologically favourable, survival of at least several years duration, and sometimes exceeding five years, having been recorded (Thompson et al. 1997). It is important to note that germination and seedling emergence is negligible from seeds buried at 2 cm. Thus, irrespective of the large scale of the L. salicaria seed bank, small disturbances in the soil surface layers are an absolute germination requirement, enabling further recruitment to the above ground population from the dormant seed bank (Welling & Becker 1990).
Fermanagh occurrence

It is represented in 207 Fermanagh tetrads, 39.2% of those in the VC. In terms of frequency, in the Fermanagh Flora Database, it ranks joint 25th with Crataegus monogyna (Hawthorn), although it is nothing like as widespread as the latter. As the distribution map indicates, it is predominantly found in the basins of Lough Erne and Lough Melvin and along the River Finn and other major feeders, while elsewhere in Fermanagh it is much more thinly scattered and local.
Irish occurrence
Although Purple-loosestrife is widespread in most lowland parts of Ireland and is particularly abundant in the milder west, it is much less frequent, local or absent in large areas of the NE of the island. Here it is more or less confined to the Lough Neagh and River Bann basin (FNEI 3). In his Flora of Lough Neagh, Harron (1986) commented that L. salicaria has a very fragmented distribution around the shore of that very large lake, usually appearing only in small quantity. He attributed the apparent population depletion of L. salicaria to past drainage operations, something The current author (RSF) considers rather unlikely, since this species very readily re-colonises suitable damp ground. McNeill (2010) commented upon the unusual distribution of L. salicaria in Co Tyrone (H36). It is entirely absent from most of the county, yet is quite abundant in three discrete areas: Lough Neagh shore extending up the River Blackwater; the Fairy Water basin; and along the rivers Finn and Foyle on the margins of the county.
British occurrence
Purple-loosestrife is locally abundant in lowland England and Wales. The New Atlas hectad map shows that the southern and western distribution tendency noticed in Ireland is even more pronounced in Britain, since here, even more so than in Ireland, the species avoids areas of predominantly acidic peat soils and critically colder early spring conditions that limit its germination and growth. Thus L. salicaria is excluded not only from upland regions of England and Wales, but also from most of Scotland except the milder SW (Preston et al. 2002).
European and world occurrence
L. salicaria is almost cosmopolitan in moist or wet, low-lying and coastal areas of Europe & Asia, except in extremely cold and arctic regions. It is native throughout Europe from Britain to C Russia, except in the extreme north. It is also indigenous around the Mediterranean basin including the islands, plus in Syria, Iran and Lebanon. It is also native in N Africa from N Morocco to Ethiopia and in W & N Asia. It is absent from NW Finland, Denmark and Iceland (Shami & Whitehead 1974).
L. salicaria is thoroughly naturalised in mild temperate areas of N America where, in the early 1800s, it was introduced to New England, most likely as either a garden ornamental, a medicinal herb, or accidently carried as seed in sailing ships' ballast (Mal et al. 1992). In the S Hemisphere, it has been introduced to Australia, Tasmania and New Zealand and has become Circumpolar (Hultén & Fries 1986, Map 1348).
A serious invasive weed in N America
Although it had gradually spread throughout NE United States and SE Canada by 1900, L. salicaria only became recognised as an aggressive wetland weed there in the 1930s. The initial spread was probably assisted by habitat disturbance at the time caused by agricultural settlement, military activities and the construction of canals, highways and railways. The species then spread explosively in N America sometime after 1944, and by 1987 was already a major, out of control, alien weed and had become a conservation nightmare. It is now known there as 'The Purple Plague' or 'The Purple Peril', since it chokes wetlands, clogs drains and infills shallow water bays used by fish for spawning. It makes conditions inhospitable to native plants, out-competing them and forming near monoculture stands that support far fewer plant and animal species than the communities they replace (Mal et al. 1992). Eradication appears impossible with present-day technology, but a great deal of research is currently tackling the problem.
Medicinal and other uses
Apparently not used at present, there are old reports of L. salicaria being used in herbal medicine in Ireland. Its astringent properties were used to treat wounds, diarrhoea and even dysentery (Allen & Hatfield 2004). Grieve (1931) says it was "highly esteemed by many herbalists" and "well established in chronic diarrhoea and dysentery, and is used in leucorrhoea and blood-spitting". It has also been employed for fevers, liver diseases, constipation and cholera infantum and for outward application to wounds and sores. It has been stated to be superior to Eyebright (Euphrasia spp.) for preserving the sight and curing sore eyes (Grieve 1931).
Pharmacologists have taken an interest in the plant as it has been discovered that stem and flower extracts produce significant hypoglycemia in hyper- and normo-glycemic rats, the extracts reducing blood sugar by increasing insulin levels. This has potential implications for weight control medication and treatment of diabetes (Lamela et al. 1985, 1986).
The species has also been regarded as a useful honey plant by beekeepers and at least ten cultivars have been selected for and improved for decorative garden use (Mal et al. 1992; Griffiths 1994).
Threats
None.